Figure 1-39.—Typical built-up girders.which are, in turn, supported by columns. The horizontalmembers or beams that connect the exterior columns arecalled spandrel beams. If you add additional rows ofcolumns and beams, there is no limitation to the area offloor and roof that can be supported using skeletonconstruction. One limitation on using skeletonconstruction, however, is the distance between columns.Oftentimes, large structures, such as aircrafthangars, may require greater distances betweensupports than can be spanned by the standard structuralsteel shapes. In this case, one of several methods oflong-span steel construction is used. One method usesbuilt-up girders to span the distances between supports.Two types of built-up girders are shown in figure 1-39.As seen in this figure, the built-up girder consists of steelplates and shapes that are combined together to meet thenecessary strength. The individual parts of these girdersare connected by welding or riveting.Another method, which is usually moreeconomical, is to use a truss to span large distances. Asyou learned in the EA3 TRAMAN, a truss is a frame-work of structural members consisting of a top chord,bottom chord, and diagonal web members that areusually placed in a triangular arrangement. (See figs.1-40 and 1-41.) As shown in figure 1-40, trusses can befabricated to conform to the shape of nearly any roofsystem.A third long-span method, although not as versatileas trusses, is the use of bar joists. Bar joists are muchlighter than trusses and are fabricated in several differenttypes. One type is shown in figure 1-42. Prefabricatedbar joists, designed to conform to specific loadrequirements, are obtainable from commercialcompanies. Other long-span construction methodsinvolve several different types of framing systems,which include steel arches, cable-hung frames, andother types of systems. These methods are beyond thescope of this TRAMAN.Figure 1-40.—Typical steel trusses.1-20
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