- Also called:
- building construction
- Related Topics:
- masonry
- carpentry
- scaffold
- rammed earth
- shoring
Long-span buildings create unobstructed, column-free spaces greater than 30 meters (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 meters (670 feet), the largest exhibition hall has a span of 216 meters (710 feet), and the largest commercial fixed-wing aircraft has a wingspread of 66.7 meters (222 feet) and a length of 69.4 meters (228 feet), requiring a 75–80-meter- (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 labor required for assembly are an important constraint along with other economic factors.
Timber 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 meters (150 feet). Its most economical forms, however, are the pure compression shapes of the multiple-arch vault, with spans up to 93 meters (305 feet), and ribbed domes, with spans up to 107 meters (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 meters (200 feet); however, they are not very efficient in their use of material. Trusses 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 meters (623 feet) in span. Two-way grids can be made of either plate girders or trusses to span square spaces up to 91 meters (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 made of rows of parabolic arches, usually in truss form for greater rigidity, have been used for spans of up to 98.5 meters (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 meters (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 meters (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 meters (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 meters (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 meters (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 meters (420 feet).
Factors in the built environment
Acoustics
Long-span auditoriums 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 odor generation from population densities of less than one square meter (11 square feet) per person. Air must be moved fairly rapidly through the population zone to maintain an acceptable air-change rate.
