building construction


building construction
Techniques and industry involved in the assembly and erection of structures.

Early humans built primarily for shelter, using simple methods. Building materials came from the land, and fabrication was dictated by the limits of the materials and the builder's hands. The erection sequence involved, as now, first placing a foundation (or using the ground). The builder erected the structural system; the structural material (masonry, mud, or logs) served as both skeleton and enclosure. Traditional bearing-wall and post-and-beam systems eventually gave way to framed structures, and builders became adept at sealing and fireproofing with a variety of claddings (exterior coverings) and finishes. Steel-framed buildings are usually enclosed by curtain walls. In modern-day construction, sheathing the skeleton of the building is only the beginning; specialists then begin the bulk of the work inside, installing plumbing, electrical wiring, HVAC (heating, ventilating, and air-conditioning), windows, floor coverings, plasterwork, moldings, ceramic tile, cabinets, and other features. See also architecture.

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Introduction

      the techniques and industry involved in the assembly and erection of structures, primarily those used to provide shelter.

 Building construction is an ancient human activity. It began with the purely functional need for a controlled environment to moderate the effects of climate. Constructed shelters were one means by which human beings were able to adapt themselves to a wide variety of climates and become a global species.

      Human shelters were at first very simple and perhaps lasted only a few days or months. Over time, however, even temporary structures evolved into such highly refined forms as the igloo. Gradually more durable structures began to appear, particularly after the advent of agriculture, when people began to stay in one place for long periods. The first shelters were dwellings, but later other functions, such as food storage and ceremony, were housed in separate buildings. Some structures began to have symbolic as well as functional value, marking the beginning of the distinction between architecture and building.

      The history of building is marked by a number of trends. One is the increasing durability of the materials used. Early building materials were perishable, such as leaves, branches, and animal hides. Later, more durable natural materials—such as clay, stone, and timber—and, finally, synthetic materials—such as brick, concrete, metals, and plastics—were used. Another is a quest for buildings of ever greater height and span; this was made possible by the development of stronger materials and by knowledge of how materials behave and how to exploit them to greater advantage. A third major trend involves the degree of control exercised over the interior environment of buildings: increasingly precise regulation of air temperature, light and sound levels, humidity, odours, air speed, and other factors that affect human comfort has been possible. Yet another trend is the change in energy available to the construction process, starting with human muscle power and developing toward the powerful machinery used today.

      The present state of building construction is complex. There is a wide range of building products and systems which are aimed primarily at groups of building types or markets. The design process for buildings is highly organized and draws upon research establishments that study material properties and performance, code officials who adopt and enforce safety standards, and design professionals who determine user needs and design a building to meet those needs. The construction process is also highly organized; it includes the manufacturers of building products and systems, the craftsmen who assemble them on the building site, the contractors who employ and coordinate the work of the craftsmen, and consultants who specialize in such aspects as construction management, quality control, and insurance.

      Building construction today is a significant part of industrial culture, a manifestation of its diversity and complexity and a measure of its mastery of natural forces, which can produce a widely varied built environment to serve the diverse needs of society. This article first traces the history of building construction, then surveys its development at the present time. For treatment of the aesthetic considerations of building design, see architecture. For further treatment of historical development, see art and architecture, Anatolian; art and architecture, Arabian; art and architecture, Egyptian; art and architecture, Iranian; art and architecture, Mesopotamian; art and architecture, Syro-Palestinian; architecture, African (African architecture); art and architecture, Oceanic; architecture, Western (Western architecture); arts, Central Asian (Central Asian arts); arts, East Asian; arts, Islamic (Islamic arts); arts, Native American (Native American art); arts, South Asian (South Asian arts); and arts, Southeast Asian (Southeast Asian arts).

The history of building construction

Primitive building: the Stone Age
      The hunter-gatherers (hunting and gathering culture) of the late Stone Age, who moved (nomadism) about a wide area in search of food, built the earliest temporary shelters that appear in the archaeological record. Excavations at a number of sites in Europe dated to before 12,000 BC show circular rings of stones that are believed to have formed part of such shelters. They may have braced crude huts made of wooden poles or have weighted down the walls of tents (tent) made of animal skins, presumably supported by central poles.

      A tent illustrates the basic elements of environmental (environment) control that are the concern of building construction. The tent creates a membrane to shed rain and snow; cold water on the human skin absorbs body heat. The membrane reduces wind speed as well; air over the human skin also promotes heat loss. It controls heat transfer by keeping out the hot rays of the sun and confining heated air in cold weather. It also blocks out light and provides visual privacy. The membrane must be supported against the forces of gravity and wind; a structure is necessary. Membranes of hides are strong in tension (stresses imposed by stretching forces), but poles must be added to take compression (stresses imposed by compacting forces). Indeed, much of the history of building construction is the search for more sophisticated solutions to the same basic problems that the tent was set out to solve. The tent has continued in use to the present. The Saudi Arabian goats' hair tent, the Mongolian yurt with its collapsible wooden frame and felt coverings, and the American Indian tepee with its multiple pole supports and double membrane are more refined and elegant descendants of the crude shelters of the early hunter-gatherers.

      The agricultural revolution, dated to about 10,000 BC, gave a major impetus to building construction. People no longer traveled in search of game or followed their herds but stayed in one place to tend their fields. Dwellings began to be more permanent. Archaeological records are scanty, but in the Middle East (Middle East, ancient) are found the remains of whole villages of round dwellings called tholoi (tholos), whose walls are made of packed clay; all traces of roofs (roof) have disappeared. In Europe, tholoi were built of dry-laid stone with domed roofs; there are still surviving examples (of more recent construction) of these beehive structures in the Alps. In later Middle Eastern tholoi a rectangular antechamber or entrance hall appeared, attached to the main circular chamber—the first examples of the rectangular plan form in building. Still later the circular form was dropped in favour of the rectangle as dwellings were divided into more rooms and more dwellings were placed together in settlements. The tholoi marked an important step in the search for durability; they were the beginning of masonry construction.

      Evidence of composite building construction of clay and wood, the so-called wattle-and-daub (wattle and daub) method, is also found in Europe and the Middle East. The walls were made of small saplings or reeds, which were easy to cut with stone tools. They were driven into the ground, tied together laterally with vegetable fibres, and then plastered over with wet clay to give added rigidity and weatherproofing. The roofs have not survived, but the structures were probably covered with crude thatch or bundled reeds. Both round and rectangular forms are found, usually with central hearths.

      Heavier timber (carpentry) buildings also appeared in Neolithic (Neolithic Period) cultures, although the difficulties of cutting large trees with stone tools limited the use of sizable timbers to frames. These frames (framed building) were usually rectangular in plan, with a central row of columns (column) to support a ridgepole and matching rows of columns along the long walls; rafters were run from the ridgepole to the wall beams. The lateral stability of the frame was achieved by burying the columns deep in the ground; the ridgepole and rafters were then tied to the columns with vegetable fibres. The usual roofing material was thatch: dried grasses or reeds tied together in small bundles, which in turn were tied in an overlapping pattern to the light wooden poles that spanned between the rafters. Horizontal thatched roofs leak rain badly, but, if they are placed at the proper angle, the rainwater runs off before it has time to soak through. Primitive builders soon determined the roof pitch that would shed the water but not the thatch. Many types of infill were used in the walls of these frame houses, including clay, wattle and daub, tree bark (favoured by American Woodland Indians), and thatch. In Polynesia (Polynesian culture) and Indonesia, where such houses are still built, they are raised above the ground on stilts for security and dryness; the roofing is often made of leaves and the walls are largely open to allow air movement for natural cooling. Another variation of the frame was found in Egypt and the Middle East, where timbers were substituted for bundles of reeds.

Bronze Age and early urban cultures
      It was the cultures of the great river valleys—including the Nile, the Tigris and Euphrates, the Indus, and the Huang Ho—with their intensive agriculture based on irrigation, that developed the first communities large enough to be called cities (city). These cities were built with a new building technology, based on the clay available on the riverbanks. The packed clay walls of earlier times were replaced by those constructed of prefabricated (prefabrication) units: mud bricks. This represented a major conceptual change from the free forms of packed clay to the geometric modulation imposed by the rectangular brick (brick and tile), and the building plans too became strictly rectangular.

      Bricks were made from mud and straw formed in a four-sided wooden frame, which was removed after evaporation had sufficiently hardened the contents. The bricks were then thoroughly dried in the sun. The straw acted as reinforcing to hold the brick together when the inevitable shrinkage cracks appeared during the drying process. The bricks were laid in walls with wet mud mortar or sometimes bitumen to join them together; openings were apparently supported by wooden lintels (post-and-lintel system). In the warm, dry climates of the river valleys, weathering action was not a major problem, and the mud bricks were left exposed or covered with a layer of mud plaster. The roofs of these early urban buildings have disappeared, but it seems likely that they were supported by timber beams and were mostly flat, since there is little rainfall in these areas. Such mud brick or adobe construction is still widely used in the Middle East, Africa, Asia, and Latin America.

 Later, about 3000 BC in Mesopotamia (Mesopotamia, history of), the first fired bricks appeared. Ceramic pottery had been developing in these cultures for some time, and the techniques of kiln-firing were applied to bricks, which were made of the same clay. Because of their cost in labour and fuel, fired bricks were used at first only in areas of greater wear, such as pavements or the tops of walls subject to weathering. They were used not only in buildings but also to build sewers to drain wastewater from cities. It is in the roofs of these underground drains that the first surviving true arches (arch) in brick are found, a humble beginning for what would become a major structural form. Corbel vaults and domes made of limestone rubble appeared at about the same time in Mesopotamian tombs (Figure 1—>). Corbel vaults are constructed of rows of masonry placed so that each row projects slightly beyond the one below, the two opposite walls thus meeting at the top. The arch and the vault may have been used in the roofs and floors of other buildings, but no examples have survived from this period. The well-developed masonry technology of Mesopotamia was used to build large structures of great masses of brick, such as the temple at Tepe Gawra and the ziggurats (ziggurat) at Ur and Borsippa (Birs Nimrud), which were up to 26 metres (87 feet) high. These symbolic buildings marked the beginnings of architecture in this culture.

      The development of bronze, and later iron, technology in this period led to the making of metal tools (tool) for working wood, such as axes and saws. Less effort was thus required to fell and work large trees. This led in turn to new developments in building technics; timbers were cut and shaped extensively, hewed into square posts, sawed into planks, and split into shingles. log cabin construction appeared in the forested areas of Europe, and timber framing became more sophisticated. Although the excavated remains are fragmentary, undoubtedly major advances were made in timber technology in this period; some of the products, such as the sawed plank and the shingle, are still used today.

Stone construction in Egypt (Egypt, ancient)
      Like the other great river valley cultures, Egypt built its cities with mud brick; fired brick did not appear there until Roman times. Timber was used sparingly, for it was never abundant. It was used mainly in roofs, where it was heavily supplemented by reeds. Only a few royal buildings were built with full timber frames.

      It was against this drab background of endless mud brick houses that a new technology of cut-stone construction emerged in the temples and pyramids of the 4th dynasty (c. 2575–c. 2465 BC). Egypt, unlike Mesopotamia or the Indus valley, had excellent deposits of stone exposed above ground; limestone, sandstone, and granite were all available. But the extracting, moving, and working of stone was a costly process, and the quarrying of stone was a state monopoly. Stone emerged as an elite construction material used only for important state buildings.

      The Egyptians developed cut stone for use in royal mortuary buildings not only for its strength but also for its durability. It seemed the best material to offer eternal protection to the pharaoh's ka, the vital force he derived from the sun-god and through which he ruled. Thus stone had both a functional and symbolic significance.

      Within the long tradition of brick masonry, stone construction appeared abruptly, with little transition. The brick mastaba tombs (tomb) of the early kings and nobles suddenly gave way to the stone technics of King Djoser's ceremonial complex at Ṣaqqārah, the construction of which is associated with his adviser and builder Imhotep. It is a structure of somewhat curious and uncertain forms but of great elegance in execution and detail. It consists mostly of massive limestone walls that enclose a series of interior courtyards. The walls have convoluted surfaces, which recall the mastaba tombs, with dummy doors, and there are even whole dummy buildings of solid stone. The complex has a large entrance hall with a roof supported by massive stone lintels that rest on rows of short wing walls projecting from the enclosing walls. There are no free-standing columns, but incipient fluted columns appear at the ends of the wing walls and engaged 3/4-columns project from the walls of the courtyards. The complex also contains the first pyramid, created from successively smaller mastabas. All these elements are built of small stones, which could be handled by one or two men. It represents a technology that was already highly developed, involving elaborate methods of quarrying (quarry), transporting, and working stone.

      The construction process began at the quarries. Most of them were open-faced, although in some cases tunnels were extended several hundred metres into cliffs to reach the best quality stone. For extracting sedimentary rock, the chief tool was the mason's pick with a 2.5-kilogram metal head and a 45-centimetre haft. With these picks vertical channels as wide as a man were cut around rectangular blocks, exposing five faces. The final separation of the sixth face was accomplished by drilling rows of holes into the rock with metal bow drills. Wooden wedges were driven into the holes to fill them completely. The wedges were doused with water, which they absorbed and which caused them to expand, breaking the stone free from its bed. In the extraction of igneous rock such as granite, which is much harder and stronger than limestone, the mason's pick was supplemented by balls of dolerite weighing up to 5 kilograms, which were used to break the rock by beating and pounding. Granite was also drilled and sawed with the help of abrasives, and expanding wooden wedges were used in splitting.

      The Egyptians were able to move blocks weighing up to 1,000,000 kilograms from quarries to distant building sites. This was an amazing accomplishment, as their only machinery was levers and crude wooden sledges worked by masses of men and draft animals. There were no wheeled vehicles before 1500 BC, and they were never widely used in building. Most quarries were near the Nile, however, and boats were also extensively used in transporting stone.

      At the building site the rough stones were precisely finished to their final forms, with particular attention to their exposed faces. This was done with metal chisels and mallets; squares, plumb bobs, and straightedges were used to check the accuracy of the work. These tools remained standard until the 19th century. After the first appearance of small stones at Ṣaqqārah, their size began to increase until they attained the cyclopean scale usually associated with Egyptian masonry at about the time of the building of the pyramids (pyramid). In spite of the heavy loads that stone structures created, foundations were of a surprisingly shoddy and improvised character, made of small blocks of poor quality stone. Not until the 25th dynasty (c. 750–656 BC) were important buildings placed on a below-grade (underground) platform of masonry several metres thick.

      The Egyptians possessed no lifting machinery to raise stones vertically. It is generally thought that the laying of successive courses of masonry was accomplished with earth or mud brick ramps, over which the stones were dragged to their places in the walls by animal and human muscle power. Later, as the ramps were removed, they served as platforms for the masons to apply the final finishes to the stone surfaces. The remains of such ramps can still be seen at unfinished temples that were begun in the Ptolemaic period. The stones were usually laid with a bed of mortar made of gypsum, sand, and water, which perhaps acted more as a lubricant to push the stone into place than as a bonding agent. There was also limited use of metal dovetail anchors between blocks.

      The great Pyramids of Giza (Giza, Pyramids of), the tallest of which rose to a height of 147 metres (481 feet), are a marvelous technological achievement, and their visual impact is stunning even today; it was not until the 19th century that taller structures would be built. But they also represent a dead end in massive stone construction, which soon moved in the direction of lighter and more flexible stone frames and the creation of larger interior spaces. The free-standing stone column supporting stone beams appeared for the first time in the royal temples associated with the pyramids of about 2600 BC. Square granite columns carrying heavy granite lintels spanned 3 to 4 metres (10 to 13 feet); the spaces between the lintels were roofed by massive granite slabs. In these structures the abstract notion of the timber frames of the early royal buildings was translated into stone.

      Although stone is more durable than timber, it is quite different in structural strength. Stone is much stronger in compression than timber but is weaker in tension. For this reason, stone works well for columns, which could be made very high—for example, 24 metres (80 feet) in the great temple of Amon-Re at Karnak. But stone lintels spanning between columns are limited by the tension they develop on their bottom surfaces; their maximum span is perhaps 5 metres (16 feet). Thus, for longer spans, another structural form was needed to exploit the higher compressive strength of stone. But the arch, which could span a longer distance in compression, remained confined to the sewers and to the underground roofs of the tombs of minor officials. So, perhaps with the image of the timber building frame still strong in their minds, the Egyptian masons were content to explore the limitations of the analogous stone frame in a series of great temples built during the New Kingdom (1539–1075 BC) at Karnak and Luxor, culminating in the elegant loggias of Queen Hatshepsut's temple at Dayr al-Baḥrī. The paradigm of the stone-frame temple that they established would endure to the end of the classical world.

Greek and Hellenistic cultures
      Use of the Egyptian stone frame diffused throughout the eastern Mediterranean after 1800 BC, and the cultures of mainland Greece (ancient Greek civilization) were particularly attracted to it. In the Greek world of the Aegean and southern Italy, many stone-frame temples were built; some have survived to the present day in various states of preservation. They were built largely of local marble or limestone; there was no granite for huge monoliths. The basic technology was little changed from that of Egypt; the major difference was in the labour force. There were no state-mobilized masses of unskilled workers to move huge stones; there were instead small groups of skilled masons who worked independently. The building accounts of the Parthenon show that each column was built under a separate contract with a master mason. There was certainly lifting machinery for handling the blocks, although its precise description is unknown; the concealed faces of stones still have grooves and holes that engaged the ropes used to lift them into place. Metal cramps and dowels were introduced for joining stones together; mortar was almost never used. There was some experimentation with iron beams (beam) to reinforce longer spans in stone, but the maximum remained about 5 to 6 metres (16 to 20 feet). Longer spans were achieved with timber beams supported by the stone frame; the solid stone roof slabs of the great Egyptian temples could not be duplicated.

      Much of the mason's effort was concentrated on the refinements of detail and optical corrections for which Greek architecture is justly famous. This same sense is also seen in the first surviving construction drawings, which were made on the unfinished surfaces of the stone walls of the Temple of Didyma. Such drawings would normally have been erased during the final finishing of the wall surfaces, and those at Didyma survived because the temple was never completed. The drawings show how the masons developed the final profiles of columns and moldings—a rare glimpse of the design processes of builders before the days of pencil and paper.

      In contrast to stone technology, which remained largely unchanged from Egyptian methods, clay masonry underwent considerable development. Although mud brick remained standard for dwellings, fired brick was more widely used and began to be laid with lime mortar, a technique borrowed from stone construction. Glazed brick also appeared in this period, particularly outside the Greek world among the Babylonians (Babylonia) and Persians (Iran), who made considerable use of it in royal palaces. A fine surviving example is the Ishtar Gate of the Palace of Nebuchadrezzar at Babylon, with a true arch spanning 7.5 metres (25 feet) and dated to 575 BC. Another major innovation was the fired clay roof tile. This was much more waterproof than thatch, and tile roofs could have the lower pitch characteristic of Greek temples. Hollow terra-cotta blocks for wall ornaments also appeared about this time, probably derived from the highly advanced pottery industry, which routinely made fired clay vessels more than one metre long.

      Although stone technology remained confined to the trabeated (column-and-beam, or post-and-lintel) frame, there were a few structures that hinted at future developments. Perhaps the most spectacular building achievement of the age was the Pharos of Alexandria, the great lighthouse built for Ptolemy II in the 3rd century BC. It was a huge stone tower nearly as high as the Great Pyramid but much smaller at the base—perhaps 30 metres (100 feet) square. Within this mass of masonry was a complex system of ramps over which pack animals carried fuel for the beacon at the top. The Pharos was the first high-rise building, but the limitations of masonry structures and the lack of a rapid way of moving people vertically precluded any further development of tall buildings until the 19th century. The Pharos remained the only example of this type long after it was demolished by the Arabs beginning in the 7th century AD.

      Another example of a new stone technology that was tried but not pursued further by the Greeks was the underground tombs of Mycenae, built about 1300 BC. These tombs have main chambers enclosed by pointed domes of corbeled stone construction, about 14 metres (47 feet) in diameter and 13 metres (43 feet) high. Crude versions of the corbel dome had appeared earlier in Mesopotamian tombs and the tholoi of Neolithic Europe, but in Mycenae the technics were refined and enlarged in scale. A corbel dome or arch does not develop the high compressive forces that characterize true arches and domes, which are built of radial segments of stone or brick. Thus it does not take full advantage of the great compressive strength of stone and cannot span long distances; 14 metres is near the upper limit. Greek masons did not choose to explore this type of structure; their buildings remained largely concerned with exterior forms. The Roman builders who followed them, however, exploited masonry to its full potential and created the first great interior spaces.

Roman achievements
      It was from the Etruscans (Etruscan), who lived in the northern part of Italy, that the Romans (ancient Rome) derived much of their early building technology. The Etruscans, probably influenced by a few rare Greek examples in southern Italy, developed the true arch in stone. A late specimen of the 3rd century BC is the Porta Marzia, an arched city gateway with a span of about 6 metres (20 feet), in Perugia. The Etruscans also had a highly developed terra-cotta technology and made excellent fired bricks.

Masonry construction
      The Romans adopted Etruscan stone construction based on the arch and built many spectacular examples of what they called opus quadratum, or structures of cut stone blocks laid in regular courses. Most of these were public works in conquered provinces, such as the late 1st-century-BC Pont du Gard (Gard, Pont du), a many-arched bridge and aqueduct spanning 22 metres (72 feet) near Nîmes, in France, or the fine bridge over the Tagus River at Alcántara in Spain, with a span of almost 30 metres (100 feet), built about AD 110. Oddly enough, such long spans in stone were never applied to buildings. The surviving Roman buildings with stone arches or vaults have typical spans of only 4 to 7 metres (15 to 25 feet); small stone domes with diameters of 4 to 9 metres were built in Roman Syria. Such arches and domes imply the existence of sophisticated timber formwork to support them during construction, as well as advanced lifting machinery, but there are no extant records of either. Many of these structures survived the fall of the empire, and they became models for the revival of stone construction in medieval Europe, when masons again sought to build “in the Roman manner.” The Romans also inherited the trabeated stone frame from the Greeks of southern Italy and continued to build temples and other public buildings with this type of construction into the 3rd century AD.

      Brickmaking, particularly in the region of Rome itself, became a major industry and finally, under the empire, a state monopoly. Brick construction was cheaper than stone due to the economies of scale in mass production and the lower level of skill needed to put it in place. The brick arch was adopted to span openings in walls, precluding the need for lintels. Mortar was at first the traditional mixture of sand, lime, and water, but, beginning in the 2nd century BC, a new ingredient was introduced. The Romans called it pulvis puteoli after the town of Puteoli (modern Pozzuoli), near Naples, where it was first found; the material, formed in Mount Vesuvius and mined on its slopes, is now called pozzolana. When mixed with lime, pozzolana forms a natural cement that is much stronger and more weather-resistant than lime mortar alone and that will harden even under water. Pozzolanic mortars were so strong and cheap, and could be placed by labourers of such low skill, that the Romans began to substitute them for bricks in the interiors of walls; the outer wythes of bricks were used mainly as forms to lay the pozzolana into place. Finally, the mortar of lime, sand, water, and pozzolana was mixed with stones and broken brick to form a true concrete, called opus caementicium. This concrete was still used with brick forms in walls, but soon it began to be placed into wooden forms, which were removed after the concrete had hardened.

Early concrete structures
      One of the earliest surviving examples of this concrete construction is the Temple of the Sybil (or Temple of Vesta) at Tivoli, built during the 1st century BC. This temple has a circular plan with a peristyle of stone columns and lintels around the outside, but the wall of the circular cella, or sanctuary room, inside is built of concrete—an uneasy confrontation of new and traditional forms of construction. An early large-scale example in Rome itself of brick-faced concrete is the plain rectangular walls of the Camp of the Praetorian Guard, built by Sejanus in AD 21–23. But the possibilities of plastic form suggested by this initially liquid material, which could easily assume curved shapes in plan and section, soon led to the creation of a series of remarkable interior spaces, spanned by domes or vaults and uncluttered by the columns required by trabeated stone construction, that showed the power of the imperial state. The first of these is the octagonal domed fountain hall of Nero's Golden House (Golden House of Nero) (AD 64–68), which is about 15 metres (50 feet) in diameter with a large circular opening, or oculus, in the top of the dome. The domed form was rapidly developed in a series of imperial buildings that culminated in the emperor Hadrian's Pantheon of about AD 118–128. This huge circular structure was entered from a portico of stone columns and was surmounted by a dome 43.2 metres (142 feet) in diameter, lighted by an oculus at the top. The walls supporting the dome are of brick-faced concrete 6 metres (20 feet) thick lightened at intervals by internal recesses; the dome is of solid concrete 1.5 metres (5 feet) in average thickness and rising 43.2 metres above the floor. This magnificent structure has survived in good condition to modern times; the diameter of its circular dome remained unsurpassed until the 19th century.

      Two large fragments of great concrete cross-vault buildings still survive from the late empire. The first of these is a portion of the Baths of Diocletian (c. 298–306) with a span of 26 metres (85 feet); it was converted into the church of Santa Maria degli Angeli by Michelangelo in the 16th century. The other is the Basilica of Maxentius (Maxentius, Basilica of) (AD 307–312), also with a span of 26 metres. All of these buildings contained stone columns, but they were purely ornamental and could have been removed at will. The brick-faced concrete walls were left exposed on the exteriors, but the interiors were lavishly decorated with a veneer of thin slabs of coloured stone held in place by metal fasteners that engaged slots cut in the edges of the slabs, a technique still used in the 20th century. These and other great Roman public spaces spanned by concrete domes and vaults made a major advance in scale over the short spans of the stone frame.

      In the late empire, concrete technology gradually disappeared, and even brickmaking ceased in western Europe. But significant developments in brick technology continued in the eastern Roman world, where the achievements of earlier periods in concrete were now duplicated in brickwork. The tomb of the emperor Galerius (now the Church of St. George) of about AD 300 at Salonika in Greece has a brick dome 24 metres (80 feet) in diameter. It probably was the model for the climactic example of late Roman building, the great church of Hagia Sophia (532–537) in Constantinople, which features a central dome spanning 32.6 metres (107 feet). Even Rome's great enemies, the Sāsānian (Sāsānian dynasty) Persians, built a large brick-vaulted hall in the palace at Ctesiphon (usually identified with Khosrow I [mid-6th century] but probably a 4th-century structure) with a span of 25 metres (82 feet) by borrowing Roman methods. These late brick structures were the last triumphs of Roman building technology and would not be equaled for the next 900 years.

Timber and metal construction
      The Romans also made major advances in timber technology. Reliefs on Trajan's Column show the timber lattice truss bridges used by Roman armies to cross the Danube. The truss, a hollowed-out beam with the forces concentrated in a triangulated network of linear members, was apparently a Roman invention. No evidence of their theoretical understanding of it exists, but nevertheless they were able to master the design of trusses in a practical way. A fine example is the Basilica of Constantine at Trier (AD 297–299), where timber king-post roof trusses (triangular frames with a vertical central strut) span a hall 23 metres (75 feet) wide; the present roof is a restoration, but the original must have been similar.

      The notion of the truss was extended from timber to metal. bronze trusses, running over three spans of about 9 metres (30 feet) each, supported the roof of the portico of the Pantheon. The choice of bronze was probably made more for durability than strength, because Pope Urban VIII was able to remove this bronze work in 1625 (to melt it down for cannon) and replace it with timber trusses. The truss remained an isolated achievement of Roman building that would not be equaled until the Renaissance.

      Metals were used extensively in Roman buildings. In addition to bronze trusses, the Pantheon had bronze doors and gilded bronze roof tiles. lead was another material introduced by the Romans for roofing; it was waterproof and could be used with very low pitches.

Building support systems
      Perhaps the most important use of lead was for pipes to supply fresh water to buildings and to remove wastewater from them (the word plumbing comes from the Latin plumbum, which means lead). The Romans provided generous water supplies (water-supply system) for their cities; all of the supply systems worked by gravity and many of them used aqueducts and syphons. Although most people had to carry their water from public fountains, there was limited distribution of water to public buildings (particularly baths) and some private residences and apartment houses; private and semiprivate baths and latrines became fairly common. The wastewater drainage system was limited, with no treatment of sewage (sewage system), which was simply discharged into a nearby river. But even these fairly modest applications of public sanitation far exceeded those of previous cultures and would not be equaled until the 19th century.

      Another material that the Romans applied to buildings was glass, which had been developed by the Egyptians who used it only for jewelry and small ornamental vessels. The Romans devised many kinds of coloured glass for use in mosaics to decorate interior surfaces. They also made the first clear window glass, produced by blowing glass cylinders that were then cut and laid flat. Seneca (c. 4 BC–AD 65) described the sensation caused by the appearance of glazed sun porches in the villas near Rome. Although no Roman glass installations have survived, glass apparently became fairly common in public buildings and was even used in middle-class apartment houses in the capital.

      In most Roman buildings, the central open fire remained the major source of heat (heating)—as well as annoying smoke—although the use of charcoal braziers made some improvement. A major innovation was the development of hypocaust, or indirect radiant, heating, by conducting heated air through flues in floors and walls. The heated masonry radiated a pleasantly uniform warmth, and smoke was eliminated from occupied spaces; the same method was used to heat water for baths. The Basilica of Constantine at Trier has a well-preserved example of hypocaust heating, where the stone slabs of the floor are supported on short brick columns, creating a continuous heating plenum beneath it.

Romanesque and Gothic
      The disappearance of Roman power in western Europe during the 5th century (Middle Ages) led to a decline in building technology. Brickmaking became rare and was not revived until the 14th century. Pozzolanic concrete disappeared entirely, and it would not be until the 19th century that man-made cements would equal it. The use of domes and vaults in stone construction was also lost. Building technics fell to Iron Age levels, exemplified by log construction, packed clay walls, mud brick, and wattle and daub.

      Advanced building technologies were developing in China in this same period during the Sui (581–618) and T'ang (618–907) dynasties. In the 3rd century BC the completion of the Great Wall (Great Wall of China), about 6,400 kilometres (4,000 miles) in length and following a sinuous path along the contours of rugged terrain, had demonstrated remarkable achievements in masonry technology, logistics, and surveying methods. The An-Chi Bridge, built about AD 610 in Hopei province, had a stone arch with a span of 37.5 metres (123 feet), far exceeding the Roman bridge at Alcántara. Extensive work was also done in the development of heavy timber framing (primarily for temples), and stone tower pagodas up to 60 metres (200 feet) high were built; fired brick was also widely used. These elements of Chinese building technology set a high standard of quality that would be maintained until the 19th century.

Stone construction
      Beginning in the 9th century, there were the first stirrings of the revival of stone construction in Europe; the Palatine Chapel of Charlemagne at Aachen (consecrated 805), with its octagonal segmented dome spanning 14.5 metres (47 feet), is an early example of this trend. But the Romanesque style, building “in the Roman manner” with stone arches, vaults (vault), and domes to span interior spaces, did not really begin until the later part of the 11th century. Vaults reappeared in such structures as the cathedral of Santiago de Compostela in Spain (begun 1078) and Saint Sernin at Toulouse (begun 1080). The cross vault raised on columns was seen again at Speyer Cathedral (1030–65, reconstructed c. 1082–1137) and Durham Cathedral (1093–1133), and the domes of St. Mark's Basilica (San Marco Basilica) in Venice (late 11th century) and the cathedral of Saint-Front in Périgueux (1120–1150) marked the recovery of the complete range of Roman structural forms. All these buildings were built by the Roman Catholic church (Roman Catholicism), which had spread its influence throughout western Europe in this period. One contemporary chronicler wrote that the earth seemed to be “clothing itself with a white robe of churches,” white because they were new and built of stone. From 1050 to 1350 more stone was quarried in France alone than in the whole history of ancient Egypt—enough to build 80 cathedrals (cathedral), 500 large churches, and tens of thousands of parish churches. The great building campaign of medieval times has been called the “cathedral crusade,” an equally impassioned counterpart of the great military adventures to recover the Holy Land. This vast undertaking required many masons, who worked as free craftsmen, organizing themselves into societies or guilds. They oversaw the quarrying of stone, supervised the process of apprenticeship by which new members were trained, and did all the cutting and placing of stone at the building site. The basic tools of the medieval masons were little changed from those of Egypt, but they had large saws driven by waterwheels to cut stone as well as considerable machinery for raising and moving materials. Their knowledge of technics was a closely held secret; it included the rules of proportion for overall planning and for determining the safe dimensions of structural members. One extant sketchbook of drawings, from the master mason Villard de Honnecourt, shows a keen sense of observation, a love of mechanical devices, and above all the notion of geometric form that underlay the work; but it gives only tantalizing bits of information about actual building construction. Jean Mignot, one of the master masons of Milan Cathedral, summed up their approach with the phrase ars sine scientia nihil est, “art without science is nothing”; that is, skill in building derived from practical experience (ars) must be tempered and guided by precise principles (scientia), which were seen as being embodied in the theorems of geometry, the only science of medieval times. But with these limited means the masons were able to realize great achievements.

      Romanesque masons had two patrons, church and state. The state built mostly for military purposes, and Roman stonework, once recovered, was adequate for castles and fortifications. But the church had other interests that propelled the development of stone construction in new and daring directions. St. Augustine had written that light is the most direct manifestation of God. It was this idea that led the search for ways to introduce more and more light into churches, opening ever larger windows in the walls until a new kind of diaphanous stone skeleton evolved.

      The Roman-inspired circular cross vaults and arches in stone were heavy and needed heavy walls and piers to receive their thrusts; the windows they offered were small. Medieval masons found that there was a more efficient form for the arch than the classical circle; this form is a catenary curve—that is, one formed by a chain when it hangs under its own weight. But the masons' belief in geometry and the perfection of circular forms led them to approximate the catenary shape with two circular segments that met in a point at the top, the so-called Gothic arch. Such arches could be made thinner since they more efficiently channeled the compressive forces that flowed through them and allowed larger openings in the walls. The heavy piers that took the lateral thrust of the roof vaults were soon hollowed out into half arches or flying buttresses, which allowed even more light to enter the nave. To absorb the forces flowing down through the stone frame, massive foundations were required; often the volume of stone below ground was greater than that above. To further lighten the loads, the vaults themselves were made thinner by introducing ribs at the intersections of their curved surfaces, called groins. The ribs were built with supporting formwork or centring made of timber; close cooperation was needed between the carpenters and the masons. The curved surfaces of stones between the ribs were probably laid with little formwork, using only mortar; brick vaults are still built this way in the Middle East. The mortar was used not only for adhesion as a construction device but also later to check for tension cracks, which were signs of possible failure; the mortar thus served as a means of quality control to help keep the structure in compression. The naves (nave) of cathedrals were made higher to gather more light; Amiens Cathedral (begun 1220) was 42 metres (140 feet) high, and finally in 1347 Beauvais Cathedral reached the maximum height of 48 metres (157 feet), but its vaults soon collapsed and had to be rebuilt. The spans of the naves of Gothic churches remained fairly small, about 13 to 16 metres (45 to 55 feet); only a few late examples have longer spans, the greatest being 23 metres (74 feet) at Gerona (Girona) Cathedral (completed 1458).

      After the enthusiasm of the cathedral crusade ebbed in the 14th century and the basic fabric of most cathedrals was completed, a new element appeared to further test the skill of masons and carpenters: the spire. The spire was more a symbol of local pride than a part of the theological quest for more light, but it raised interesting technical problems. At Salisbury Cathedral the spire was built over the crossing of the nave and transept, which had not been designed to accommodate it; the tall crossing piers began to buckle under the added weight. Strainer arches had to be added between the piers to brace them against buckling; this was apparently the first time that stone columns were slender and heavily loaded enough to be observed to bend or buckle—later, such action would be a major concern in the design of metal columns (column). Salisbury's spire is an ingenious composite structure of stone cladding laid over a timber frame and tied together at the base with iron bands to resist spreading; it rose to a total height of 123 metres (404 feet) when it was finished in 1362. Strasbourg Cathedral added a 144-metre (475-foot) spire in 1439, and the upper limit was reached at Beauvais Cathedral in 1569 when its 157-metre (516-foot) spire was completed; the Beauvais spire collapsed in 1573 and was never rebuilt, a last sad epilogue to the cathedral crusade.

Construction in timber and brick
      Timber construction underwent slow development in this period. Scandinavian stave churches (stave church) of heavy timber were built from the 11th through the 14th century, prior to the triumph of the stone church, and about 30 have survived to the present day. In western Europe, particularly from the 14th century onward, half-timber (half-timber work) construction emerged as a new form of house building. The continental type had a frame of squared timbers, with vertical posts spaced about one metre apart and horizontal girts spaced at the same distance; diagonal braces were run through the outside walls for lateral stability. The roof beams spanned between the ridgepole and the walls; floor beams were supported on the walls and interior partitions. The English half-timber frame was similar, but it eliminated the horizontal girts and diagonal bracing by using closely spaced verticals about one-half metre apart. In both systems the space in the outside wall was filled with an enclosure material to impart added rigidity to the frame; brick or wattle and daub were often used. All the timbers of the frame were attached together by elaborate dovetail, or mortise-and-tenon, joints. Half-timber framing would remain the standard way of building with wood in Europe until the 19th century. There was also considerable use of heavy timber for the roofs and floors of masonry buildings, which was influenced by shipbuilding technology. A particular instance of this is the English hammer-beam roof, which was a kind of corbeled truss that could span quite long distances. The roof of King Richard II's Westminster Hall in London (1402), with a 21-metre (70-foot) span, is an excellent example of this type.

      Fired brick began to be made again in Europe in the 14th century, preceded in many areas by the use of salvaged Roman brick. The 14th-century bricks were not as precise as the Roman and were often distorted in firing. Therefore, large lime-mortar joints were needed for regular course lines. Bricks became nearly standardized at something close to the present size, about 20.3 × 9.5 × 5.7 centimetres (8 × 3.75 × 2.25 inches), and bonding (bond) systems based on this approximately 2:1 proportion were developed. These bonding patterns reduced continuous vertical mortar joints, because the mortars were of substantially lower strength than the bricks and vertical joints could form planes of weakness in the walls where cracks might develop. The best bonding pattern was English bond, in which all the bricks in each course overlapped the ones below and vertical joints were entirely eliminated. Brick remained quite expensive because of the cost of the fuel needed to fire it, and it was used mainly where there was no readily available stone. In the late medieval period and mostly in northern Europe, brick was adapted to Gothic stone forms to build so-called hall churches (hall church), with naves and aisles of equal height.

Building services
      Although Roman hypocaust heating disappeared with the empire, a new development in interior heating appeared in western Europe at the beginning of the 12th century: the masonry fireplace and chimney began to replace the central open fire. The large roof openings over central fires let in wind and rain, so each house had only one and larger buildings had as few as possible. Therefore, heated rooms tended to be large and semipublic, where many persons could share the fire's warmth; the roof opening did not effectively remove all the smoke, some of which remained to plague the room's occupants. The chimney did not let in much air or water and could remove most of the smoke. Although much of the heat went up the flue, it was still a great improvement, and, most significantly, it could be used to heat both small and large rooms and multistory buildings as well. Houses, particularly large ones, were broken up into smaller, more private spaces each heated by its own fireplace, a change that decisively altered the communal lifestyle of early medieval times.

Reintroduction of dome construction
      The waning of the cathedral crusade in the late 14th century led to a decline in the International Gothic style practiced by the master masons. In this period the emerging nation-states of Europe began to compete with the church as centres of power. To these new nations, the Roman Empire was the model nation-state, and it seemed appropriate that they use Roman building forms as symbols of their power—particularly the round arch, the vault, and, above all, the dome, following the powerful example of the Pantheon. From 1350 until 1750 much of building technology was focused on the domed church, which developed as a symbol not only of religious belief but also of national and urban pride. There was a conscious rejection of Gothic forms in favour of the ideological appeal of Rome. This attitude led to a split between the processes of design and construction and to the appearance of the first architects (a word derived from the Greek architekton, meaning a chief craftsman), who conceived a building's form, as opposed to the builder, who executed it. The first building in which the designer and the builder were separate persons was the Campanile, or bell tower, of the cathedral of Florence. The design was made by the painter Giotto and constructed by cathedral masons from 1334 to 1359.

      The cathedral of Florence itself had been begun in the Gothic style by Arnolfo di Cambio in 1296. But in 1366 the City of Florence, following the advice of certain painters and sculptors, decided that the Gothic should no longer be used and that all new work should follow Roman forms, including an octagonal dome 42 metres (138 feet) in span to be built at the east end of the nave. The dome was not built until the early 15th century, when Filippo Brunelleschi (Brunelleschi, Filippo), a goldsmith and sculptor, began to make statues for the cathedral. Gradually he became interested in the building itself and built some smaller parts of it. In about 1415 he prepared a design for the dome that he daringly proposed to build without the aid of formwork, which had been absolutely necessary in all previous Roman and Gothic construction. He built a 1:12 model of the dome in brick to demonstrate his method; the design was accepted and built under his supervision from 1420 to 1436. Brunelleschi was thus the first real architect to conceive the building's form and the methods to execute it and to guarantee its performance; he pointedly refused membership in both the masons' and carpenters' guilds. Brunelleschi's dome consists of two layers, an inner dome spanning the diameter and a parallel outer shell to protect it from the weather and give it a more pleasing external form. Both domes are supported by 24 stone half arches, or ribs, of circular form, 2.1 metres (7 feet) thick at the base and tapering to 1.5 metres (5 feet), which meet at an open stone compression ring at the top. To resist outward thrust, tie rings of stone held together with metal cramps run horizontally between the ribs. There are also tie rings of oak timbers joined by metal connectors. The spaces between the ribs and tie rings are spanned by the inner and outer shells, which are of stone for the first 7.1 metres (23 feet) and brick above. The entire structure was built without formwork, the circular profiles of the ribs and rings being maintained by a system of measuring wires fixed at the centres of curvature. Brunelleschi obviously understood enough about the structural behaviour of the dome to know that, if it were built in horizontal layers, it would always be stable and not require timber centring. He also designed elaborate wooden machines to move the needed building materials both vertically and horizontally. Having all but equaled the span of the Pantheon in stone, Brunelleschi was hailed as the man who “renewed Roman masonry work”; the dome was established as the paragon of built form.

      The next great dome of the Renaissance was that of St. Peter's Basilica (Saint Peter's Basilica) in Rome, begun by Pope Julius II in 1506. The technology was very similar to that of Brunelleschi, and the diameter is nearly the same. The dome's design went through many changes and extended over a period of nearly 80 years. The major contributors to the design were the painter and sculptor Michelangelo, who served as architect from 1546 to 1564, and the architects Giacomo della Porta and Domenico Fontana, under whose direction it was finally built during the 1580s. The dome was considerably thinner than that of Florence and was reinforced by three tie rings made of continuous iron chains. It developed numerous cracks, and in the 1740s five more chains were added to further stabilize it. Since the dome used a proven technology, most of the design was done on paper with drawings.

      Another large dome of this period was that of St. Paul's Cathedral (Saint Paul's Cathedral) in London, which was built from 1675 to 1710 by the English architect Sir Christopher Wren (Wren, Sir Christopher). In the early stages of the design process only two physical models were used; later efforts included extensive drawings and apparently also mathematical modeling with numerical calculations. Wren had begun his career as a mathematician and physical scientist and was professor of astronomy at Oxford from 1661 to 1673 before becoming a full-time architect. With this background he was thus able to profit from the first theoretical determination of the catenary curve as the most efficient profile of the arch and dome, which was published by the Scottish mathematician David Gregory in 1697. Wren's solution to the dome, which has a diameter of 34.5 metres (113 feet), was a series of three nested shells, of which the middle one is the true structure. This middle dome is built of brick in a nearly conical catenary form, owing to the large concentrated load of the lantern on top, and constrained by iron chains; it supports a triangularly braced timber framework to which is attached the exterior surfacing of lead sheets. Within the middle dome is a shallower catenary dome that carries only its own weight and serves as a ceiling for the interior space. Wren's concealed structure, to which were applied the desired internal and external forms, has become a standard architectural technique.

Revival of Roman technics and materials
      In addition to Roman forms in masonry, the Renaissance recovered other Roman technologies, including timber trusses (truss). Giorgio Vasari (Vasari, Giorgio) used king-post timber trusses for a 20-metre (66-foot) span in the roof of the Uffizi (Uffizi Gallery), or municipal office building, in Florence in the mid-16th century. At the same time, the Venetian architect Andrea Palladio (Palladio, Andrea) used a fully triangulated timber truss for a bridge with a span of 30.5 metres (100 feet) over the Cimone River. Palladio clearly understood the importance of the carefully detailed diagonal members, for in his diagram of the truss in his Four Books on Architecture he said that they “support the whole work.” The tension connections of the timber members in the truss were joined with iron cramps and bolts.

      Trussed spans in the range of 20–26 metres (65–85 feet) became fairly common in building roofs. In 1664 Wren used timber trusses with a span of about 22 metres (73 feet) in the roof of the Sheldonian Theatre at Oxford. But a precise theoretical understanding of the truss, and major use of it in buildings, would not come until the 19th century.

      Another Roman material that was revived and much improved in the Renaissance was clear glass. A new technique for making it was perfected in Venice in the 16th century. It was known as the crown glass method and was originally used for making dinner plates. Glassblowers spun the molten glass into flat disks up to a metre in diameter; the disks were polished after they had cooled and were cut into rectangular shapes. The first record of crown glass windows is their installation in double-hung counterweighted sliding-sash frames, at Inigo Jones's Banqueting House in London in 1685. Large areas of such glass became common in the 1700s, pointing the way toward the great glass and iron buildings of the 19th century.

      The efficiency of interior heating was improved by the introduction of cast-iron and clay-tile stoves, which were placed in a free-standing position in the room. The radiant heat they produced was uniformly distributed in the space, and they lent themselves to the burning of coal—a new fuel that was rapidly replacing wood in western Europe. When European builders had recovered the technology of the classical world in brick, stone, and timber, a stable plateau was reached in the development of the building arts; these materials and technics were well suited to the churches, palaces, and fortifications that their patrons required. The Industrial Revolution, however, brought new materials and the demand for new building types that completely transformed building technology.

The first industrial age
Development of iron technology (industrialization)
      The last half of the 18th century saw the unfolding of a series of events, primarily in England, that later historians would call the first Industrial Revolution, which would have a profound influence on society as a whole as well as on building technology. Among the first of these events was the large-scale production of iron, beginning with the work of Abraham Darby (Darby, Abraham), who in 1709 was the first to use coke as a fuel in the smelting process. The ready availability of iron contributed to the development of machinery, notably James Watt's double-acting steam engine of 1769. Henry Cort (Cort, Henry) developed the puddling process for making wrought iron in 1784, and in the same year he built the first rolling mill, powered by a steam engine, to produce rolled lengths of wrought-iron bars, angles, and other shapes. cast iron, which has a higher carbon content than wrought iron but is more brittle, was also produced on a large scale. Standard iron building elements soon appeared, pointing the way to the development of metal buildings.

      Early applications of iron in construction are found several centuries prior to the industrial age. There are records of iron chain suspension bridges (suspension bridge) with timber decks in China from the early Ming dynasty (1368–1644); some of them, such as the Liu-Tung Bridge—the object of a famous battle on Mao Zedong's Long March in 1935—have survived in a much-restored condition. The iron tension chains in the domes of St. Peter's and St. Paul's cathedrals are other examples. But the first large cast-iron structure of the industrial age was the bridge over the River Severn at Ironbridge. Built by the iron founder Abraham Darby III between 1777 and 1779, it has a span of 30 metres (100 feet), using five circular-form arches that are reduced to a spidery web of slender iron ribs. Each arch was cast in two pieces with a maximum dimension of 21 metres (70 feet), which were difficult to move from the foundry to the site and to set in place. Smaller, more easily handled pieces characterized the rapid application of iron to buildings that followed. Solid cast-iron columns (column) were used in St. Anne's Church in Liverpool as early as 1772, and hollow tubular columns of increased efficiency were developed in the 1790s. The first use of wrought-iron trusses, which were made of flat bars riveted together, was in a 28-metre (92-foot) span for the roof of the Théâtre-Français (Comédie-Française) in Paris in 1786 by the architect Victor Louis; there iron was used not so much for its strength as its noncombustibility, which, it was hoped, would reduce the hazard of fire. For the same reason, about 1800 the British textile industry began to use partial metal framing in mill buildings up to seven stories high. Hollow cast-iron cylindrical columns were spaced at about 3 metres (10 feet) on centre and supported cast-iron tee beams spanning up to 4.5 metres (15 feet); the floors were bridged by brick arches resting on the bottom flanges of the tee beams; at the perimeter the beams rested on masonry bearing walls, which gave the structure its lateral stability. This prototype of the iron-frame building with exterior masonry walls soon set a standard that would continue to the end of the century.

      The completely independent iron frame without masonry adjuncts emerged slowly in a series of special building types. The first modest example was Hungerford Fish Market (1835) in London. Timber was forbidden because of sanitation requirements; the cast-iron beams spanned 9.7 metres (32 feet) with 3-metre (10-foot) cantilevers on either side, and the hollow cast-iron columns also served as roof drains. All lateral stability was provided by the rigid joints between columns and beams. The next type to use the full iron frame was the greenhouse, which provided a controlled luminous and thermal environment for exotic tropical plants in the cold climate of northern Europe. Among the first of these was the Palm House at Kew Gardens near London; it was built by the architect Decimus Burton in the 1840s.

      A spectacular series of iron and glass buildings for conservatories and exhibition halls continued to the end of the century. The most important of these was the Crystal Palace, built in London's Hyde Park to house the Great Exhibition of 1851. This vast building, 564 metres (1,851 feet) long, was built entirely of standardized parts. Cast-iron columns carried iron trusses of three different spans—7.3 metres (24 feet), 14.6 metres (48 feet), and 21.9 metres (72 feet)—in riveted wrought iron; spanning between the trusses were ingenious “Paxton gutters” made of wooden compression members above iron tension rods that prestressed the wood to reduce deflection. All these prefabricated elements were simply bolted or clipped together on the site to enclose a space of 90,000 square metres (1,000,000 square feet) in only six months. But the major triumph of the Crystal Palace was its all-glass enclosure, made of standard panes 25 × 124 centimetres (10 × 49 inches) in size; the huge space was flooded with light that was scarcely interrupted by the diaphanous metal framing—it resembled a great secular cathedral realizing the ultimate ambition of the medieval masons.

      The French also produced a number of fine iron and glass exhibition halls, including one with a 48-metre (160-foot) span in 1855. Others with somewhat smaller spans, but larger enclosed areas than the Crystal Palace, followed in 1867 and 1878. Iron trusses with glazed roofs were also used in the train sheds of railway stations that were built throughout western Europe. The New Street Station in Birmingham, Eng. (1854), had a train shed with an iron truss roof spanning 64 metres (211 feet). It was apparently the first building to exceed the span of the Pantheon. One of the largest was St. Pancras Station (1873) in London, which featured a glazed hall spanned by 74-metre (243-foot) trussed iron arches. After the brilliant successes of midcentury, iron and glass construction was applied in a more prosaic series of buildings that continued to be built until 1900.

Manufactured building materials
      The production of brick (brick and tile) was industrialized in the 19th century. The laborious process of hand-molding, which had been used for 3,000 years, was superseded by “pressed” bricks. These were mass-produced by a mechanical extrusion process in which clay was squeezed through a rectangular die as a continuous column and sliced to size by a wire cutter. There was also a proliferation of elaborately shaped and stamped masonry units. Periodically fired beehive kilns (stoked by coke) continued to be used, but the continuous tunnel kiln, through which bricks were moved slowly on a conveyor belt, had appeared by the end of the century. The new methods considerably reduced the cost of brick, and it became one of the constituent building materials of the age.

      Timber technology underwent rapid development in the 19th century in North America, where there were large forests of softwood fir and pine trees that could be harvested and processed by industrial methods; steam- and water-powered sawmills began producing standard-dimension timbers in quantity in the 1820s. The production of cheap machine-made nails (nail) in the 1830s provided the other necessary ingredient that made possible a major innovation in building construction, the balloon frame (balloon framing); the first example is thought to be a warehouse erected in Chicago in 1832 by George W. Snow. There was a great demand for small buildings of all types as the North American continent was settled, and the light timber frame provided a quick, flexible, and inexpensive solution to this problem. In the balloon frame system, traditional heavy timbers and complex joinery were abandoned. The building walls were framed with 5 × 10-centimetre (2 × 4-inch) vertical members, or studs, placed at 40 centimetres (16 inches) on centre (that is, measured between the centre points of each); these in turn supported the roof and floor joists, usually 5 × 25 centimetres (2 × 10 inches) also placed 40 centimetres (16 inches) apart and capable of spanning up to 6 metres (20 feet). Lateral stability was achieved by light diagonal braces let into the studs or, more commonly, by 2-centimetre- (0.75-inch-) thick diagonal boards applied to all exterior walls and to floor and roof joists, creating a rigid, light box. Openings were cut through the framing and sheathing as required. All connections were made with machine-made nails, which were easily driven through the soft, thin timbers. A wide variety of interior and exterior surfacing materials could be applied to the frame, including timber siding, stucco, and brick veneer. The balloon frame building, made with manufactured materials and requiring only a few hand tools and little skill to build, has remained a popular and inexpensive form of construction to the present day.

Building science
      A significant achievement of the first industrial age was the emergence of building science, particularly the elastic (elasticity) theory of structures. With it, mathematical models could be used to predict structural performance with considerable accuracy, provided there was adequate quality control of the materials used. Although some elements of the elastic theory, such as the Swiss mathematician Leonhard Euler's theory of column buckling (1757), were worked out earlier, the real development began with the English scientist Thomas Young's (Young, Thomas) modern definition of the modulus of elasticity in 1807. Louis Navier published the elastic theory of beams in 1826, and three methods of analyzing forces in trusses were devised by Squire Whipple, A. Ritter, and James Clerk Maxwell between 1847 and 1864. The concept of a statically determinate structure—that is, a structure whose forces could be determined from Newton's laws of motion alone—was set forth by Otto Mohr in 1874, after having been used intuitively for perhaps 40 years. Most 19th-century structures were purposely designed and fabricated with pin joints to be statically determinate; it was not until the 20th century that statically indeterminate structures became readily solvable. The elastic theory formed the basis of structural analysis until World War II, when bomb-damaged buildings were observed to behave in unpredicted ways and the underlying assumptions of the theory were found to require modification.

Emergence of design professionals
      The coming of the industrial age also marked a major change in the role of the architect. The artist-architects of the Renaissance had the twin patrons of church and state upon whom they could depend for commissions. In the rising industrial democracies the market for large-scale buildings worthy of an architect's attention widened, and the different users asked for a bewildering range of new building types. The response of the architect was to develop the new role of licensed professional on the model of professions such as law and medicine. In addition, with the coming of building science, there was a further division of labour in the design process; structural engineering appeared as a separate discipline specializing in the application of mathematical models in building. One of the first buildings for which the architect and engineer (civil engineering) were separate persons was the Granary (1811) in Paris. Societies representing the building design professions were founded, including the Institution of Civil Engineers (1818) and the Royal Institute of British Architects (1834), both in London, and the American Institute of Architects (1857). Official government licensing of architects and engineers, a goal of these societies, was not realized until much later, beginning with the Illinois Architects Act of 1897. Concurrent with the rise of professionalism was the development of government regulation, which took the form of detailed municipal and national building codes specifying both prescriptive and performance requirements for buildings.

Improvements in building services
      Environmental control technologies began to develop dramatically in the first industrial age. The first major advance was the use of coal gas for lighting. Coal gas was first made in the 1690s by heating coal in the presence of water to yield methane, and in 1792 William Murdock (Murdock, William) developed the gas jet lighting fixture. The first large building to have gas lighting (from a small gas plant on the site) was James Watt's foundry in Birmingham in 1803. The Gas Light and Coke Company was founded in London in 1812 as the first real public utility, producing coal gas as a part of the coking process in large central plants and distributing it through underground pipes to individual users; soon many major cities had gasworks and distribution networks. Gas was expensive, however, and was used mainly for lighting, not for heating or cooking; it also contained many impurities that produced undesirable products of combustion (particularly carbon soot) in occupied spaces. Relatively pure methane in the form of natural gas would not be available until the exploitation of large oil fields in the 20th century.

      The stove and fireplace continued as the major sources of space heating throughout this period, but the development of the steam engine and its associated boilers led to a new technology in the form of steam heating. James Watt heated his own office with steam running through pipes as early as 1784. During the 19th century, systems of steam and later hot-water heating were gradually developed; these used coal-fired central boilers connected to networks of pipes that distributed the heated fluid to cast-iron radiators and returned it to the boiler for reheating. Steam heat was a major improvement over stoves and fireplaces because all combustion products were eliminated from occupied spaces, but heat sources were still localized at the radiators.

       plumbing and sanitation systems in buildings advanced rapidly in this period. Public water-distribution systems were the essential element; the first large-scale example of a mechanically pressurized water-supply system was the great array of waterwheels installed by Louis XIV at Marley on the Marne River in France to pump water for the fountains at Versailles, about 18 kilometres (10 miles) away. The widespread use of cast-iron pipes in the late 18th century made higher pressures possible, and they were used by Napoleon in the first steam-powered municipal water supply for a section of Paris in 1812. Gravity-powered underground drainage systems were installed along with water-distribution networks in most large cities of the industrial world during the 19th century; sewage-treatment plants were introduced in the 1860s. Permanent plumbing fixtures appeared in buildings with water supply and drainage, replacing portable basins, buckets, and chamber pots. Joseph Bramah (Bramah, Joseph) invented the metal valve-type water closet as early as 1778, and other early lavatories, sinks, and bathtubs were of metal also; lead, copper, and zinc were all tried. The metal fixtures proved difficult to clean, however, and in England during the 1870s Thomas Twyford developed the first large one-piece ceramic lavatories as well as the ceramic washdown water closet. At first these ceramic fixtures were very expensive, but their prices declined until they became standard, and their forms remain largely unchanged today. The bathtub proved to be too large for brittle ceramic construction, and the porcelain-enamel cast-iron tub was devised about 1870; the double-shell built-in type still common today appeared about 1915.

The second industrial age
Introduction of steel building technology
 If the first industrial age was one of iron and steam, the second industrial age, which began in about 1880, could be called one of steel and electricity. Mass production of this new material and of this new form of energy also transformed building technology. Steel was first made in large quantities for railroad rails. Rolling of steel rails (which was adapted from wrought-iron rolling technology) and other shapes such as angles and channels began about 1870; it made a much tougher, less brittle metal. Steel was chosen as the principal building material for two structures built for the Paris Exposition of 1889: the Eiffel Tower and the Gallery of Machines. Gustave Eiffel's tower was 300 metres (1,000 feet) high, and its familiar parabolic curved form has become a symbol of Paris itself; its height was not exceeded until the topping off of the 318.8-metre- (1,046-foot-) tall Chrysler Building in New York City in 1929. The Gallery of Machines was designed by the architect C.-L.-F. Dutert and the engineer Victor Contamin with great three-hinged arches spanning 114 metres (380 feet) and extending more than 420 metres (1,400 feet). Its glass-enclosed clear span area of 48,727 square metres (536,000 square feet) has never been equaled; in fact, it was so large that no regular use for it could be found after the exposition closed, and this magnificent building was demolished in 1910.

Early steel-frame (steel) high-rises
      While these prodigious structures were the centre of attention, a new and more significant technology was developing: the steel-framed (framed building) high-rise (skyscraper) building. It began in Chicago, a city whose central business district was growing rapidly. The pressure of land values in the early 1880s led owners to demand taller buildings. The architect-engineer William Le Baron Jenney (Jenney, William Le Baron) responded to this challenge with the 10-story Home Insurance Company Building (1885), which had a nearly completely all-metal structure. The frame consisted of cast-iron columns supporting wrought-iron beams (beam), together with two floors of rolled-steel beams that were substituted during construction; this was the first large-scale use of steel in a building. The metal framing was completely encased in brick or clay-tile cladding for fire protection, since iron and steel begin to lose strength if they are heated above about 400 °C (750 °F). Jenney's Manhattan Building (1891) had the first vertical truss bracing to resist wind forces; rigid frame or portal wind bracing was first used in the neighbouring Old Colony Building (1893) by the architects William Holabird and Martin Roche. The all-steel frame finally appeared in Jenney's Ludington Building (1891) and the Fair Store (1892).

      The foundations of these high-rise buildings posed a major problem, given the soft clay soil of central Chicago. Traditional spread footings, which dated back to the Egyptians, proved to be inadequate to resist settlement due to the heavy loads of the many floors, and timber piles (a Roman invention) were driven down to bedrock. For the 13-story Stock Exchange Building (1892), the engineer Dankmar Adler (Adler, Dankmar) employed the caisson foundation used in bridge construction. A cylindrical shaft braced with board sheathing was hand-dug to bedrock and filled with concrete to create a solid pier to receive the heavy loads of the steel columns.

      By 1895 a mature high-rise building technology had been developed: the frame of rolled steel I beams with bolted or riveted connections, diagonal or portal wind bracing, clay-tile fireproofing, and caisson foundations. The electric-powered elevator provided vertical transportation, but other environmental technologies were still fairly simple. Interior lighting was still largely from daylight, although supplemented by electric light. There was steam heating but no cooling, and ventilation was dependent on operating windows; thus these buildings needed narrow floor spaces to give adequate access to light and air. Of equal importance in high-rise construction was the introduction of the internal-combustion engine (which had been invented by Nikolaus Otto in 1876) at the building site; it replaced the horse and human muscle power for the heaviest tasks of lifting. Over the next 35 years, higher steel-frame buildings were built; in Chicago the Masonic Temple (1892) of Daniel Burnham and John Root reached 22 stories (91 metres or 302 feet), but then the leadership shifted to New York City with the 26-story Manhattan Life Building (1894). The Singer Building (1907) by the architect Ernest Flagg rose to 47 stories (184 metres or 612 feet), Cass Gilbert's Woolworth Building (1913) attained a height of 238 metres (792 feet) at 55 stories, and Shreve, Lamb & Harmon's 102-story Empire State Building (1931) touched 381 metres (1,250 feet). The race for higher buildings came to an abrupt halt with the Great Depression and World War II, and high-rise construction was not resumed until the late 1940s.

Steel long-span construction
      Long-span structures in steel developed more slowly than the high-rise in the years from 1895 to 1945, and none exceeded the span of the Gallery of Machines. Two-hinge (made of a single member hinged at each end) and three-hinge (made of two members hinged at each end and at the meeting point at the crown) trussed arches were widely used, the largest examples being two great airship hangars for the U.S. Navy in New Jersey—the first built in 1922 with a span of 79 metres (262 feet), the second in 1942 with a span of 100 metres (328 feet). The flat truss was used also, reaching a maximum span of 91 metres (300 feet) in the Glenn L. Martin Co. Aircraft Assembly Building (1937) in Baltimore. Electric arc welding, another important steel technology, was applied to building construction at this time, although the principle had been developed in the 1880s. The first all-welded multistory buildings were a series of factories for the Westinghouse Company, beginning in 1920. The welded rigid frame became a new structural type for medium spans, reaching a length of 23 metres (77 feet) in the Cincinnati Union Terminal (1932), but widespread use of welding did not come until after 1945.

Reintroduction of concrete
      The second industrial age also saw the reemergence of concrete in a new composite relationship with steel, creating a technology that would rapidly assume a major role in building construction. The first step in this process was the creation of higher-strength artificial cements. Lime mortar—made of lime, sand, and water—had been known since ancient times. It was improved in the late 18th century by the British engineer John Smeaton (Smeaton, John), who added powdered brick to the mix and made the first modern concrete by adding pebbles as coarse aggregate. Joseph Aspdin patented the first true artificial cement, which he called portland cement, in 1824; the name implied that it was of the same high quality as Portland stone. To make portland cement, Aspdin burned limestone and clay together in a kiln; the clay provided silicon compounds, which when combined with water formed stronger bonds than the calcium compounds of limestone. In the 1830s Charles Johnson, another British cement manufacturer, saw the importance of high-temperature burning of the clay and limestone to a white heat, at which point they begin to fuse. In this period, plain concrete was used for walls, and it sometimes replaced brick in floor arches that spanned between wrought-iron beams in iron-framed factories. Precast concrete blocks also were manufactured, although they did not effectively compete with brick until the 20th century.

The invention of reinforced concrete
      The first use of iron-reinforced concrete was by the French builder François Coignet in Paris in the 1850s. Coignet's own all-concrete house in Paris (1862), the roofs and floors reinforced with small wrought-iron I beams, still stands. But reinforced concrete development began with the French gardener Joseph Monier's (Monier, Joseph) 1867 patent for large concrete flowerpots reinforced with a cage of iron wires. The French builder François Hennebique (Hennebique, François) applied Monier's ideas to floors, using iron rods to reinforce concrete beams and slabs; Hennebique was the first to realize that the rods had to be bent upward to take negative moment near supports. In 1892 he closed his construction business and became a consulting engineer, building many structures with concrete frames composed of columns, beams, and slabs. In the United States Ernest Ransome paralleled Hennebique's work, constructing factory buildings in concrete. High-rise structures in concrete followed the paradigm of the steel frame. Examples include the 16-story Ingalls Building (1903) in Cincinnati, which was 54 metres (180 feet) tall, and the 11-story Royal Liver Building (1909), built in Liverpool by Hennebique's English representative, Louis Mouchel. The latter structure was Europe's first skyscraper, its clock tower reaching a height of 95 metres (316 feet). Attainment of height in concrete buildings progressed slowly owing to the much lower strength and stiffness of concrete as compared with steel.

      Between 1900 and 1910 the elastic theory of structures was at last applied to reinforced concrete in a scientific way. Emil Morsch, the chief engineer of the German firm of Wayss and Freitag, formulated the theory, which was verified by detailed experimental testing at the Technical University of Stuttgart. These tests established the need for deformed bars for good bonding with concrete and demonstrated that the amount of steel in any member should be limited to about 8 percent of the area; this assures the slow elastic failure of the steel, as opposed to the abrupt brittle failure of the concrete, in case of accidental overloading. In 1930 the American engineer Hardy Cross (Cross, Hardy) introduced relaxation methods for the approximate analysis of rigid frames, which greatly simplified the design of concrete structures. In the Johnson-Bovey Building (1905) in Minneapolis, the American engineer C.A.P. Turner employed concrete floor slabs without beams (called flat slabs or flat plates) that used diagonal and orthogonal patterns of reinforcing bars. The system still used today—which divides the bays between columns into column strips and middle strips and uses only an orthogonal arrangement of bars—was devised in 1912 by the Swiss engineer Robert Maillart (Maillart, Robert).

The concrete dome
      Concrete was also applied to long-span buildings, an early example being the Centennial Hall (1913) at Breslau, Ger. (now Wrocław, Pol.), by the architect Max Berg and the engineers Dyckerhoff & Widmann; its ribbed dome spanned 65 metres (216 feet), exceeding the span of the Pantheon. More spectacular were the great airship hangars at Orly constructed by the French engineer Eugène Freyssinet (Freyssinet, Eugène) in 1916; they were made with 9-centimetre- (3.5-inch-) thick corrugated parabolic vaults spanning 80 metres (266 feet) and pierced by windows. In the 1920s Freyssinet made a major contribution to concrete technology with the introduction of pretensioning. In this process, the reinforcing wires were stretched in tension, and the concrete was poured around them; when the concrete hardened, the wires were released, and the member acquired an upward deflection and was entirely in compression. When the service load was applied, the member deflected downward to a flat position, remaining entirely in compression, and it did not develop the tension cracks that plague ordinary reinforced concrete. Widespread application of pretensioning was not made until after 1945.

      Shell construction in concrete also began in the 1920s; the first example was a very thin (6 centimetres) hemispherical shell for a planetarium (1924) in Jena, Ger., spanning 25 metres (82 feet). In 1927 an octagonal ribbed shell dome with a span of 66 metres (220 feet) was built to house a market hall in Leipzig. Many variations of thin shells were devised for use in industrial buildings. The shell emerged as a major form of long-span concrete structure after World War II.

Development of building service and support systems

Vertical transportation
      Elisha Graves Otis (Otis, Elisha Graves) developed the first safe steam-powered roped elevators (elevator) with toothed guide rails and catches in the late 1850s. The steam-powered hydraulic elevator, which was limited to buildings of about 15 stories, was developed in 1867 by the French engineer Léon Édoux. The development of the electric motor by George Westinghouse in 1887 made possible the invention of the high-speed electric-powered roped elevator (called “lightning” elevators in comparison to the slower hydraulics) in 1889 and the electric-powered moving staircase, or escalator, in the 1890s.

      In the second industrial age, environmental technologies developed rapidly. Most of these technologies involved the use of electric power, which declined in cost during this period. The carbon-arc electric light was demonstrated as early as 1808, and the British physicist Michael Faraday devised the first steam-powered electric generator to operate a large carbon-arc lamp for the Foreland Lighthouse in 1858. But the carbon-arc lamp was so bright and required so much power that it was never widely used and was rapidly superseded by the simultaneous invention of the carbon-filament bulb by Thomas Edison (Edison, Thomas Alva) and Joseph Swan in 1879. The carbon-filament bulb was highly inefficient, but it banished the soot and fire hazards of coal-gas jets and soon gained wide acceptance. It was succeeded by the more efficient tungsten-filament incandescent (incandescent lamp) bulb, developed by George Coolidge of the General Electric Company, which first appeared in 1908; the double-coiled filament used today was introduced about 1930.

      Edison experimented with gas-discharge light tubes (electric discharge lamp) in 1896, and Georges Claude (Claude, Georges) in France and Moore in England produced the first practical discharge tubes using noble gases such as neon and argon; these tubes were first used to outline the facade of the West End Cinema in London in 1913 and were rapidly exploited for signs and other decorative purposes. In 1938 General Electric and Westinghouse produced the first commercial fluorescent (fluorescent lamp) discharge lamps using mercury vapour and phosphor-coated tubes to enhance visible light output. Fluorescent tubes had roughly double the efficiency of tungsten lamps and were rapidly adopted for commercial and office use. Light intensity increased in all buildings as electric costs decreased, reaching a peak in about 1970. Gaseous-discharge lamps using high-pressure mercury and sodium vapour were developed in the 1960s but found only limited application in buildings; they are of such high intensity and marked colour that they are used mostly in high-ceilinged spaces and for exterior lighting.

heating and cooling systems
      Steam and hot-water heating systems of the late 19th century provided a reasonable means for winter heating, but no practical methods existed for artificial cooling, ventilating, or humidity control. In the forced-air system of heating, air replaced steam or water as the fluid medium of heat transfer, but this was dependent on the development of powered fans to move the air. Although large, crude fans for industrial applications in the ventilation of ships and mines had appeared by the 1860s, and the Johns Hopkins Hospital in Baltimore had a successful steam-powered forced-air system installed in 1873, the widespread application of this system to buildings only followed the development of electric-powered fans in the 1890s.

      Important innovations in cooling (cooling system) technology followed. The development of refrigeration machines for food storage played a role, but the key element was Willis Carrier's (Carrier, Willis Haviland) 1906 patent that solved the problem of humidity removal by condensing the water vapour on droplets of cold water sprayed into an airstream. Starting with humidity control in tobacco and textile factories, Carrier slowly developed his system of “man-made weather,” finally applying it together with heating, cooling, and control devices as a complete system in Graumann's Metropolitan Theater, Los Angeles, in 1922. The first office building air-conditioned (air-conditioning) by Carrier was the 21-story Milam Building (1928) in San Antonio, Texas. It had a central refrigeration plant in the basement that supplied cold water to small air-handling units on every other floor; these supplied conditioned air to each office space through ducts in the ceiling; the air was returned through grills in doors to the corridors and then back to the air-handling units. A somewhat different system was adopted by Carrier for the 32-story Philadelphia Savings Fund Society Building (1932). The central air-handling units were placed with the refrigeration plant on the 20th floor, and conditioned air was distributed through vertical ducts to the occupied floors and horizontally to each room and returned through the corridors to vertical exhaust ducts that carried it back to the central plant. Both systems of air handling, local and central, are still used in high-rise buildings. The Great Depression and World War II reduced the demand for air-conditioning systems, and it was not until the building of the United Nations Secretariat in New York City in 1949 that Carrier produced a method of air conditioning that could deal effectively with the large heat loads imposed by the building's all-glass curtain walls. The conditioned air was delivered not only from the ceiling but also through pipe coil convector units just inside the glass wall. The pipe coil convectors contained centrally supplied warm or cold water to further temper the heat loss or gain at the perimeter; conditioned air and water were centrally supplied from four mechanical floors spaced within the building's 39-story height.

      Carrier's “Weathermaster” system was energy-intensive, appropriate to the declining energy costs of the time, and it was adopted for most of the all-glass skyscrapers that followed in the next 25 years. In the 1960s the so-called dual-duct system appeared; both warm and cold air were centrally supplied to every part of the building and combined in mixing boxes to provide the appropriate atmosphere. The dual-duct system also consumed much energy, and, when energy prices began to rise in the 1970s, both it and the Weathermaster system were supplanted by the variable air volume (VAV) system, which supplies conditioned air at a single temperature, the volume varying according to the heat loss or gain in the occupied spaces. The VAV system requires much less energy and is widely used.

      In the early 1950s, air-conditioning systems were reduced to very small electric-powered units capable of cooling single rooms. These were usually mounted in windows to take in fresh air and to remove heat to the atmosphere. These units found widespread application in the retrofitting of existing buildings—particularly houses and apartment buildings—and have since found considerable application in new residential buildings.

      The relatively high energy costs of the 1970s also prompted interest in various forms of solar (solar energy) heating, both for interior spaces and for domestic hot water, but, except for residential passive solar heating, the relative decline in energy prices in the 1980s made such systems unattractive.

      The study of thermodynamics in the late 19th century included the heat-transfer properties of materials and led to the concept of thermal insulation (insulator)—that is, a material that has a relatively low rate of heat transfer. As building atmospheres became more carefully controlled after 1900, more attention was given to the thermal insulation of building enclosures (envelopes). One of the best insulators is air, and materials that trap air in small units have low heat-transfer rates; wool and foam are excellent examples. The first commercial insulations, in the 1920s, were mineral wools and vegetable-fibreboards; fibreglass wool appeared in 1938. foam glass, the first rigid insulating foam, was marketed in the 1930s, and after 1945 a wide variety of plastic foam insulations was developed. Since the 1970s most building codes have set minimum requirements for insulation of building envelopes, and these have proved to be very cost-effective in saving energy.

glass as a building material
      Glass underwent considerable development in the second industrial age. The making of clear plate glass was perfected in the late 19th century, as were techniques of sandblasting and etching it. In the United States in 1905 the Libbey Owens Glass Company (New England Glass Company) began making sheet glass by a continuous drawing process from a reservoir of molten glass; its surface was somewhat distorted, but it was much cheaper than plate glass. Prefabricated panels of double glazing about 2.5 centimetres (1 inch) thick were first made in the 1940s, although the insulating principle of air trapped between two layers of glass had been recognized much earlier. Hollow glass blocks were introduced by the Corning Company in 1935. In 1952 the Pilkington Brothers in England developed the float glass process, in which a continuous 3.4-metre- (11-foot-) wide ribbon of glass floated over molten tin and both sides were fire finished, avoiding all polishing and grinding; this became the standard method of production. Pilkington also pioneered the development of structural glass mullions in the 1960s. In the 1950s the rise of air conditioning led to the marketing of tinted glass that would absorb and reduce solar gain, and in the 1960s reflective glass with thin metallic coatings applied by the vacuum plating process was introduced, also to reduce solar gain. Heat-mirror glass, which has a transparent coating that admits the short-wavelength radiation from the sun but tends to reflect the longer-wavelength radiation from within occupied spaces, was introduced in 1984; when combined with double glazing, its insulating value approaches that of a wall.

High-rise construction since 1945

Use of steel and other metals
      The second great age of high-rise buildings began after the end of World War II, when the world economy and population again expanded. It was an optimistic time with declining energy costs, and architects embraced the concept of the tall building as a glass prism. This idea had been put forward by the architects Le Corbusier (Corbusier, Le) and Ludwig Mies van der Rohe (Mies van der Rohe, Ludwig) in their visionary projects of the 1920s. These designs employed the glass curtain wall, a non-load-bearing “skin” attached to the exterior structural components of the building. The earliest all-glass curtain wall, which was only on a single street facade, was that of the Hallidie Building (1918) in San Francisco. The first multistory structure with a full glass curtain wall was the A.O. Smith Research Building (1928) in Milwaukee by Holabird and Root; in it the glass was held by aluminum frames, an early use of this metal in buildings. But these were rare examples, and it was not until the development of air conditioning, fluorescent lighting, and synthetic rubber sealants after 1945 that the glass prism could be realized.

      The paradigm of the glass tower was defined by the United Nations Secretariat Building (1949) in New York City; Wallace Harrison was the executive architect, but Le Corbusier also played a major role in the design. The UN building, which featured a Weathermaster air-conditioning system and green-tinted glass walls, helped set the standard for tall buildings around the world. Several other influential buildings—such as Mies van der Rohe's 26-story 860–880 Lake Shore Drive Apartments (1951) in Chicago and Skidmore, Owings & Merrill's 21-story Lever House (1952) in New York City—helped to further establish the technology of curtain walls. Perhaps the most important element was the development of extruded- aluminum mullion and muntin shapes to support the glass. Aluminum began to be produced in quantity in the United States by the Hall process (Hall effect) in 1886; this process for separating the metal from the ore required large amounts of electricity, and declining energy costs after World War II influenced the development of this building technology. Aluminum forms a coating of transparent oxide that protects it against corrosion; this oxide layer can be artificially thickened and coloured through a process called anodizing. Anodized aluminum was first used in the windows of the Cambridge University Library in England in 1934. Aluminum became the principal material of curtain-wall framing because of its corrosion resistance and ease of forming by means of the extrusion process, in which the metal is forced through a series of dies to create complex cross-sectional shapes. Formed sheet aluminum is also used for opaque curtain-wall panels. Other metals used in curtain walls are stainless steel (a compound of 82 percent iron and 18 percent chromium) and so-called weathering steel, copper-bearing steel alloys that form an adherent oxide layer. The bronze curtain wall of Mies van der Rohe's Seagram Building (1954–58) in New York City proved to be an isolated example. Probably of equal importance in curtain-wall construction was the development of cold-setting rubbers (rubber) during World War II; these form the elastic sealants that successfully seal the joints between glass and metal and between metal and metal against wind and rain. In the late 1970s the development of artificial diamonds (diamond) made possible cutting tools that slice stone wafer-thin, and it became an important component of curtain walls.

      Following the development of the curtain wall, new forms of structure appeared in high-rise buildings. As environmental control systems increased in cost, economic pressures worked to produce more efficient structures. In 1961 the 60-story Chase Manhattan Bank Building, designed by Skidmore, Owings & Merrill, had a standard steel frame with rigid portal wind bracing, which required 275 kilograms of steel per square metre (55 pounds of steel per square foot), nearly the same as the Empire State Building of 30 years earlier. Economy of structure in tall buildings was demonstrated by the same firm only nine years later in the John Hancock Building in Chicago. It used a system of exterior diagonal bracing to form a rigid tube devised by the engineer Fazlur Khan (Khan, Fazlur R.); although the Hancock building is 100 stories, or 343 metres (1,127 feet), high, its structure is so efficient that it required only 145 kilograms of steel per square metre (29 pounds per square foot). The framed tube, which Khan developed for concrete structures, was applied to other tall steel buildings. Khan used a steel system of nine bundled tubes of different heights—each 22.5 metres (75 feet) square with columns spaced at 4.5 metres (15 feet)—to form the structure of the 110-story, 442-metre (1,450-foot) Sears Tower (1973), also in Chicago. (See Researcher's Note: Height of the Sears Tower.) Considerably taller buildings are possible with current technology, but their erection also depends on general economic considerations and the resulting marketability of floor space.

Use of reinforced concrete
      Parallel to the development of tall steel structures, substantial advancements in high-rise structural systems of reinforced concrete have been made since 1945. The first of these was the introduction of the shear wall as a means of stiffening concrete frames against lateral deflection, such as results from wind or earthquake loads; the shear wall acts as a narrow deep cantilever beam to resist lateral forces. In 1958 the architect Milton Schwartz and engineer Henry Miller used shear walls to build the 39-story Executive House in Chicago to a height of 111 metres (371 feet). Of equal importance was the introduction of the perimeter-framed tube form in concrete by Fazlur Khan in the DeWitt–Chestnut Apartments (1963) in Chicago; the building rises 43 stories (116 metres, or 387 feet). Lateral stability was achieved by closely spaced columns placed around the building perimeter and connected together by deep beams. The next step in concrete high-rise construction was the combination of the perimeter-framed tube with a largely solid-walled interior tube or shear walls to give further lateral stability. This was employed by Eero Saarinen and Kevin Roche in the 35-story CBS Building (1964) in New York City, and the system was further developed by Khan in the 221-metre (725-foot) Shell Oil Building (1967) in Houston. Another new structural form in concrete was introduced by Khan in the 174-metre (570-foot) 780 Third Avenue Office Building (1983) in New York City. This is a framed (framed building) tube with diagonal bracing achieved by filling in diagonal rows of window openings to create exterior bracing members; this is a very efficient system and may lead to yet taller buildings of this type. Three further innovations helped the rapid rise in height of concrete buildings. One was the development of lightweight concrete, using blast-furnace slag in place of stone as aggregate for floor construction; this reduced the density of the concrete by 25 percent, with a corresponding reduction in the loads the building columns needed to carry. The second was the increase in the ultimate strength of concrete used for columns. Third, the use of pumps to move liquid concrete to the upper floors of tall buildings substantially reduced the cost of placement.

      Another important technique developed for concrete high-rise construction is slipforming. In this process, a continuous vertical element of planar or tubular form is continuously cast using a short section of formwork that is moved upward with the pouring process. Slipforming has been used to build a number of very tall structures in Canada, including several industrial chimneys 366 metres (1,200 feet) high and the CN Tower in Toronto, which contains an observation deck and a massive television antenna and has a total height of 553 metres (1,815 feet). Concrete has shown itself to be a serious competitor with steel in high-rise structures; it is now used for the great majority of tall residential buildings and for a substantial number of tall office buildings.

Postwar developments in long-span construction
      After 1945 the dome and the shell vault continued to be the major forms of long-span structures. One innovation was the geodesic dome, which was devised by the architect and engineer R. Buckminster Fuller (Fuller, R Buckminster) in the 1940s; in this form the ribs are placed in a triangular or hexagonal pattern and lie on the geodesic lines, or great circles, of a sphere. A very shallow spherical form with aluminum trussed (truss) members was used by Freeman Fox & Partners for the Dome Discovery built in London in 1951. Fuller's own patented forms were used in 1958 to build two large hemispheric domes 115.3 metres (384 feet) in diameter using steel tube members. These are used as workshops for the Union Tank Car Company in Wood River, Ill., and Baton Rouge, La. The largest geodesic dome is the Poliedro de Caracas, in Venezuela, built of aluminum tubes spanning 143 metres (469 feet).

 Another form of steel trussed dome is the lamella dome, which is made of intersecting arches hinged together at their midpoints to form an interlocking network in a diamond pattern. It was used for the first two examples of the great covered sports stadiums built in the United States since the 1960s: the Harris County Stadium, or Astrodome (see photograph—>), built in Houston, Texas, in 1962–64 with a span of 196 metres (642 feet) and the 207-metre- (678-foot-) diameter Superdome in New Orleans, La., designed by Sverdrup and Parcel and completed in 1973. The steel truss continued to be used and was extended to three dimensions to form space trusses. The longest span of this type was the Narita Hangar at Tokyo International Airport, which used a tied portal truss to span 190 metres (623 feet) supporting a space-truss roof spanning 90 metres (295 feet).

      The concrete dome or shell developed rapidly in the 1950s. The St. Louis Lambert Airport Terminal (1954), designed by Hellmuth, Yamasaki and Leinweber, has a large hall 36.6 metres (120 feet) square, spanned by four intersecting thin-shell concrete barrel vaults supported at the four corners; the thickness of the shell varies from 20 centimetres (8 inches) at the supports to 11.3 centimetres (4.5 inches) at the centre. Another example is the King Dome, in Seattle, Wash., which covers a sports stadium with a thin single shell concrete parabolic dome stiffened with ribs 201 metres (661 feet) in diameter.

      New forms of the long-span roof appeared in the 1950s based on the steel cables (cable) that had long been used in suspension bridges. One example was the U.S. Pavilion at the 1958 Brussels World's Fair, designed by the architect Edward Durell Stone. It was based on the familiar principle of the bicycle wheel; its roof had a diameter of 100 metres (330 feet), with a steel tension ring at the perimeter from which two layers of radial cables were tightly stretched to a small tension ring in the middle—the double layer of cables gave the roof stability against vertical movement. The Oakland–Alameda County Coliseum (1967), by Skidmore, Owings & Merrill, extended this system to 126 metres (420 feet) in diameter, but only a single layer of cables, stiffened by encasing ribs of concrete, connects the inner and outer rings.

      Another system derived from bridge construction is the cable-stayed roof. An early example is the TWA (Trans World Airlines, Inc.) Hangar (1956) at Kansas City, Mo., which shelters large aircraft under a double cantilever roof made of semicylindrical shells that reach out 48 meters (160 feet); deflection is reduced and the shells kept in compression by cables that run down from central shear walls to beams in the valleys between the shells. Another example of the cable-stayed roof is the McCormick Place West Exhibition Hall (1987) in Chicago, by Skidmore, Owings & Merrill. Two rows of large concrete masts rise above the roof, supporting steel trusses that span 72 metres (240 feet) between the masts and cantilever 36 metres (120 feet) to either side; the trusses are also supported by sets of parallel diagonal cables that run back to the masts.

      A third form of long-span roof structures in tension are air-supported plastic membranes, which were devised by Walter Bird of Cornell University in the late 1940s and were soon in use for swimming pools, temporary warehouses, and exhibition buildings. The Osaka World's Fair of 1970 included many air-supported structures, the largest of which was the U.S. Pavilion designed by the engineers Geiger Berger Associates; it had an oval plan 138 × 79 metres (460 × 262 feet), and the inflated domed roof of vinyl-coated fabric was restrained by a diagonally intersecting network of steel cables attached to a concrete compression ring at the perimeter. The system of the Osaka Pavilion was adapted for two large sports stadiums built in the 1980s: the Silverdome at Pontiac, Mich., and the Hubert H. Humphrey Metrodome in Minneapolis. Air-supported structures are perhaps the most cost-effective type of structure for very long spans.

      Building construction has settled into a period of relative calm after the explosive innovations of the 19th century. steel, concrete, and timber have become fairly mature technologies, but there are other materials—such as fibre composites—that may yet play a major role in building.

Modern building practices

The economic context of building construction
      Buildings, like all economic products, command a range of unit prices based on their cost of production and their value to the consumer. In aggregate, the total annual value of building construction in the various national economies is substantial. In 1987 in the United States, for example, it was about 10 percent of the gross domestic product, a proportion that is roughly applicable for the world economy as a whole. In spite of these large aggregate values, the unit cost of buildings is quite low when compared to other products. In the United States in 1987, new building cost ranged from about $0.50 to $2.50 per pound. The lowest costs are for simple pre-engineered metal buildings, and the highest represent functionally complex buildings with many mechanical and electrical services, such as hospitals and laboratories. These unit costs are at the low end of the scale of manufactures, ranking with inexpensive foodstuffs, and are lower than those of most other familiar consumer products. This scale of cost is a rough index of the value or utility of the commodity to society. Food, although essential, is relatively easy to produce; aircraft, at the high end of the scale, perform a desirable function but do so with complex and expensive mechanisms that command much higher unit prices which reflect not only the materials and labour required to produce them but also substantial capital and research investments. Buildings fall nearer to food in value; they are ubiquitous and essential, yet the services consumers expect them to provide can be supplied with relatively unsophisticated technology and inexpensive materials. Thus there has been a tendency for building construction to remain in the realm of low technology, for there has been relatively little incentive to invest in research given consumer expectations.

      Within this general economic context, there are a number of specific parameters that affect the cost of buildings. First are government building codes (building code), which are enacted to protect public health and safety; these take the form of both prescriptive and performance requirements. Structural requirements include description of the loads buildings must support, beginning with the constant everyday loads of building contents imposed by gravity and extending to the less frequent but more extreme loadings of wind and earthquake forces. These are specified on a statistical basis, usually the maximum expected to occur with a 100-year frequency. safety factors for materials are specified to allow for accidental overloading and lapses of quality control. Economic considerations are also reflected; for example, buildings must perform well under normal gravity loads, but no code requires a building to resist direct exposure to the wind and low-pressure effects of a tornado, for its cost would be prohibitive.

      Planning and zoning requirements provide for height and floor area limitations and building setbacks from lot lines to ensure adequate light and air to adjoining properties. Zoning regulations also establish requirements for permitted building usages, parking spaces, and landscaping and even set standards for the visual appearance of buildings. Another example is requirements for building atmosphere conditions; these include minimum (but not maximum) temperatures and rates of air change to dilute odours and provide an adequate oxygen supply. Life-safety requirements include adequate stairways for emergency exits, emergency lighting, smoke detection and control systems, and fire-resistant building materials. Sanitation requirements include adequate numbers of plumbing fixtures and proper pipe sizes. Electrical requirements include wire sizes, construction requirements for safety, and location of outlets.

      Beyond the government standards there are market standards, which reflect user expectations for buildings. One example is elevator systems; elevators are not required by building codes, but in the United States, for example, the number of elevators in office buildings is calculated based on a maximum waiting period of 30 seconds. Cooling of building atmospheres is also not required by code but is provided in climates and building types where the marketplace has shown it to be cost-effective.

      Building systems and components are perceived as having two dimensions of value. One is the purely functional dimension: the structure is expected to resist loads, the roof must keep out rain. The other is the aesthetic or psychic dimension: stone is perceived as more durable than wood; an elevator system with a waiting time of 30 seconds is preferable to one with a waiting time of two minutes. For these perceived differences many users are willing to pay more. When symbolic buildings such as temples, cathedrals, and palaces play an important role in society, the aesthetic dimension is important in valuing buildings; for example, the Parthenon of Athens or Chartres Cathedral commanded a level of investment in their economies that might be roughly compared to the U.S. Apollo space program. But in most buildings the functional dimension of value is dominant.

      Because of its relatively low level of technology, wide geographic distribution, highly variable demand, and wide variety of building products, the building industry in industrialized countries is subdivided into many small enterprises. This lack of centralization tends to discourage research and keeps building components sturdy and simple, following well-tried formulas. Within this diversity there are a number of fairly well-defined markets based on building types; these include low-rise residential buildings, low-rise commercial, institutional, and industrial buildings, high-rise buildings, and long-span buildings.

      A somewhat similar pattern is found in eastern Europe, although the building industry there is more centralized. There is also a much smaller low-rise residential market, with most new housing being provided in high-rise buildings.

      In developing countries the major market is for low-rise residential buildings to house rapidly growing populations. Much of the construction is undertaken by local craftsmen using simple building products. Local timber is widely used, and masonry materials still include the ancient mud brick. More sophisticated long-span and high-rise technologies are found only in major cities.

Building design and construction
Design programming
      The design of a building begins with its future user or owner, who has in mind a perceived need for the structure, as well as a specific site and a general idea of its projected cost. The user, or client, brings these facts to a team of design professionals composed of architects and engineers, who can develop from them a set of construction documents that define the proposed building exactly and from which it can be constructed.

      Building design professionals include those licensed by the state—such as architects and structural, mechanical, and electrical engineers—who must formally certify that the building they design will conform to all governmental codes and regulations. Architects are the primary design professionals; they orchestrate and direct the work of engineers, as well as many other consultants in such specialized areas as lighting, acoustics, and vertical transportation.

      The design professionals draw upon a number of sources in preparing their design. The most fundamental of these is building science, which has been gradually built up over the past 300 years. This includes the parts of physical theory that relate to building, such as the elastic theory of structures and theories of light, electricity, and fluid flow. There is a large compendium of information on the specific properties of building materials that can be applied in mathematical models to reliably project building performance. There is also a large body of data on criteria for human comfort in such matters as thermal environment, lighting levels, and sound levels that influence building design.

      In addition to general knowledge of building science, the design team collects specific data related to the proposed building site. These include topographic and boundary surveys, investigations of subsoil conditions for foundation and water-exclusion design, and climate data and other local elements.

      Concurrently with the collection of the site data, the design team works with the client to better define the often vague notions of building function into more precise and concrete terms. These definitions are summarized in a building space program, which gives a detailed written description of each required space in terms of floor area, equipment, and functional performance criteria. This document forms an agreement between the client and the design team as to expected building size and performance.

Design development
      The process by which building science, site data, and the building space program are used by the design team is the art of building design. It is a complex process involving the selection of standard building systems, and their adaptation and integration, to produce a building that meets the client's needs within the limitations of government regulations and market standards. These systems have become divided into a number of clear sectors by the building type for which they are intended. The design process involves the selection of systems for foundations, structure, atmosphere, enclosure, space division, electrical distribution, water supply and drainage, and other building functions. These systems are made from a limited range of manufactured components but permit a wide range of variation in the final product. Once the systems and components have been selected, the design team prepares a set of contract documents, consisting of a written text and conventionalized drawings, to describe completely the desired building configuration in terms of the specified building systems and their expected performance. When the contract documents have been completed, the final costs of the building can usually be accurately estimated and the construction process can begin.

      Construction of a building is usually executed by a specialized construction team; it is normally separate from the design team, although some large organizations may combine both functions. The construction team is headed by a coordinating organization, often called a general contractor, which takes the primary responsibility for executing the building and signs a contract to do so with the building user. The cost of the contract is usually an agreed lump sum, although cost-plus-fee contracts are sometimes used on large projects for which construction begins before the contract documents are complete and the building scope is not fully defined. The general contractor may do some of the actual work on the building in addition to its coordinating role; the remainder of the work is done by a group of specialty subcontractors who are under contract to the general contractor. Each subcontractor provides and installs one or more of the building systems—e.g., the structural or electrical system. The subcontractors in turn buy the system components from the manufacturers. During the construction process the design team continues to act as the owner's representative, making sure that the executed building conforms to the contract documents and that the systems and components meet the specified standards of quality and performance.

Low-rise residential buildings
      Low-rise residential buildings include the smallest buildings produced in large quantities. Single-family detached houses, for example, are in the walk-up range of one to three stories and typically meet their users' needs with about 90 to 180 square metres (about 1,000 to 2,000 square feet) of enclosed floor space. Other examples include the urban row house and walk-up apartment buildings. Typically these forms have relatively low unit costs because of the limited purchasing power of their owners. The demand for this type of housing has a wide geographic distribution, and therefore most are built by small local contractors using relatively few large machines (mostly for earth moving) and large amounts of manual labour at the building site. The demand for these buildings can have large local variations from year to year, and small builders can absorb these economic swings better than large organizations. The building systems developed for this market reflect its emphasis on manual labour and its low unit costs. A proportion of single-family detached houses are “factory-built”; that is, large pieces of the building are prefabricated and then transported to the site, where considerable additional work is required to complete the finished product.

Foundations
      All foundations must transmit the building loads to a stable stratum of earth. There are two criteria for stability: first, the soil under the foundations should be able to receive the imposed load without more than about 2.5 centimetres (one inch) of settlement and, second, the settlement should be uniform under the entire building. It is also important that the bottom of the foundation be below the maximum winter frost level. Wet soil expands as it freezes, and repeated freeze–thaw cycles can move the building up and down, leading to possible displacement and damage. Maximum frost depth varies with climate and topography. It can be as deep as 1.5 metres (five feet) in cold continental climates and is zero in tropical and some subtropical areas. The foundation systems for low-rise residential buildings are suitable for their light loads; nearly all are supported on spread footings, which are of two types—continuous footings that support walls and isolated pad footings that support concentrated loads. The footings themselves are usually made of concrete poured directly on undisturbed soil to a minimum depth of about 30 centimetres (12 inches). If typical continuous concrete footings are used, they usually support a foundation wall that acts either as a retaining wall to form a basement or as a frost wall with earth on both sides. Foundation walls can be built of reinforced concrete or masonry, particularly concrete block. Concrete blocks are of a standard size larger than bricks and are hollow, forming a grid of vertical planes. They are the least expensive form of masonry—using cheap but strong material—and their large size economizes on the labour required to lay them. Their appearance and weathering properties are inferior to those of fired masonry, but they are satisfactory for foundation walls. In some places timber foundation walls and spread footings are used. Excavation for foundations is the most highly mechanized operation in this building type; it is done almost entirely with bulldozers and backhoes.

Structural systems

Timber frames (framed building)
      In these small buildings the ancient materials of timber and masonry are still predominant in the structural systems. In North America, which has abundant softwood forests, light timber frames descended from the 19th-century balloon frame are widely used. These present-day “platform” frames are made of standard-dimension timbers, usually two or four centimetres (0.75 or 1.5 inch) thick, which are joined together by machine-made nails and other metal fasteners using hand tools.

      The first step is to construct a floor, which rests on the foundation wall. A heavy timber sill is attached to the wall with anchor bolts, and on top of it are nailed the floor joists (joist), typically 4 × 28 centimetres (1.5 × 11.25 inches) and spaced 40 centimetres (16 inches) apart. The span of the floor joists is usually about 3.6 metres (12 feet), which is the common maximum length of available timbers. The floor may need intermediate supports in the form of interior foundation walls or, if there is a basement, intermediate beams of wood or steel supported by the foundation walls and columns. For longer spans, floor trusses can be made, with members joined by nail grids or nailed plywood gussets or with wood chords and diagonal metal web members. On top of the joists is nailed plywood subflooring, which forms the deck and gives lateral stability to the floor plane.

      The exterior bearing walls (wall) are made of 4 × 9-centimetre (1.5 × 3.5-inch; “2 × 4”) timber verticals, or studs, spaced 40 or 60 centimetres (16 or 24 inches) apart, which rest on a horizontal timber, or plate, nailed to the floor platform and support a double plate at the top. The walls are sheathed on the outside with panels of plywood or particleboard to provide a surface to attach the exterior cladding and for lateral stability against wind. Plywood and particleboard are fabricated in panels of standard sizes. Plywood is made of thin layers of wood, rotary-cut from logs and glued together with the wood grain running perpendicularly in adjoining layers. Particleboard consists of fine wood chips mixed together in an adhesive matrix and allowed to harden under pressure. On top of the wall plate is placed either a second floor or the roof.

      Since most of the roofing materials used in these buildings are not fully watertight, the roofs must have sloped surfaces to rapidly drain off rainwater. Sloped forms are created by two methods. The traditional method uses joists similar to those of floor construction to span between exterior walls. Rafters are nailed to the ends of each joist and the rafters meet at a central ridge member, forming a triangular attic space. Where no attic space is needed, it has proved more economical to span the roof with triangular trusses (truss) with interior web members. These roof trusses are usually made of narrow timbers joined by nails, glue, or metal connectors, and they are often prefabricated in a workshop. Plywood or particleboard sheathing is then nailed to the roof surfaces to receive the roofing and to provide lateral stability, making the entire frame into a rigid box.

      Light timber frames are quite flammable, but small one- or two-story buildings are easy to evacuate in case of a fire (fire prevention and control), and building codes permit the use of these frames with such features as fire-resistant gypsum board on the interiors and fire-stops (short wooden members) between the studs. Timber structures are attacked by certain species of insects—such as termites and carpenter ants—as well as certain fungi, particularly in warm, moist climates. Wood can be chemically treated to discourage these attacks; other precautions include raising the timber above the ground and keeping it dry.

Masonry walls
      Structural masonry walls are also used in this building type, primarily in multistory buildings, where they offer greater load-bearing capacity and fire resistance. Brick (brick and tile) and concrete block are the major materials, brick being favoured for exterior surfaces because of its appearance and durability. Solid brick walls are rarely used, due to the higher labour and material costs; composite walls of brick and block or block alone are common. Cavity walls are used in colder climates; in these, two wythes (vertical layers) of masonry are built on either side of a layer of rigid insulation. The wythes are joined together by steel reinforcement that runs through the insulation and is laid in the horizontal masonry joints at intervals. Cavity walls have a heat-flow rate that is 50 percent of that of a solid wall. Timber floor and roof construction, similar to balloon framing, is used with masonry construction; and there is also some use of precast prestressed hollow concrete panels, which are fireproof and can span up to nine metres (30 feet).

Enclosure systems
      Enclosure systems for this building type are varied. For roofs, traditional wood shingles (shingle) or, more commonly, felt asphalt shingles are used, as are semicylindrical clay tiles and standing-seam metal roofs. Rainwater from roofs is usually caught in metal gutters and directed to exterior downspouts that discharge onto splash blocks or into underground drains connected to storm sewers.

      The wall surfaces of low-rise residential buildings are clad with a range of different materials. Traditional wood elements such as shingles and horizontal shiplap, or clapboard siding, are used on light timber frames as are vertical tongue-and-groove siding and boards and battens. Aluminum and vinyl sidings have been adapted from these wooden forms. Brick and stone veneer are also applied over timber and anchored to it with metal fasteners. Cement plaster, or stucco (stuccowork), is another traditional material used to enclose both timber and masonry structures, and its semiliquid application allows great plasticity of form. A more recent development is a very thin synthetic resin stucco applied directly to the surface of rigid plastic foam insulation.

      Insulation (insulator), which slows the rate of heat transfer through the enclosure, is usually applied at all exterior building surfaces that are exposed to air. There are two major types of insulation, rigid and nonrigid. Rigid insulations are primarily plastic foams (the dead air in the foam cells is the true insulator), which vary in thickness from 2.5 to five centimetres (one to two inches). They include styrofoam, used primarily below grade behind frost walls due to its low fire resistance; urethane foam; isocyanurate foam, which has the best fire resistance; and foam glass. Nonrigid insulations are usually made of fibre (fibreglass)—glass fibre being the most common—often with a foil-backed paper on one side. Fibre insulations are made in thicknesses up to 23 centimetres (9.25 inches). The effectiveness of an insulation material is measured in terms of its heat-transfer rate, or U-value, often expressed as the number of BTUs passing through a given unit of insulating material each hour at an expressed temperature differential across the material. Low U-values indicate good insulating properties of the material. U-value is an inverse function of thickness, so that there is a limit to the cost-effectiveness of increasing the amount of insulation on a surface. Rigid insulation panels are applied to vertical wall sheathing and the surfacing material is fastened through the insulation, or it is applied to horizontal roof decks. Glass fibre is usually applied in the spaces between wall studs and between roof joists or the bottom chords of roof trusses.

      Most low-rise residential buildings have a limited number of transparent openings in their exteriors, because of the traditional requirements of interior privacy and the relatively higher cost of windows (window) compared to opaque walls. The traditional wooden frames of domestic windows are often clad in extruded vinyl or aluminum cladding, and frames made entirely of extruded aluminum are common. Residential windows are a major means of ventilation, and there are a variety of operating actions for their movable sections: sliding or double-hung windows are still the major form, but hinged types—including casement, hopper, and awning forms—are also used. Sliding glass panel doors are also used, particularly in warmer regions. Glazing is still largely of clear glass. Double glazing, with two panes bonded to a metal tubular separator that contains a desiccant, is cost-effective in northern climates, but triple glazing is used commonly only in regions above about 55° to 60° latitude. A recent development is heat-mirror glass, in which a low-emissivity coating enhances the relative opacity of the glass to infrared radiation and slows the rate of internal heat loss in winter.

      Interior finishes and space-division systems define the living spaces within residential buildings with a range of both natural and synthetic materials. The most widely used wall finish is gypsum board (wallboard), a prefabricated form of traditional wet plaster. Wet gypsum plaster is cast between paper facings to form large panels that are nailed to light timber or metal frameworks. The joints between the panels are filled with a hard-setting resin compound, giving a smooth seamless surface that has considerable fire resistance. Gypsum board forms the substrate to which a number of other materials, including thin wood-veneered plywood and vinyl fabrics, can be applied with adhesives. In wet areas such as kitchens and bathrooms, water-resistant gypsum board is used, sometimes with the addition of adhesive-applied ceramic tile.

      Doors (door) in residential buildings are usually of the hollow-core type, with thin veneers of wood glued over a honeycomb paper core and solid wood edge strips; door frames are typically made of machined timber shapes. Plastic laminates bonded to particleboard are extensively used for built-in cabinets and countertops. The most common floor finish is carpeting (rug and carpet), most of which is now made of synthetic fibres, displacing the traditional wool and cotton. It can be easily maintained, and its soft visual and tactile texture, as well as its sound-absorbing qualities, make it attractive for residential use. Hardwoods—primarily oak, birch, and maple—are also used for floors, both in the traditional narrow planks nailed to plywood decks and as prefabricated parquet elements, which are applied with adhesives. In wet or hard-use areas vinyl-composition tiles or ceramic tiles are used.

      Domestic water-supply systems (water-supply system) for low-rise residential buildings have two sources, either municipal water-distribution systems or, where these are not available, wells that are drilled to underground aquifers which are free of contamination. Water is drawn from the wells with small submersible electric pumps, which are lowered through the well casing to the intake. Underground exterior water-supply pipes are usually cast-iron with threaded connections to contain the pressures applied to the fluid, which is typically sufficient to raise it four stories. Within the building, copper tubing with soldered connections is used for distribution because of its corrosion resistance and ease of fabrication; in some areas plastic pipe is also used. The domestic water supply is divided into cold and hot systems, the cold water being piped directly to the fixtures. The hot-water system first draws the supply through a hot-water heating tank, which raises its temperature to about 60° C (140° F) using electric resistance or gas heat. Domestic water heaters that use solar radiation to heat water in coils exposed to the sun on a glass-covered black metal plate (flat-plate solar collectors) are found in areas where there is ample sunshine and relatively high energy costs. The hot water is then distributed from the heater to the fixtures in a recirculating loop pipe system, in which gravity and temperature differentials maintain a constant temperature in period of low demand.

      The primary residential use of water is in the bathroom, which typically includes a bathtub of cast iron or pressed steel with a ceramic porcelain coating (although fibre-glass-reinforced resin is also used), a ceramic lavatory, and a ceramic tank-type water closet. The bath and lavatory are supplied with hot and cold water through faucets with lever or screw-type valve controls. The valve of the water closet supply is also lever-operated and relies on the gravity power of the water in the tank for its flushing action. Shower baths are also common, often incorporated into bathtub recesses or in a separate compartment finished with ceramic tile. In some countries a bidet is included.

      Other widely used plumbing fixtures include kitchen sinks, usually of cast iron or pressed steel with a ceramic porcelain coating, or of stainless steel; automatic dishwashing machines; and automatic washing machines for laundry. Kitchen sinks can be fitted with garbage disposals, which grind solid waste into a fluid slurry that is flushed out with wastewater. Where the possibility of back siphonage of wastewater into the water supply exists, a vacuum breaker must be provided at the supply to prevent this happening, but most domestic plumbing fixtures are designed to avoid this possibility.

      Drainage systems to remove wastewater are made of cast-iron pipe with threaded joints or bell-and-spigot joints sealed with molten lead or with plastic pipe with solvent-welded joints. The waste pipe of every plumbing fixture is provided with a semicircular reverse curve, or trap, which remains constantly filled with water and prevents odours from the drainage system from escaping into occupied spaces. Immediately downstream from each trap is an opening to a vent pipe system, which lets air into the drainage system and protects the water seals in the traps from removal by siphonage or back pressure. When wastewater leaves the building, it is drained through a backflow-prevention valve and into underground ceramic pipes. It then flows by gravity to either a private sewage treatment plant, such as septic tank and tile field, or to the public sewer system. If the discharge level of the wastewater is below the level of the sewer, a sewage ejector pump is required to raise the wastewater to a higher level, where gravity carries it away.

heating and cooling (cooling system)
      Atmosphere-control systems in low-rise residential buildings use natural gas, fuel oil, or electric resistance coils as central heat sources; usually the heat generated is distributed to the occupied spaces by a fluid medium, either air or water. Electric resistance coils are also used to heat living spaces directly with radiant energy. Forced-air distribution moves the heat-bearing air through a treelike system of galvanized sheet-metal ducts of round or rectangular cross section; electric-powered fans provide a pressure differential to push the air from the heat source (or furnace) to the living spaces, where it is expelled from grills located in the walls or floors. The negative pressure side of the fan is connected to another treelike system of return air ducts that extract air from living spaces through grills and bring it back to the furnace for reheating. Fresh outside air can be introduced into the system airstream from an exterior intake, and odour-laden interior air can be expelled through a vent, providing ventilation, usually at the rate of about one complete air change per hour. To conserve energy, air-to-air heat exchangers can be used in the exhaust–intake process. The heated air is usually supplied in constant volume, and the ambient temperature is varied in response to a thermostat located in one room. Central humidity control is rarely provided in this building type.

      Another common heating system is the radiant hot-water type. The heat source is applied to a small boiler, in which water is heated and from which it is circulated by an electric pump in insulated copper pipes similar to a domestic hot-water system. The pipes can be connected to cast-iron or finned tube steel radiators within the living spaces. The radiators are placed near the areas of greatest heat loss (such as windows or outside walls) where their radiant energy heats the surrounding air and creates a convection cycle within the room, producing a roughly uniform temperature within it. The hot water can also be conducted through narrow pipes placed in a continuous looping pattern to create a large radiant surface; this pattern of pipes may be cast in a concrete floor slab or placed above a ceiling to heat the adjoining living space. Temperature control in hot-water systems uses a thermostat in the living space to adjust the pumped flow rate of the water to vary the heat supplied.

      Radiant electric resistance heating systems use coils in baseboard units in the rooms, which create convection cycles similar to hot-water radiators, or resistance cables in continuous looped patterns embedded in plaster ceilings. Local temperature control can be much more precise with electric heating, because it is possible to install a thermostatically controlled rheostat to vary the energy output of relatively small sections of baseboard units or cable.

      A type of space heating that is increasing in use in residential buildings is passive solar radiation. On sunny winter days, south-facing windows let in substantial amounts of energy, often enough to heat the entire building. Wood-burning fireplaces with chimneys are still widely provided in residential buildings, but their use is mostly for aesthetic effect.

      The cooling of atmospheres in low-rise residential buildings is often done locally with unit air conditioners (air-conditioning), which penetrate the exterior wall of the space to be cooled; this permits the intake of fresh air when desired and the ejection of heat pumped from the space to the exterior air. Less often, forced-air heating systems have cooling coils introduced into the airstream to provide a centrally cooled interior. A compressive cooling process is used, similar to that in a domestic refrigerator. A refrigerant, which is a liquid at room temperature, is pumped through a closed system of coiled copper tubes. An electric pump maintains a low pressure in the cooling coils, and the liquid refrigerant passes through an expansion valve from a region of high pressure to the low-pressure coils. This change in pressure results in a phase change of the refrigerant; it turns from a liquid into a gas and absorbs heat in the process, just as water absorbs heat when it is boiled and converted into steam. The heat absorption of the liquid-to-gas transition cools air passing over the cooling coils. The cooled air is circulated through the building by the forced-air system. When the low-pressure gaseous refrigerant leaves the cooling coils, it goes through the pump and is pressurized. The refrigerant travels through condensing coils, which are located outside the building; there the phase change is reversed as the gas turns to a high-pressure liquid and liberates heat (heat pump) to the exterior air passing over the condensing coils. The liquid refrigerant returns to the expansion valve to repeat the cooling cycle. The refrigeration machine is thus a “heat pump” that moves heat out of the building to the exterior atmosphere. Heat pumps can also be run in reverse in the winter months to pump heat from the outside air into the building interior; they work best in mild climates with fairly warm winter temperatures. The use of heat pumps in cold climates poses many difficult technological problems.

      Interior atmospheres are also ventilated by operating windows, as well as by unintended leakage at all types of exterior openings. Bathrooms, kitchens, and laundries generate odours and heat and often have separate exhaust systems powered by electric fans that are operated intermittently as required. Residential atmosphere quality is also protected by the smoke detector, which sounds an alarm to warn of possible danger when smoke reaches even a very low level in living spaces.

      Electrical systems in residential buildings are supplied from public utility power grids, starting from a step-down transformer near the building that reduces the high line voltage to a safer level. An underground or overhead cable from the transformer leads to the building, where it is connected to a meter that records the energy used by the subscriber. Immediately beyond the meter is a fused main switch to protect the building against an accidental power surge from the grid. The main service is then broken down into a number of circuits by a panelboard, each circuit having a fused switch. From the panelboard the wires (wire) of each circuit distribute the electricity to different areas of the building. The wires are usually copper, although aluminum is also used, and are covered with thermoplastic insulation. The wires must be contained in conduit, which is either metal or plastic tubing, to protect against damage and reduce the possibility of fire in the case of accidental overloading of the wires. Conduits are usually concealed in finished spaces within the framing of partition walls or above ceilings and terminate in junction boxes flush with a wall surface. The junction boxes contain terminal devices such as the convenience outlet, control switches, or the connection point for built-in light fixtures.

      Residential lighting is provided primarily by movable incandescent fixtures plugged into convenience outlets, but there is often built-in lighting in kitchens, bathrooms, corridors, and closets, mostly of the incandescent type. There is also some use of fluorescent lighting, particularly in built-in fixtures. Overall interior light levels in residential uses are low, about 20–40 footcandles. Exterior lighting is used for entrances, walkways, and exterior living spaces.

      The power densities of dwelling units are fairly low and are declining because of the increased use of fluorescent lighting fixtures and improvement of efficiency in electrical appliances. The decline in power consumption enhances the prospect of the widespread appearance of dwellings—particularly detached houses—with their own independent electric power generation and storage systems, unconnected to public utility grids. Photovoltaic cells, which convert sunlight directly into electricity, in combination with storage batteries can offer these residences a new kind of energy autonomy.

Low-rise commercial, institutional, and industrial buildings
      The size of buildings in the commercial, institutional, and industrial market segment ranges from a few hundred to as much as 45,000 square metres (500,000 square feet). All of these buildings have public access and exit requirements, although their populations may differ considerably in density. The unit costs are generally higher than those for dwellings (although those of simple industrial buildings may be lower), and this type includes buildings with the highest unit cost, such as hospitals and laboratories. Residential buildings are fairly static in their function, changing only at long intervals. By contrast, most commercial, institutional, and industrial buildings must respond to fairly rapid changes in their functions, and a degree of flexibility is required in their component systems. In addition, these buildings are built by contractors who utilize heavy mechanized equipment not only for foundations (pile drivers and caisson augers) but also for lifting heavy components (a wide variety of cranes and hoists). Semimanual machines such as cement finishers, terrazzo grinders, and welding generators are also used, but a large percentage of the work is done manually; the human hand and back remain major instruments of the construction industry, well adapted to the nonrepetitive character of building.

Foundations
      The foundations in these buildings support considerably heavier loads than those of residential buildings. Floor loadings range from 450 to 1,500 kilograms per square metre (100 to 300 pounds per square foot), and the full range of foundation types is used for them. Spread footings are used, as are pile foundations, which are of two types, bearing and friction. A bearing pile is a device to transmit the load of the building through a layer of soil too weak to take the load to a stronger layer of soil some distance underground; the pile acts as a column to carry the load down to the bearing stratum. Solid bearing piles were originally made of timber, which is rare today; more commonly they are made of precast concrete, and sometimes steel H-piles are used. The pile length may be a maximum of about 60 metres (200 feet) but is usually much less. The piles are put in place by driving them into the ground with large mechanical hammers. Hollow steel pipes are also driven, and the interiors are excavated and filled with concrete to form bearing piles; sometimes the pipe is withdrawn as the concrete is poured. An alternative to the bearing pile is the caisson. A round hole is dug to a bearing stratum with a drilling machine and temporarily supported by a steel cylindrical shell. The hole is then filled with concrete poured around a cage of reinforcing bars; and the steel shell may or may not be left in place, depending on the surrounding soil. The diameter of caissons varies from one to three metres (three to 10 feet). The friction pile of wood or concrete is driven into soft soil where there is no harder stratum for bearing beneath the site. The building load is supported by the surface friction between the pile and the soil.

      When the soil is so soft that even friction piles will not support the building load, the final option is the use of a floating foundation, making the building like a boat that obeys Archimedes' principle—it is buoyed up by the weight of the earth displaced in creating the foundation. Floating foundations consist of flat reinforced concrete slabs or mats or of reinforced concrete tubs with walls turned up around the edge of the mat to create a larger volume.

      If these buildings do not have basements (cellar), in cold climates insulated concrete or masonry frost walls are placed under all exterior nonbearing walls to keep frost from under the floor slabs. Reinforced concrete foundation walls for basements must be carefully braced to resist lateral earth pressures. These walls may be built in excavations, poured into wooden forms. Sometimes a wall is created by driving interlocking steel sheet piling into the ground, excavating on the basement side, and pouring a concrete wall against it. Deeper foundation walls can also be built by the slurry wall method, in which a linear series of closely spaced caissonlike holes are successively drilled, filled with concrete, and allowed to harden; the spaces between are excavated by special clamshell buckets and also filled with concrete. During the excavation and drilling operations, the holes are filled with a high-density liquid slurry, which braces the excavation against collapse but still permits extraction of excavated material. Finally, the basement is dug adjoining the wall, and the wall is braced against earth pressure.

      The structures of these buildings are mostly skeleton frames of various types, because of the larger spans their users require and the need for future flexibility. Timber is used, but on a much-reduced scale compared to residential buildings and primarily in regions where timber is readily available. The public nature of commercial and institutional buildings and the hazards of industrial buildings generally require that they be of noncombustible construction, and this largely excludes the use of light timber frames. Heavy timber construction can be used where the least dimensions of the members exceed 14 centimetres (5.5 inches); when timbers are this large they are charred but not consumed in a fire and are considered fire-resistant. Because most harvested trees are fairly small, it is difficult to obtain solid heavy timbers, and most large shapes are made up by glue laminating smaller pieces. The synthetic glues used are stronger than the wood, and members with cross sections up to 30 × 180 centimetres (12 × 72 inches) are made; these may be tapered or otherwise shaped along their length. Skeletons of glue-laminated beams and columns, joined by metal connectors, can span 30 to 35 metres (100 to 115 feet). Heavy decking made of tongue-and-groove planks up to 9.4 centimetres (3.75 inches) thick is used to span between beams to support floors and roofs.

      Steel is a major structural material in these buildings. It is a strong and stiff material and yet relatively inexpensive, and it can be quickly fabricated and erected, which saves construction time. Although steel is noncombustible, it starts to lose strength when heated above 400° C (750° F), and building codes require it to be fireproofed in most multistory buildings; in small and low-hazard buildings, however, it can be left unprotected.

      Nearly all structural steel—including sheets, round or square bars, tubes, angles, channels, and I beam or wide flange shapes—is formed by the hot-rolling process. Steel roof and floor deck panels are fabricated from sheet metal by further cold-rolling into corrugated profiles four to eight centimetres (1.5 to three inches) deep and 60 centimetres (24 inches) wide. They are usually welded to the supporting steel members and can span up to 4.5 metres (15 feet). The lightest and most efficient structural shape is the bar (or open web) joist, a standard truss made with angles for the top and bottom chords, joined by welding to a web made of a continuous bent rod. It is used almost exclusively to support roofs and can span up to 45 metres (150 feet). The standard rolled shapes are frequently used as beams (beam) and columns (column), the wide flange, or W shape, being the most common. The widely separated flanges give it the best profile for resisting the bending action of beams or the buckling action of columns. W shapes are made in various depths and can span up to 30 metres (100 feet). Where steel beams support concrete floor slabs poured onto a metal deck, they can be made to act compositely with the concrete, resulting in considerable economies in the beam sizes.

      The connections of steel shapes are of two types: those made in the workshop and those made at the building site. Shop connections are usually welded, and site or field connections are usually made with bolts due to the greater labour costs and difficulties of quality control in field welding. Steel columns are joined to foundations with base plates welded to the columns and held by anchor bolts embedded in the concrete. The erection of steel frames (framed building) at the building site can proceed very rapidly, because all the pieces can be handled by cranes and all the bolted connections made swiftly by workers with hand-held wrenches.

      A large proportion of steel structures are built as prefabricated, pre-engineered metal buildings, which are usually for one-story industrial and commercial uses. They are manufactured by companies that specialize in making such buildings of standard steel components—usually rigid steel bents or light trusses—which are assembled into frames and enclosed with corrugated metal siding. The configurations can be adapted to the needs of individual users. The metal building industry is a rare example of a successful application of prefabrication techniques in the construction industry in the United States, where its products are ubiquitous in the suburban and rural landscape.

      Reinforced concrete is also a major structural material in these buildings. Indeed, outside of North America and western Europe, it is the dominant industrialized building material. Its component parts are readily available throughout the world at fairly low cost. portland cement is easily manufactured by burning shale and limestone; aggregates such as sand and crushed limestone can be easily obtained. Steel minimills, which use scrap iron to feed their electric furnaces, can mass-produce reinforcing bars for regional use. In industrialized countries the mixing and delivery of liquid concrete to building sites has been mechanized with the use of central plants and mixing trucks, and this has substantially reduced its cost. In barely 100 years, reinforced concrete has risen from an experimental material to the most widespread form of building construction.

      There are two methods of fabricating reinforced concrete. The first is to pour the liquid material into forms at the building site; this is so-called in situ concrete. The other method is called precast concrete, in which building components are manufactured in a central plant and later brought to the building site for assembly. The components of concrete are portland cement, coarse aggregates such as crushed stone, fine aggregates such as sand, and water. In the mix, water combines chemically with the cement to form a gel structure that bonds the stone aggregates together. In proportioning the mix, the aggregates are graded in size so the cement matrix that joins them together is minimized. The upper limit of concrete strength is set by that of the stone used in the aggregate. The bonding gel structure forms slowly, and the design strength is usually taken as that occurring 28 days after the initial setting of the mix. Thus there is a one-month lag between the time in situ concrete is poured and the time it can carry loads, which can significantly affect construction schedules.

      In situ concrete is used for foundations and for structural skeleton frames. In low-rise buildings, where vertical gravity loads are the main concern, a number of framing systems are used to channel the flow of load through the floors to the columns (column) for spans of six to 12 metres (20 to 40 feet). The oldest is the beam and girder system, whose form was derived from wood and steel construction: slabs rest on beams, beams rest on girders, and girders rest on columns in a regular pattern. This system needs much handmade timber formwork, and in economies where labour is expensive other systems are employed. One is the pan joist system, a standardized beam and girder system of constant depth formed with prefabricated sheet-metal forms. A two-way version of pan joists, called the waffle slab, uses prefabricated hollow sheet-metal domes to create a grid pattern of voids in a solid floor slab, saving material without reducing the slab's strength. The simplest and most economical floor system is the flat plate, where a plain floor slab about 20 centimetres (eight inches) thick rests on columns spaced up to 6.7 metres (22 feet) apart. If the span is larger, the increasing load requires a local thickening of the slab around the columns. When these systems are applied to spans larger than nine to 12 metres (30 to 40 feet), a technique called posttensioning is often used. The steel reinforcing takes the form of wire cables, which are contained in flexible tubes cast into the concrete. After the concrete has set and gained its full strength, the wires are permanently stretched taut using small hydraulic jacks and fastening devices, bending the entire floor into a slight upward arch. This reduces deflection, or sagging, and cracking of the concrete when the service load is applied and permits the use of somewhat shallower floor members. Concrete columns are usually of rectangular or circular profile and are cast in plywood or metal forms. The reinforcing steel never exceeds 8 percent of the cross-sectional area to guard against catastrophic brittle failure in case of accidental overloading.

      Precast concrete structural members are fabricated under controlled conditions in a factory. Members that span floors and roofs are usually pretensioned, another prestressing technique, which is similar in principle to posttensioning. The reinforcement is again steel wire, but the wires are put into tension (stretched) on a fixed frame, formwork is erected around the taut wires, and concrete is poured into it. After the concrete has set and gained its full strength, the wires are cut loose from the frame. As in posttensioning, this gives the precast floor members a slight upward arch, which reduces deflection and permits the use of shallower members. Precast prestressed floor elements are made in a number of configurations. These include beams of rectangular cross section, hollow floor slabs 15 to 30 centimetres (six to 12 inches) deep and spanning up to 18 metres (60 feet), and single- and double-stem T shapes up to 1.8 metres (six feet) deep and spanning up to 45 metres (150 feet). Precast concrete columns are usually not prestressed and have projecting shelves to receive floor members. At the building site, precast members are joined together by a number of methods, including welding together metal connectors cast into them or pouring a layer of in situ concrete on top of floor members, bonding them together. Precast prestressed construction is widely used, and it is the dominant form of construction in the Soviet Union and eastern Europe.

      Masonry finds only a limited structural use in these buildings. Concrete block walls with brick facing and punched openings (discrete windows entirely surrounded by the facing material) spanned by concealed steel lintels can be used for exterior bearing walls where the interior is a skeleton frame of steel or timber. The use of interior bearing walls so greatly reduces the flexibility needed in these buildings that they are only rarely found.

Enclosure systems
      Enclosure systems in these buildings range from rather simple forms in industrial uses to quite sophisticated assemblies in the commercial and institutional sectors. Most have in common the use of flat roofs with highly water-resistant coverings, the traditional one being a built-up membrane of at least four layers of coal-tar pitch and felt, often weighted down with a gravel ballast. Such roofs are pitched at slopes of 1 : 100 to 1 : 50 toward interior drains. In recent years the single-ply roof, made of plastic membranes of various chemistries, has found wide application. The seams between the pieces of membrane are heat- or solvent-welded together, and they are either ballasted with gravel or mechanically fastened to the underlying substrate, which is usually rigid foam insulation. Sometimes standing-seam sheet-metal roofs are also used; the best quality is continuously welded stainless steel.

      The choice of transparent surfaces in these enclosures is based on three major considerations: conductive heat transfer, radiant energy transfer, and safety. All the transparent materials used in the low-rise residential sector are found, plus a number of others. In buildings with fully controlled atmospheres, double glazing is common to reduce heat transfer and both interior and exterior condensation on the glass. Commercial and institutional buildings tend to have large internal sources of heat gain, such as people and lighting, so it is desirable to exclude at least some solar gain through the transparent surfaces to reduce energy consumption in cooling. This can be done by reducing the light transmission or shading coefficient of the glass by integrally tinting it in various colours; grey, bronze, and green are common tints. This can also be accomplished by vacuum-plating partial reflective coatings of varying densities to an inner surface of double glazing; this can reflect up to 90 percent of the incident energy. Two kinds of reflecting metal are used: aluminum, which is silver in tone, and rubidium, which is gold-toned. These coatings are perceived as strong tints when the outside world is viewed through them by day: grey for aluminum and green for rubidium.

      Skylights or horizontal transparent surfaces have found wide application in these types of buildings. These installations range from purely functional daylighting in industrial uses to elaborate aesthetic forms in commercial structures. In horizontal applications, and in vertical walls where people might blunder into glazed panels, safety (safety glass) glazing is required. Safety glazing is of four types: certain plastics that are flexible and difficult to break; wire-embedded glass, which holds together when broken; tempered glass, which is very strong and breaks into tiny and relatively harmless fragments; and laminated glass, which consists of two layers of glass heat-welded together by an intermediate plastic film. Laminated glass can also be made with tinted lamination film, producing many colours not available in integrally coloured glass.

      Because many of these buildings have skeleton structures, their vertical surfaces are enclosed in nonstructural curtain walls that resist wind forces and provide weatherproofing. Curtain walls are of several types; the most common is one supported by a metal (typically aluminum) gridwork attached to the building structure. The vertical members, called mullions, are attached to the building at every floor and are spaced 1.5 to three metres (five to 10 feet) apart; the horizontal members, called muntins, are attached between the mullions. The rectangles between the grid of mullions and muntins are filled with transparent or opaque panels. The transparent surfaces can be any of those just described, and the opaque panels include opaque coloured glass, painted or anodized aluminum sheets, porcelain enameled steel sheets, fibreglass-reinforced cement, and stone wafers of granite, marble, or limestone cut with diamond-edged tools. All of these materials are usually backed up by rigid insulation to slow heat transfer. Metal sandwich panels are also used for economy of material; two thin layers of metal are separated by a core of different material, often with a high U-value for insulating effect. The separation of the thin layers of strong metal greatly increases the overall stiffness of the panel. The joints between panels and the supporting grid are weatherproofed with elastomeric sealants (cold-setting synthetic rubbers) or by prefabricated rubber gaskets. In glazed areas of curtain walls, mullions of structural glass are an alternative to metal mullions; they are more expensive, but they give an effect of greater transparency where this is desired.

      Another type of curtain wall is the panel type. It has no gridwork of mullions and muntins but is made of large prefabricated rigid panels connected to the floors and spanning between them, with transparent openings made as holes cut out of the panel. The panels can be made of precast concrete, aluminum, or steel, often in sandwich form; elastomeric sealants are used to close the joints.

      The finishes of metals in curtain walls include anodizing of aluminum, an electrolytic process that builds up the natural colourless oxide of aluminum into a thick adherent layer; it often includes the introduction of colour into the oxide layer itself. Durable paint coatings (with lifetimes of up to 40 years) can be applied to the metal in the factory; more conventional paints that must be renewed at shorter intervals are also used.

Partitions (wall)
      Space-division systems in these buildings make use of gypsum board partitions, usually applied to a framework of formed sheet-metal members attached to the building structure. They are readily demolished and rebuilt at relatively low cost, meeting the need for flexibility in such buildings. They are often used for fire-resistive protective enclosures, for which a number of layers are laminated to achieve the specified fire resistance. Transparent and translucent partitions are also used, with different types of glass set in metal frames. Office buildings may contain prefabricated movable metal partitions, which typically use metal sandwich panel construction to create panels with both transparent and opaque surfaces as well as doors. These partitions are expensive compared with gypsum board and must be moved often to justify the greater initial cost. Concrete block is used in unfinished spaces and for fire-resistive partitions. Glazed ceramic block or ceramic tile applied over concrete block or gypsum board is used in wet areas and where cleanliness is a problem, such as in kitchens and toilet rooms. Occasionally walls with wood paneling or stone veneer are used for aesthetic effect. Doors (door) are usually set in formed sheet-metal frames, although some wood frames are used. The doors themselves are usually made of solid timbers glue-laminated together and covered with thin decorative wood veneers; painted hollow sheet-metal doors are used for exterior doors and in areas of hard use.

ceiling finishes
      Ceiling finishes in these buildings create a sandwich space below the roof or floor slab above, which conceals projecting structural elements, recessed light fixtures, electrical wiring conduits, and air-handling ductwork. The ceiling must be accessible to change or maintain the service elements located above it, and the most common ceiling system is composed of wet felted mineral fibre panels, painted and perforated on one side for sound absorption. The removable panels are supported on a grid of formed sheet-metal tee bars or zee tracks, which are suspended by wires from the structure above. Where accessibility is not important and a smooth finish is desired, suspended gypsum board ceilings can be used.

Floor finishes
      Floor finishes in commercial and institutional uses make considerable use of synthetic-fibre carpeting and vinyl composition tile. In areas of higher traffic harder surfaces may be used—for example, cut stone tiles of marble or granite, ceramic tile applied with epoxy adhesive to the substrate, or terrazzo. Terrazzo is made in two ways, traditional and thin-set. In the traditional form a four-centimetre (1.5-inch) layer of cement and sand grout is poured over the substrate; a grid of metal divider strips to control shrinkage cracks is set on the hardened surface, and grout mix of coloured cement and marble chips is poured between the strips. After hardening, the surface is machine polished to expose the marble chips and metal dividers. Thin-set terrazzo is made by placing the metal strips and pouring the binder and marble chips directly onto the subfloor, without the underbed of cement and sand. It is generally possible only when epoxy resins are used in place of cement binders. Terrazzo is available in many colours, and it forms a hard, smooth, and durable surface that is easily cleaned.

Life-safety systems
      Most important in the hierarchy of interior elements are life-safety systems to protect and evacuate the building population in emergencies. These include life-threatening events, such as fire and smoke and earthquakes, and less critical ones, such as electric power failures. To deal with the threat of fire and smoke there is an array of fire-detection and fire-suppression systems (fire prevention and control). These include electronic heat and smoke detectors (smoke detector) that can activate audible alarm devices to warn the building population and automatically notify local fire departments. For fire suppression hand-operated fire extinguishers (fire extinguisher) must be provided, but many buildings have a separate piping system (sprinkler system) to provide water for fire fighting. If public water mains cannot provide adequate water pressure, an electric pump is included, and there is also a connection outside the building to attach portable fire truck pumps. The piping terminates in an array of sprinkler heads located throughout the building in the ceiling plane in a density ranging from eight to 18 square metres (90 to 200 square feet) per head. Typically there is always water in the pipes (a wet system), though dry systems are used in unheated buildings or where leakage might damage the contents. The head is opened to spray water by a fusible link made of metal that melts at a fairly low temperature when the air surrounding it is heated by a fire. Sprinkler systems have proved to be a highly reliable and effective means of fire suppression. Smoke can be as dangerous as fire to building occupants, and protective measures include the automatic shutdown of mechanical ventilating systems and the division of the building into smokeproof compartments to prevent the spread of smoke.

      The evacuation of occupants in emergencies is accomplished by a system of protected exits (fire escape) leading to the exterior; all building areas must be within a specified travel distance of such an exit, varying from 30 to 90 metres (100 to 300 feet). For one-story buildings the exit usually consists simply of exterior doors, but for multistory buildings the exits are enclosed stairways that also lead to the exterior. The stairways have fire-rated enclosures and are often pressurized to exclude smoke; their width is determined by the maximum predicted number of occupants per floor. Travel paths to the exit must be clearly marked by illuminated directional exit signs, and battery-powered emergency lighting is required in the travel path and in the exit itself, in case of power failure. Some buildings of this type, such as hospitals, have large diesel- or natural gas-powered emergency electric generating systems that provide power and lighting for critical areas (such as operating rooms).

      Another of the life-safety elements in these buildings is the fire-resistance requirements for building materials. These include the application of cementitious fireproofing or insulation to structural steel frames, the fire-resistive construction of the enclosures around exits, the flame-spread ratings of finish materials such as carpeting and wall coverings, and the use of such inherently fire-resistant materials as reinforced concrete and heavy timber. The fire-resistive ratings of various construction materials and assemblies are established by laboratory fire tests.

Vertical transportation
      Vertical transportation systems in these low buildings include stairways (staircase), sometimes only those provided as life-safety exits but more often open, well-lighted ones as well. Where large numbers of people need to be moved vertically a short distance, escalators (escalator), or moving stairways, powered by electric motors are often provided. For moving smaller volumes of people and freight, hydraulic elevators (elevator) are used; the cabs of these elevators are moved by a telescoping tubular piston underneath, which is raised and lowered by pumping oil in and out of it with an electric pump. Hydraulic elevators move slowly, but they are the least expensive type and are well suited for low buildings.

      Plumbing systems for water supply and wastewater removal are very similar to those used in residential buildings, but the higher population densities of commercial, institutional, and industrial buildings require larger toilet rooms for public multiperson use. These often include pressure-valve water closets placed in partitioned cubicles and urinals in men's toilet rooms. Some fixtures in each toilet room must be carefully arranged for easy access by handicapped persons.

      The internal drainage of large flat roofs introduces another piping system, similar to that for sanitary wastewater, to carry away storm water to separate underground storm sewers. Heavy rainstorms can introduce huge influxes of water into storm sewers, and sometimes this surge effect is tempered by the use of storm water retention ponds on the building site; runoff from the roof and paved areas is temporarily stored in these ponds while it flows into the sewer at a slower rate. Hospitals, laboratories, and factories have many other types of plumbing systems for various gases and liquids; these require special materials and construction. The sites of commercial, institutional, or industrial buildings may have underground networks of irrigation piping that terminate in flush sprinkler heads to water grass and plantings.

Environmental control
      The atmosphere systems of industrial buildings are usually simple, involving only winter heating and possibly humidity control if the manufacturing process is sensitive to it. A commonly used element is the unit heater, in which an electric fan blows air through a coil heated by hot water, steam, electric resistance, or gas combustion and provides a directed supply of warm air where needed. Another system involves radiant heating using electric resistance coils backed by reflectors or continuous reflector-backed metal pipes that radiate heat from gas burned inside them. Ventilation (ventilating) in industrial buildings is sometimes done with operable windows but more often with unit ventilators, which penetrate walls or roofs and use electric fans to exhaust interior air that is replaced by air flowing in through operable louvres.

      Commercial and institutional low-rise buildings generally have fully controlled atmospheres with heating, cooling, and humidification. An economical method of providing this controlled atmosphere is with rooftop single or multizone package units. Each unit contains an electric fan to move conditioned air; heating elements, which can be gas or oil-fired or electric resistance coils; cooling coils, which use the compressive cooling cycle with compressor, cooling coils, and condensor coils to liberate heat; as well as a fresh-air intake and air exhaust. All of these elements are prefabricated in a rectangular enclosed unit that is simply set on the roof over an opening through which it is connected to the supply and exhaust ducts. The airflow over the heating and cooling elements can be partitioned to provide different conditioned airstreams to serve different zones of the building. The conditioned air is fed at a constant volume into treelike systems of insulated sheet-metal ductwork for transmission to the zones served. The conditioned air enters the occupied space through diffusers placed in the ceiling system and connected to the ducts by flexible spiral reinforced fabric tubes. Thermostats (thermostat) within the space sense temperatures and send signals by electricity or compressed airflow to the unit to adjust heating and cooling as required; relative humidity is held to a range of 20 to 40 percent. The return of air from the occupied space to the unit for reconditioning is sometimes done through a reverse tree of ductwork leading back to the unit, but more often in commercial buildings this is accomplished by placing the entire sandwich space between the ceiling and the structural deck above under negative pressure to make what is called a return-air plenum. The negative pressure is created by an opening into the plenum from the return side of the rooftop unit, and perforated openings or grills in the ceiling plane admit the return air from the occupied space. Return air can also be made to enter the plenum by passing over the lamps of fluorescent light fixtures; this permits the direct recovery of heat generated by the lamps, which can be recycled to the occupied space in winter.

      The rooftop unit is best used in one-story buildings or smaller multistory ones. For larger multistory buildings, centralized atmosphere systems are used. These are built up of separate components, most of which are housed in mechanical equipment rooms or in penthouses at roof level. The components include fans for moving air, humidification devices, air-filtering devices, and refrigeration machines. Where large refrigeration machines are used, the condensor coils that liberate heat are no longer placed outside the building as in residential units or rooftop units but are located in a water jacket near the compressor. This water is circulated through a piping system to carry away the heat to a cooling tower outside the building where the water is sprayed into the atmosphere and partially evaporated to liberate heat, then recovered and returned at a lower temperature to the condensor coil jacket. Mechanical equipment rooms for atmosphere systems require a minimum of 5 percent of the floor space in a commercial building and can range up to 20 percent in hospitals and 40 percent in laboratory buildings; if the building is large, there can be more than one fan room with centralized refrigeration machines and cooling towers. The distribution of conditioned air in buildings with centralized atmosphere systems is usually done through an insulated ductwork tree, using the variable air volume (VAV) method. This method supplies conditioned air in variable amounts as required to maintain the desired temperature in occupied spaces; it results in considerable energy economies over constant volume air supply methods. Separate exhaust systems are used for areas generating heat and odours, such as kitchens, laboratories, and toilet rooms.

      Electrical systems in these buildings begin at a step-down transformer provided by the utility company and located within or very close to the building. The transformer reduces the standard line potential to two dual voltage systems, which then pass through master switches and electric meters to record the subscriber's usage. Each of the voltages provided serves a separate category of use; different levels are required for incandescent lights and small appliances, large appliances, ceiling-mounted non-incandescent lighting, and heavy machinery. Each voltage pair has a separate distribution system of wiring leading from the meters and master switches to circuit breaker panels, where it is further broken down into circuits similar to residential uses. Because high-voltage wiring is considered hazardous, the switches controlling overhead lighting use lower voltages, and each heavy machine has its own fused switch. From the circuit breaker panel, low-voltage power conduit and wiring is typically distributed through partitions and ceiling sandwich spaces, but, in large open areas of commercial buildings, there may be wireways embedded in the floor slab. These wireways can be either rectangular metal tubes inserted into the concrete slab before pouring or closed cells of formed steel deck; the wireways are tapped where desired to provide convenience outlets at floor level.

      Lighting in these buildings is predominantly fluorescent. Lamps range in size and wattage, and the available colours can range from warm white to cool white. Incandescent (incandescent lamp) tungsten-filament lamps are used mostly for accent lighting, since their light-output efficiency is low. Mercury-vapour (mercury lamp) and metal halide-vapour lamps have the same efficiency as fluorescent lamps (fluorescent lamp), but certain types may have longer operating lives. High-pressure sodium-vapour lamps (sodium-vapour lamp) have even higher efficiencies and are used in industrial applications; their marked orange colour and high intensity has limited their commercial and institutional use, however. Each of these types of lamp is used in a variety of fixtures to produce different lighting conditions. Incandescent lamps can be placed in translucent glass globes for diffuse effects, or in recessed ceiling-mounted fixtures with various types of reflectors to evenly light walls or floors. Fluorescent lamps are typically installed in recessed rectangular fixtures with clear prismatic lenses, but there are many other fixture types, including indirect cove lights and luminous ceilings with lamps placed above suspended plastic or metal eggcrate diffuser grids. Mercury-vapour and high-pressure sodium-vapour lamps are placed in simple reflectors in high-ceilinged industrial spaces, in pole-mounted light fixtures for outdoor applications on parking lots and roadways, and in indirect up-lighting fixtures for commercial applications.

      Mathematical models (mathematical model) can accurately predict the performance of lighting in most applications. The zonal cavity method, which takes into account the lamps, fixtures, shape of room, and colours of room surfaces, is one example. The usual measure of light intensity is in footcandles on a horizontal surface, such as the floor of a room or a desk. The intensity ranges from 15 footcandles for a minimum ambient light level to 70 footcandles for an office or classroom and 100–200 footcandles for very precise visual tasks such as drafting; direct sunlight at noon, by comparison, is about 1,000 footcandles. In most of these buildings, the required lighting level is achieved with fixtures mounted at ceiling level; having all lighting at ceiling level allows flexibility in using building spaces. But the intensity of light varies inversely with the square of the distance from the source; thus, if a light fixture gives an intensity of 40 footcandles at a distance of one metre, it will produce an intensity of 10 footcandles at two metres. Therefore, considerable energy savings can be realized by having a minimal ambient light level (say 15 footcandles) produced by ceiling-mounted fixtures and providing task lighting close to work surfaces where higher intensities are needed. Daylighting from windows and skylights is also utilized in these buildings, and mathematical models have been developed that accurately predict its performance.

      Communications systems are of growing significance and complexity in commercial, institutional, and industrial buildings. Thus communications wires for telephones, public-address systems, and computer data are free to take many paths through the building, including vertical risers, ceiling sandwich spaces, and wireways in floor slabs similar to those of electrical power wires. Where the density of wires rises to very high levels—for example, in computer rooms or where many small computer terminals are installed—raised floor systems are used. Removable floor panels are mounted on tubular metal frameworks resting on the structural floor slab, creating a plenum space to carry the necessary wiring.

      A number of building systems are controlled by computers (computer) or microprocessors. In certain atmosphere systems both the interior sensors (such as thermostats) and the exterior weather sensors feed data to a computer that adjusts the system for minimal energy expenditure. Other examples include security, fire, and emergency alarm systems.

High-rise buildings (high-rise building)
      The high-rise building (skyscraper) is generally defined as one that is taller than the maximum height which people are willing to walk up; it thus requires mechanical vertical transportation. This includes a rather limited range of building uses, primarily residential apartments, hotels, and office buildings, though occasionally including retail and educational facilities. A type that has appeared recently is the mixed-use building, which contains varying amounts of residential, office, hotel, or commercial space. High-rise buildings are among the largest buildings built, and their unit costs are relatively high; their commercial and office functions require a high degree of flexibility.

Foundations
      The foundations of high-rise buildings support very heavy loads, but the systems developed for low-rise buildings are used, though enlarged in scale. These include concrete caisson columns bearing on rock or building on exposed rock itself. Bearing piles and floating foundations are also used.

Structural systems

wind loads
      The structural systems of tall buildings must carry vertical gravity loads, but lateral loads, such as those due to wind and earthquakes, are also a major consideration. Maximum 100-year-interval wind forces differ considerably with location; in the interiors of continents they are typically about 100 kilograms per square metre (20 pounds per square foot) at ground level. In coastal areas, where cyclonic storms such as hurricanes and typhoons occur, maximum forces are higher, ranging upward from about 250 kilograms per square metre (50 pounds per square foot). Wind forces also increase with building height to a constant or gradient value as the effect of ground friction diminishes. The maximum design wind forces in tall buildings are about 840 kilograms per square metre (170 pounds per square foot) in typhoon areas.

      The effect of wind forces on tall buildings is twofold. A tall building may be thought of as a cantilever beam with its fixed end at the ground; the pressure of the wind on the building causes it to bend with the maximum deflection at the top. In addition, the flow of wind past the building produces vortices near the corners on the leeward side; these vortices are unstable and every minute or so they break away downwind, alternating from one side to another. The change of pressure as a vortex breaks away imparts a sway, or periodic motion, to the building perpendicular to the direction of the wind. Thus, under wind forces there are several performance criteria that a high-rise structure must meet. The first is stability—the building must not topple over; second, the deflection, or sidesway at the top, must not exceed a maximum value (usually taken as 1/500 of the height) to avoid damage to brittle building elements such as partitions; and, third, the swaying motion due to vortex shedding must not be readily perceptible to the building occupants in the form of acceleration, usually stated as a fraction of gravity, or g. The threshold of perception of lateral motion varies considerably with individuals; a small proportion of the population can sense 0.003 g or 0.004 g. The recommendation for motion perception is to limit acceleration to 0.010 g for wind forces that would recur in 10-year intervals. The fourth criterion involves the natural period of the building structure. This is the vibration period at which the swaying cantilever motions of the building naturally reinforce and enhance each other and could become large enough to damage the building or even cause it to collapse. The natural period of the building should be less than one minute, which is the period of vibration due to the shedding of wind vortexes.

earthquake loads
      Earthquake or seismic forces, unlike wind forces, are generally confined to relatively small areas, primarily along the edges of the slowly moving continental plates that form the Earth's crust. When abrupt movements of the edges of these plates occur, the energy released propagates waves through the crust; this wave motion of the Earth is imparted to buildings resting on it. Timber frame buildings are light and flexible and are usually little damaged by earthquakes; masonry buildings are heavy and brittle and are susceptible to severe damage. Continuous frames of steel or reinforced concrete fall between these extremes in their seismic response, and they can be designed to survive with relatively little damage.

      In two major earthquakes involving large numbers of high-rise buildings, in Los Angeles in 1971 and Mexico City (Mexico City earthquake of 1985) in 1985, lateral accelerations due to ground motions in a number of tall buildings were measured with accelerometers and were found to be of the order of 0.100 to 0.200 g. In Los Angeles, where the period of the seismic waves was less than one second, most steel-frame high rises performed well with relatively little damage; continuous concrete frames also generally performed well, but there was considerable cracking of concrete, which was later repaired by the injection of epoxy adhesive. In Mexico City, however, the period of the seismic waves was quite long, on the order of a few seconds. This approached the natural frequency of many tall structures, inducing large sidesway motions that led to their collapse. Based on this experience, determination of the seismic performance criteria of buildings involves the lateral resistance of forces of 0.100 to 0.200 g and consideration of the natural period of the building in relation to the period of seismic waves that can be expected in the locality. Another important factor is the ductility of the structure, the flexibility that allows it to move and absorb the energy of the seismic forces without serious damage. The design of buildings for seismic forces remains a complex subject, however, and there are many other important criteria involved.

Classification of structural systems
      The types of structures used for high-rise buildings must meet the lateral load performance criteria outlined above, and they must be reasonably efficient in the use of material and of reasonable cost. The most efficient high-rise structure would meet the lateral load criteria using no more material than would be required for carrying the building gravity load alone; in other words, it would have no premium for height. This economic criterion of “no premium for height” has led to a classification of high-rise structures, each of which has only a small premium for a particular range of height.

      High-rise structures begin at the lowest range with the rigid frame in both steel and concrete. Some or all of the joints between the beams and columns are rigidly joined together by welding the steel or pouring the concrete in situ, and lateral resistance is provided by the rigid joints; this system can rise about 90 metres (300 feet) with little premium. The next type is the rigid frame with a vertical shear truss in steel or a shear wall in concrete to provide greater lateral rigidity; it has a range of 38 to 150 metres (125 to 500 feet). The framed tube structure in both steel and concrete brings more gravity load and more structural material to closely spaced columns at the building's perimeter, again increasing lateral rigidity; this type is reasonably efficient from 38 to 300 metres (125 to 1,000 feet) in height. The trussed tube with interior columns, which can also be executed in both steel and concrete, introduces diagonal bracing on all sides of the building's perimeter. The bracing also carries gravity loads and further raises the lateral rigidity, making this a low-premium structure for the region of 240 to 360 metres (800 to 1,200 feet). The bundled tube, which consists of a number of framed tubes joined together for even greater lateral rigidity, begins to be practical at about 75 metres (250 feet). It was the form of the steel structure used for the Sears Tower in Chicago. Beyond this height there is another system that appears to have a low premium: the superframe. In this structure much of the building's gravity load, and therefore its material, is brought to a diagonally braced superframe tube at the perimeter by interior transfer trusses of various configurations. No true superframes have yet been built.

Enclosure systems
      The enclosure systems for high-rise buildings are usually curtain walls similar to those of low-rise buildings. The higher wind pressures and the effects of vortex shedding, however, require thicker glazing and more attention to sealants. The larger extent of enclosed surfaces also requires consideration of thermal movements, and wind- and seismic-induced movements must be accommodated. Window washing in large buildings with fixed glass is another concern, and curtain walls must provide fixed vertical tracks or other attachments for window-washing platforms. Interior finishes in high-rise buildings closely resemble those used in low-rise structures.

Life-safety systems
      Life-safety systems are similar to those in low-rise buildings, with stairways serving as vertical emergency exits; in case of fire all elevators are automatically shut down to prevent the possibility of people becoming trapped in them. Emergency generator systems are provided to permit the operation of one elevator at a time to rescue people trapped in them by a power failure. Generators also serve other vital building functions such as emergency lighting and fire pumps. Fire-suppression systems often include sprinklers, but, if none are required by building codes, a separate piping system is provided with electric pumps to maintain pressure and to bring water to fire-hose cabinets throughout the building. There are also exterior connections at street level for portable fire-truck pumps. The fire hoses are so placed that every room is accessible; the hoses are intended primarily for professional fire fighters but may also be used by the building occupants.

Vertical transportation
      Vertical transportation systems are of vital importance in high-rise buildings. Escalators (escalator) are used on lower floors for moving high volumes of people over short distances. A few retail or educational buildings have escalators for up to 10 stories. The principal means of vertical transport in tall buildings is the roped elevator. It moves by a direct current electric motor, which raises and lowers the cab in a shaft with wire ropes running over a series of sheaves at the motor and the cab itself; the ropes terminate in a sliding counterweight that moves up and down the same shaft as the cab, reducing the energy required to move the elevator. Each elevator cab is also engaged by a set of vertical guide tracks and has a flexible electric cable connected to it to power lighting and doors and to transmit control signals. Passenger elevators range in capacity from 910 to 2,275 kilograms (2,000 to 5,000 pounds) and run at speeds from 90 to 510 metres per minute; freight elevators hold up to 4,500 kilograms (10,000 pounds). The speed of elevators is apparently limited to the current value of 510 metres per minute by the acceleration passengers can accept and the rate of change of air pressure with height, which at this speed begins to cause eardrum discomfort.

      Elevator movements are often controlled by a computer that responds to signals from call buttons on each floor and from floor-request buttons in each cab. The number of elevators in a building is determined by the peak number of people to be moved in a five-minute period, usually in the early morning; for example, in an office building this is often set at 13 percent of occupancy. The average waiting time for an elevator between pressing the call button and arrival must be less than 30 seconds in an office building and less than 60 seconds in an apartment building. The elevators are usually arranged in groups or banks ranging from one to 10 elevators serving a zone of floors, with no more than five elevators in a row to permit quick access by passengers. In a few very tall buildings the sky lobby system is used to save elevator-shaft space. The building is divided vertically into subbuildings, each with its own sky lobby floor. From the ground floor large express elevators carry passengers to the sky lobby floors, where they transfer to local elevator banks that take them to the individual floors within the subbuildings.

       plumbing systems in tall buildings are similar to those of low-rise buildings, but the domestic water-supply systems require electric pumps and tanks to maintain pressure. If the building is very tall, it may require the system to be divided into zones, each with its own pump and tank.

Environmental control
      The atmosphere systems in high-rise office buildings are similar to those of low-rise, with conditioned air distributed by a ductwork tree using the VAV system and return air removed through ceiling plenums. The placement of air-handling equipment can be done in two ways. One uses centralized fans placed about every 20 floors, with air moved vertically through trunk ducts to and from each floor; the other uses floor-by-floor fan rooms to provide air separately for each floor. There is usually a central refrigeration plant for the entire building connected with cooling towers on the roof to liberate heat. The central refrigeration machines produce chilled water, which is circulated by electric pumps in a piping system to the air-handling fans in order to cool incoming air as required. Incoming air is heated in winter either by piping coils through which hot water is circulated by pumps and piping from a central boiler, or by electric resistance coils in the air-handling units. In residential high-rise buildings cooling is typically provided by window air-conditioning units, and heating by hot-water or electric resistance radiant systems. There is limited use of centralized cooling, in which chilled water from a central refrigeration plant is circulated to fan-coil units near the building perimeter; a small electric fan within the unit circulates the air of the room over the chilled water coil to absorb heat.

Electrical systems
      Electrical (electricity) systems for high-rise buildings are also very similar to low-rise types. The major difference is that, if the building is exceptionally tall, the utility company may bring its high-voltage lines inside the building to a number of step-down transformers located in mechanical equipment spaces. From each step-down transformer the distribution of electricity is similar to that of a smaller building.

Long-span buildings
      Long-span buildings create unobstructed, column-free spaces greater than 30 metres (100 feet) for a variety of functions. These include activities where visibility is important for large audiences (auditoriums and covered stadiums), where flexibility is important (exhibition halls and certain types of manufacturing facility), and where large movable objects are housed (aircraft hangars). In the late 20th century, durable upper limits of span have been established for these types: the largest covered stadium has a span of 204 metres (670 feet), the largest exhibition hall has a span of 216 metres (710 feet), and the largest commercial fixed-wing aircraft has a wingspread of 66.7 metres (222 feet) and a length of 69.4 metres (228 feet), requiring a 75–80-metre- (250–266-foot-) span hangar. In these buildings the structural system needed to achieve these spans is a major concern.

Structural systems

Structural types
      Structural systems for long-span buildings can be classified into two groups: those subject to bending, which have both tensile and compressive forces, and funicular structures, which experience either pure tension or pure compression. Since bridges are a common type of long-span structure, there has been an interplay of development between bridges and long-span buildings. Bending structures include the girder, the two-way grid, the truss, the two-way truss, and the space truss. They have varying optimum depth-to-span ratios ranging from 1 : 5 to 1 : 15 for the one-way truss to 1 : 35 to 1 : 40 for the space truss. The funicular structures include the parabolic arch, tunnel vault, and dome, which act in pure compression and which have a rise-to-span ratio of 1 : 10 to 1 : 2, and the cable-stayed roof, the bicycle wheel, and warped tension surfaces, which act in pure tension. Within these general forms of long-span structure, the materials used and labour required for assembly are an important constraint along with other economic factors.

Timber (wood) structures
      Glue-laminated timber can be used as a long-span material. It can be prefabricated using metal connectors into trusses that span up to 45 metres (150 feet). Its most economical forms, however, are the pure compression shapes of the multiple-arch vault, with spans up to 93 metres (305 feet), and ribbed domes, with spans up to 107 metres (350 feet). These are often used as industrial storage buildings for materials such as alumina, salt, and potash that would corrode steel or concrete. Such timber structures are usually found only near forested areas; transportation of timber to other areas increases its cost.

steel structures
      Steel is the major material for long-span structures. Bending structures originally developed for bridges, such as plate girders and trusses, are used in long-span buildings. Plate girders are welded from steel plates to make I beams that are deeper than the standard rolled shapes and that can span up to 60 metres (200 feet); however, they are not very efficient in their use of material. Trusses (truss) are hollowed-out beams in which the stresses are channeled into slender linear members made of rolled shapes that are joined by welding or bolting into stable triangular configurations. The members of trusses act either in pure compression or pure tension: in the top and bottom horizontal members the forces are greatest at the centre of the span, and in the verticals and diagonals they are greatest at the supports. Trusses are highly efficient in bending and have been made up to 190 metres (623 feet) in span. Two-way grids can be made of either plate girders or trusses to span square spaces up to 91 metres (300 feet) in size; these two-way structures are more efficient but more expensive to build.

      The highly efficient funicular forms are used for the longest spans. Vaults (vault) made of rows of parabolic arches, usually in truss form for greater rigidity, have been used for spans of up to 98.5 metres (323 feet). Steel truss domes, particularly the Schwedler triangulated dome, have been the choice for several large covered stadiums, with the greatest span being 204.2 metres (669 feet). Cable-stayed roof construction is another structural system derived from bridge building. A flat roof structure in bending is supported from above by steel cables radiating downward from masts that rise above roof level; spans of up to 72 metres (236 feet) have been built. Another funicular form is the bicycle-wheel roof, where two layers of radiating tension cables separated by small compression struts connect a small inner tension ring to the outer compression ring, which is in turn supported by columns.

      Tension-cable networks use a mesh of cables stretched from masts or continuous ribs to form a taut surface of negative curvature, such as a saddle or trumpet shape; the network of cables can be replaced by synthetic fabrics to form the tension surface. Another fabric structure using tension cables is the air-supported membrane. A network of cables is attached by continuous seams to the fabric, and the assembly of cables and fabric is supported by a compression ring at the edge. The air pressure within the building is increased slightly to resist exterior wind pressure. The increase can be as slight as 1.5 percent of atmospheric pressure, and it is possible to maintain this even in large buildings with relatively small compressors. The cables stiffen the fabric against flutter under uneven wind pressure and support it in case of accidental deflation.

concrete structures
      Reinforced concrete, because of its inherent strength in compression, is primarily used for long spans in funicular compression forms, including vaults, shells, and domes. Thin parabolic shell vaults stiffened with ribs have been built with spans up to about 90 metres (300 feet). More complex forms of concrete shells have been made, including hyperbolic paraboloids, or saddle shapes, and intersecting parabolic vaults. An example of the latter is the CNIT Exhibition Hall in Paris, which consists of six intersecting double-shell parabolic vaults built to span a triangular space 216 metres (708 feet) on a side with supports only at the apexes of the triangle. Reinforced concrete domes, which are usually also of parabolic section, are built either in ribbed form or as thin shells. The maximum span of these domes is about 200 metres (660 feet).

      Another funicular form used in concrete, though it is really a composite structure, is the inverted dome, or dish. As in the steel bicycle wheel, a concrete compression ring resting on columns at the perimeter of the structure supports radial steel cables that run inward and downward to a small steel tension ring at the centre, forming the dish shape. The cable network is stiffened against wind forces by encasing it in a poured concrete dish; structures of this type have been built with spans of up to 126 metres (420 feet).

Factors in the built environment

      Long-span auditoriums (auditorium) involve considerations in acoustics: audiences wish to hear speakers clearly and to hear music with appropriate tonality. Unfortunately, acoustic requirements for speech quality often conflict with those for music, and it is difficult to design an auditorium that is satisfactory for both. The best single measure of acoustic performance for auditoriums is the reverberation time, which is directly proportional to the volume of the hall and inversely proportional to the amount of sound absorbency within it, including wall and ceiling surfaces and the audience itself. Measured in the sound range of 500–1,000 hertz, rooms with short reverberation times of one to 1.5 seconds are good for the intelligibility of speech, while longer reverberation times of 1.5 to 2.5 seconds add richness of tone to musical performances. Thus, adding sound-absorbent material to a hall improves it for speech but detracts from its musical qualities. People are excellent sound absorbers, and thus the audience has a distinct impact on auditorium acoustics; to keep this effect constant with varying audience size, auditorium seats are usually upholstered to serve as surrogate spectators of the same sound absorbency. Curved surfaces, which tend to focus sound, are either avoided in auditoriums or covered with sound-absorbent material. Electronic sound-amplification systems can be used to assist speakers in large halls but generally are not satisfactory for music. Other long-span buildings, such as covered stadiums and exhibition halls, receive only minor acoustical treatment.

Environmental control systems
      Atmosphere systems in long-span buildings must handle the considerable heat and odour generation from population densities of less than one square metre (11 square feet) per person. Air must be moved fairly rapidly through the population zone to maintain an acceptable air-change rate.

Additional Reading

General
Hugh Brooks, Encyclopedia of Building and Construction Terms (1983), provides useful reference information in nontechnical language; Henry J. Cowan (ed.), Encyclopedia of Building Technology (1988), is a collection of informative articles by authoritative contributors from 15 countries; and Joseph A. Wilkes and Robert T. Packard (eds.), Encyclopedia of Architecture: Design, Engineering & Construction, 5 vol. (1988–89), offers an extensive coverage of a broad spectrum of topics. See also Cyril M. Harris (ed.), Dictionary of Architecture and Construction (1975); and Kornelis Smit (ed.), Means Illustrated Construction Dictionary (1985).

History
Banister Fletcher, Sir Banister Fletcher's A History of Architecture, 19th ed., edited by John Musgrove (1987), is the most comprehensive exploration of comparative methods. The early history of building materials and building techniques is covered in Norman Davey, A History of Building Materials (1961, reissued 1971); René Alleau, History of the Great Building Constructions (1967; originally published in French, 1966); Henry J. Cowan, The Master Builders: A History of Structural and Environmental Design from Ancient Egypt to the Nineteenth Century (1977, reprinted 1985); and John Fitchen, Building Construction Before Mechanization (1986). Specific urban cultures are the basis of the analyses in Jean Gimpel, The Cathedral Builders, trans. by Teresa Waugh (1983; originally published in French, 1958); John Fitchen, The Construction of Gothic Cathedrals: A Study of Medieval Vault Erection (1961, reissued 1981); Frank D. Prager and Gustina Scaglia, Brunelleschi: Studies of His Technology and Inventions (1970); and E. Baldwin Smith, The Dome: A Study in the History of Ideas (1950; reissued 1978). For the industrial age, see Carl W. Condit, American Building Art: The Nineteenth Century (1960), American Building Art: The Twentieth Century (1961), American Building: Materials and Techniques from the First Colonial Settlements to the Present, 2nd ed. (1982), and The Rise of the Skyscraper (1952); Sigfried Giedion, Mechanization Takes Command: A Contribution to Anonymous History (1948, reprinted 1969), and Space, Time, and Architecture: The Growth of a New Tradition, 5th rev. ed. (1967); Reyner Banham, The Architecture of the Well-Tempered Environment, 2nd ed. (1984); and Henry J. Cowan, Science and Building: Architectural and Environmental Design in the Nineteenth and Twentieth Centuries (1978).

Modern practices
Standards regulating building construction are covered in Building Officials And Code Administrators International, The BOCA Basic/National Building Code (triennial); International Conference Of Building Officials, Uniform Building Code (triennial); Southern Building Code Congress, Southern Standard Building Code (irregular); and American Society Of Heating, Refrigerating, And Air-conditioning Engineers, ASHRAE Guide and Data Book (annual). See also Frederick S. Merritt (ed.), Building Design and Construction Handbook, 4th ed. (1982); and Charles A. Herubin, Construction Site Planning and Development (1988). Standards and data pertaining to architecture, building, and allied fields are illustrated in John Ray Hoke, Jr. (ed.), Ramsey/Sleeper Architectural Graphic Standards, 8th ed. (1988). See also Albert G.H. Dietz, Dwelling House Construction, 4th rev. ed. (1974); Joseph D. Falcone, Principles and Practices of Residential Construction (1987); Francis D.K. Ching, Building Construction Illustrated (1975); Harold B. Olin, John L. Schmidt, and Walter H. Lewis, Construction: Principles, Materials & Methods, 5th ed. (1983), focusing on low-rise construction; and, for nontraditional building, Lester L. Boyer and Walter T. Grondzik, Earth Shelter Technology (1987).B.S. Benjamin, Structures for Architects, 2nd rev. ed. (1984), analyzes the behaviour of structural systems based on physical principles and properties of materials; Mario Salvadori, Why Buildings Stand Up: The Strength of Architecture (1980), is an illustrated introduction to structural engineering; and Mario Salvadori and Robert Heller, Structure in Architecture: The Building of Buildings, 3rd ed. (1986), is a broader survey of foundations and structural systems. See also Ronald C. Smith and Cameron K. Andres, Principles and Practices of Heavy Construction, 3rd ed. (1986); Irving Engel, Structural Steel in Architecture and Building Technology (1988); E. Eranti and G.C. Lee, Cold Region Structural Engineering (1986); and Wolfgang Schueller, High-Rise Building Structures, 2nd ed. (1986).The following works focus on building materials: Ronald C. Smith and Cameron K. Andres, Materials of Construction, 4th ed. (1988); Paul Bianchina, Illustrated Dictionary of Building Materials and Techniques (1986); Edward Allen, Fundamentals of Building Construction: Materials and Methods (1985); and Henry J. Cowan and Peter R. Smith, The Science and Technology of Building Materials (1988).Mechanical and electrical systems are studied in Richard D. Rush, The Building Systems Integration Handbook (1986); Building Services Research And Information Association, Building Services Materials Handbook: Heating, Sanitation, and Fire Protection (1987); Roger W. Haines, Control Systems for Heating, Ventilating, and Air Conditioning, 4th ed. (1987); Benjamin Stein, John S. Reynolds, and William J. McGuinness, Mechanical and Electrical Equipment for Buildings, 7th ed. (1986); and Robert H. Perry and Tyler G. Hicks (eds.), Building Systems Reference Guide (1987).Alfred Swenson Pao-Chi Chang

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