Building construction involves the techniques and procedures by which a building is erected. A building is generally classed by the material used to create its frame. The basic structural component of most large-scale modern buildings is steel; (for buildings whose structure is primarily wood, see \Thouse\t). The principles used in steel construction have evolved from the era of \Tcast-iron architecture\t, beginning in the late 18th century. Although steel has replaced iron as the primary structural element, the engineering principles associated with the development of the iron \Ttruss\t and the \Tarch\t, beam, and \Tcolumn\t are still in use today. SKELETON STEEL FRAMING Skeleton steel construction evolved in Chicago in the 1880s and '90s and has continued to be one of the most important techniques used in the construction of tall buildings. A skeleton steel frame comprises a braced steel framework of columns and beams connected by bolts, rivets, or electric welding (see RIVETING; \Twelding and soldering\t). This steel framework, or skeleton, supports all floors and walls. The exterior, or curtain, walls are attached to the steel framework. Interior partitions, called nonbearing walls, do not add to the rigidity or strength of the building. Built floor by floor, steel frame structures can be erected to great heights. For many years the standard practice was to rivet the beams to the columns, thus forming a semirigid connection. Electric arc-welding had been perfected by the 1920s to a point where the entire frame could be welded into a rigid, homogeneous grid of interconnecting columns and beams. The most recent technique in joining steel structural members is the use of high-strength structural bolts. These bolts, made from special steels and finished to exact dimensions, are installed in predrilled holes and tightened by a pneumatic wrench to help form an immobile joint. Tall buildings rest on \Tfoundations\t usually of reinforced concrete piers drilled or dug to sufficient depth to support the building. \Tpiles\t of wood, concrete, or steel are driven into the dense soils deep under the surface and then used to support reinforced concrete on which the steel skeleton rests. The erection of structural steel is accomplished with cranes or \Lderrick\ls. Cranes mounted on trucks or tractors can be used on buildings up to 12 stories high. Above this height, a tower crane is often used. This comprises a horizontal boom mounted on top of a steel tower positioned next to the building. This counterweighted boom is long enough to reach wherever material is to be placed in the structural frame. The steel is erected floor by floor. Each piece of structural steel is fabricated in a factory, where it is marked to show exactly where it will go in the finished frame. Beams and columns are hoisted into place, aligned, and temporarily bolted. When the steel members of a complete floor are in position, they are braced and checked for perfect alignment, and the joints are then permanently fixed by riveting, welding, or bolting. As the construction of the structural steel skeleton proceeds upward, lower floor joists and floors are emplaced as working platforms. When the steel skeleton has been completed and beams and floors are in place, the curtain wall panels are hoisted and attached to the structural frame. The installation of mechanical equipment and interior finish material can then be completed. CURTAIN WALLS A curtain wall is an exterior wall that carries no floor loads; it usually is made principally of metal, stone, glass, or precast concrete. Prefabricated into panels, the curtain is attached to the structural frame by clips or anchors to form a continuous wall. Curtain walls may also be made of sandwich panels. In this case a metal exterior panel, a layer of insulation, and a layer of interior finish material are prefabricated into a single unit by joining the three layers; this unit then forms both the interior and exterior walls. The joints between the panels of curtain walls must be sealed to prevent water seepage. FLOOR AND ROOF SYSTEMS Floor and roof systems are closely related to the structural frame. The oldest type of floor used on steel skeleton framed buildings comprised reinforced concrete slabs about 10 cm (4 in) thick, laid between wrought-iron I beams. Today reinforced concrete slabs that span 3.7 to 5.5 m (12 to 18 ft) can be poured into place. Spaces of greater width may be spanned by ribbed reinforced concrete slabs, which are poured into place over hollow clay tile or metal pans to form reinforced concrete beams that span distances between the major steel beams. One of the lightest floor- and roof-support members for long spans is the open-web steel joist, a variety of truss. It can be fabricated to span up to 44 m (144 ft). The top and bottom chords usually comprise two steel angles separated by diagonal round steel bars or steel angles. The top chord rests on the structural beam, where it is bolted or welded into place. The most commonly used material for spanning between open-web joists is steel decking, a light-gauge sheet steel prefabricated with corrugations to furnish stiffening. Cellular steel decking comprises two sheets of steel held apart to form hexagonally shaped open cells. These cells may be used as conduits for electrical and other wiring. The decking is usually welded to the structural beams, and a 50- to 100-cm (2- to 4-in) concrete fill completes the rough floor. FIREPROOFING Although steel is an incombustible material, it loses its strength and buckles or collapses when heated to 650 deg-950 deg C (1,200 deg-1,700 deg F). Early steel buildings were protected from the effects of fire by clay tile, masonry, or concrete surrounding each column and beam. Although these materials are still used to a limited extent, other methods have been developed that use lighter, thinner substances. Contact fireproofing uses \Tgypsum\t or portland cement plaster mixed with \Tperlite\t or vermiculite; this is sprayed directly onto columns, beams, and the underside of steel decking. H columns may be wrapped with metal lath, which is then plastered or sprayed with gypsum or portland cement to make a square column. Steel decks and floor beams may be protected by membrane fireproofing, a thin, lightweight ceiling attached to or hung underneath decks and beams on steel wires. The membrane is usually made of vermiculite or perlite and gypsum plaster. The space between the floor and the hung ceiling is used for ducts, piping, electrical wiring, and other mechanical equipment. Traditionally, all exterior structural members were encased in insulation as fire protection. Three new forms of protection are now in use: hollow steel columns and beams are filled with liquid; flame shields are placed around window openings to direct the heat of a fire away from the structural steel; the structural steel is separated from building walls so that the air space between wall and steel prevents heat buildup. LONG-SPAN STEEL FRAMING Large buildings such as auditoriums, airports, and sports arenas require greater distances between supports than can be supplied by traditional column-and-beam framing. Large areas may be covered instead by long-span steel framing. The most common types of long-span framing include built-up girders--steel plates and shapes welded or riveted together to form a structural member of far greater strength than the single girder. Built-up girders offer great flexibility, because they may be arched or tapered to suit different design requirements and may be pierced in low-stress areas to allow pipes, ducts, and conduits to pass through. Trusses are also used to span large areas. Usually prefabricated in the shop, they are hoisted into place by cranes or derricks. As the trusses are set, they are held in alignment by horizontal beams called purlins, which span between trusses. When very large clear spans are needed, open-web arches may be used. The arch may start from the ground and furnish support for the walls as well as for the roof. The bottom of the arch is connected to the foundation by heavy pin connections. These act as hinges and allow the bottom of the arch to move, thus relieving stresses that could build up in individual members of the arch. This type of arch is called a two-hinged arch. A three-hinged arch has a third pin connection at the center of the span. For long spans, tie rods, either in or under a concrete floor, tie the bottom of the two sides of the arch together. Arched trusses may radiate from a central point, forming a dome. The outward thrust is resisted by a tension ring on the outside of the circle thus formed. Covers over athletic facilities with spans as great as 200 m (660 ft) use this type of framing. Space frames, known also as lattice structures, comprise three-dimensional, trusslike triangular frameworks used to span square or rectangular areas. The individual members are arranged to carry loads to four or more supports on all sides of the rectangle. Space frames are used for structures such as exhibition halls, where long spans and a column-free interior space are desired. Space frames with parallel top and bottom surfaces are usually constructed of standardized, prefabricated rods or tubes that can be quickly assembled and erected by simple methods. Joints, which may serve to connect as many as nine identical members, may be designed for bolted or welded connections. The framing members are fabricated and joined together in the shop in sections that can be conveniently transported to the job site and hoisted into place by a crane. The matching joints are finished on the site by welding or bolting. Prefabricated metal or precast concrete roof deck, insulation, and roofing can then be applied to the top of the space frame to complete the assembly. The underside of the space frame may be left exposed, whereby it becomes an interior design feature. CABLE-SUPPORTED STRUCTURES In recent years a substantial upsurge has occurred in the design and construction of cable-supported structures. This type of system uses a wire-cable suspension bridge, which had reached a high state of development in the 1860s. Two types of cable-supported structures are in use today: the single-story hanging roof, and multistory systems using cables for vertical supports. A cable is the most economical form of steel with which to span large areas. A steel cable has an extremely high tensile strength and flexibility. Roof decks supported entirely by steel cables take on a characteristic shape because the cables naturally sag downward. Cable-supported roofs introduce a new problem into design because they tend to be aerodynamically unstable (affected by wind). Several techniques have been used to overcome flutter in suspended roofs. One of the first major one-story, cable-supported roofs erected in the United States was the State Fair and Exhibition building in Raleigh, N.C., built in 1954. In this building the roofing rests on a tightly drawn two-layered grid of cables at right angles to one another. The cables are anchored to two parabolic reinforced concrete arches on a common axis. The arches slope outward and are joined at the low point of the roof to form a stable structure, putting all cables in tension. Another form of suspended roof construction is Eero \TSaarinen\t's Dulles International Airport in Washington, D.C., completed in 1962. This building has a cable suspension system covered with heavy precast concrete decking. In the multistory suspension system a central reinforced concrete core or tower is erected first. The roof and floors are hung from a steel truss above the roof. Exterior curtain walls are then hung from the floors. REINFORCED CONCRETE CONSTRUCTION Concrete is a material with great compressive strength. It can be designed to resist a compressive load of 69 kilonewtons a square meter (10,000 lb/sq in). Concrete, however, has little resistance to a pulling force or tension. Steel is produced with a tensile strength of 345 kN/sq m (50,000 lb/sq in). Concrete and steel are combined in reinforced concrete to take advantage of the strengths of each material. Modern reinforced concrete construction uses many smaller steel reinforcing bars exactly shaped and placed in each structural member. The first high-rise concrete buildings were constructions of columns and beams similar to skeleton steel framing. The concrete was hoisted up temporary steel towers and poured into wooden forms to shape the columns and beams. When the concrete had set and gained sufficient strength, the forms were removed and reused. As the concrete skeleton progressed upward, new forms were set, and the floors were poured over the beams. The refinement of flat slab framing in the 1920s greatly reduced the weight of buildings by eliminating many of the heavy beams. In this system the reinforced floor slabs act as a continuous beam attached to, and spanning between, columns. A recent innovation in concrete construction is the design of exterior walls as load-bearing columns and beams. Flat slabs span from the exterior framework to solid interior walls that act as shear panels. This type of construction eliminates the use of a curtain wall. The full potential of concrete in building construction resulted from the development of prestressing, which is the method of inducing a controlled stress in a beam before it is subjected to a load. In the first type of prestressing, called pretensioning, high-quality steel cables are imbedded in the lower portion of the beam or in areas where tension exists. The steel cables are placed in forms and stretched by heavy jacks; the concrete is poured into the form and allowed to set; and the jacks are then released, putting the concrete in compression. In the other type of prestressing, called posttensioning, the cables may be located in hollow tubes placed within the forms. The concrete is poured into the forms and allowed to set; and the cables are put in tension by screws or jacks and wedged into place against steel plates at opposite ends of the beam. The labor of building forms and the equipment necessary to transport and place concrete in high structures are major cost items. Concrete formed and placed in factories or at ground level is simpler and less expensive. Intricate shapes and textures can be obtained in forms lined with metal, plastic, or plaster. Two major methods have been developed to erect precast walls or floors: tilt-up and lift-slab construction. Tilt-up construction comprises casting wall panels in a horizontal position and, when they have gained sufficient strength, tilting them up into position by cranes. Individual panels are generally joined by poured-in-place concrete pilasters. Steel reinforcing rods projecting from the edges of adjacent panels are welded together, wood forms are erected, and the pilaster is poured to make a rigid wall. Lift-slab construction is similar to tilt-up construction. Concrete or steel columns are erected, and the ground-floor slab is cast. The second, third, and subsequent floors are cast on top of the first floor; the final floor is the roof slab. The roof and floor slabs are lifted into position one at a time by hydraulic jacks at a rate of 2 to 3 m/h (7 to 10 ft/h). As each slab reaches its designated position, it is attached to the column. Buildings more than 12 stories high have been constructed by this method. SHELL STRUCTURES A shell structure is formed of single or double curved surfaces where the ratio of thickness to span is 1:500 or less. Column, beam, and slab structures resist bending and shearing forces by their strength, and shell structures have stresses uniformly distributed over their entire surface and resist loading by their shape. The simplest of the shell structures comprise barrel vaults and domes. Builders in Europe and Latin America used thin shell structures in a variety of shapes derived from the geometry of double-warped surfaces, such as the conoid and hyperbolic paraboloid. Outstanding shell structures have been designed and constructed by Felix \Tcandela\t of Mexico, Luigi \TNervi\t of Italy, Oscar \TNiemeyer\t of Brazil, and Eduardo Torroja of Spain. Relatively few outstanding structures have been constructed in North America and the United Kingdom. One of the most unusual shell structures, designed by the Danish architect Jorn Utzon, is the Opera House in Sydney, Australia. Most shell structures can be constructed on forms made of straight pieces of lumber. The forms are erected on falsework, and the reinforcing, which comprises an intricate web of small reinforcing rods or wire, is spaced in such a way that it will be in the center of the finished slab. The concrete is pumped into place through flexible hoses. Sections of the shell may be precast in a shop or on the job site for future erection. FACTORY-PRODUCED MODULES In the future, building construction may be revolutionized by a new technology using preformed modular structures. Several systems have been developed that use factory-produced modules that can be assembled to form a complete high-rise building. In France more than 100,000 dwelling units are produced annually that use precast concrete walls and slabs in room-sized units containing all utilities. The box-shaped units are assembled on the ground and hoisted into place in a structural steel frame by a crane. In London many multistory flats are now produced whose units are machine-molded from materials reinforced with glass fiber and are hoisted into place in modular steel-framed structures. Scandinavia has produced several industrialized building systems that are used throughout the world under licensing arrangements. One example is the Skarne system developed in Sweden. This system is used to construct multistory housing units up to 25 stories high. All components are delivered to the site prefabricated; vinyl coverings are on the floors, ceilings are painted, the partitions snap into premolded floor and ceiling tracks, and cabinet and closet walls are prefinished. These components are precision manufactured with tolerances of a few millimeters. Although Europe has moved toward modular buildings, progress has been slow in the United States, partially because of the multiplicity of building codes and regulations. Prefabricated modular systems will be successful only when such regulations are modified to the point where mass production is feasible. \TDon\t A. \TWatson\t Bibliography: Cowan, J. J., and Smith, P. R., Dictionary of Architectural and Building Technology (1986); Olin, H. B., et al., Construction Principles, Materials and Methods (1983); Smith, R. C., and Honkala, T. L., Principles and Practices of Light Construction, 4th ed. (1986); Watson, Don A., Construction Materials and Methods (1978). See also: \Tarchitecture\t; \Tcement and concrete\t; \Telectrical wiring\t; \Tgeodesic dome\t; \Tskyscraper\t.