type of construction, all live and dead loads are carried by the structural-frame skeleton. For this reason, the exterior walls are nonbearing curtain walls. Roof and floor loads are transmitted to beams and girders, which are, in turn, supported by columns. The horizontal members or beams that connect the exterior columns are called spandrel beams. If you add additional rows of columns and beams, there is no limitation to the area of floor and roof that can be supported using skeleton construction. However, one limitation of using skeleton construction is the distance between columns.

Oftentimes, large structures, such as aircraft hangars, may require greater distances between supports than can be spanned by the standard structural steel shapes. In this case, one of several methods of long-span steel construction is used. One method uses built-up girders to span the distances between supports. Two types of built-up girders are shown in figure 8-50. As shown in the figure, the built-up girder consists of steel plates and shapes that are combined together to meet the necessary strength. The individual parts of these girders are connected by welding or riveting.

Another method, usually more economical, is the use of a truss to span large distances. A TRUSS is the framework of a structural member consisting of a top chord, bottom chord, and diagonal web members that are usually placed in a triangular arrangement. Figure 8-51 shows many different types of trusses that can be fabricated to conform to the shape of nearly any roof system.

A third long-span method, although not as versatile as trusses, is the use of bar joists. BAR JOISTS are much lighter than trusses and are fabricated in several different types. One type is shown in figure 8-52. Prefabricated bar joists, designed to conform to specific load requirements, are obtainable from commercial companies. Other long-span construction methods

Figure 8-50. - Typical built-up girders.

Figure 8-51. - Typical steel trusses.