12.5 EXAMPLE LOAD-FACTOR DESIGN OF COMPOSITE PLATE-GIRDER BRIDGE

The "Standard Specifications for Highway Bridges" of the American Association for State Highway and Transportation Officials (AASHTO) allow load-factor design as an alternative method to allowable-stress design for design of simple and continuous beam and girder structures of moderate length and it is widely used for highway bridges.

Load-factor design (LFD) is a method of proportioning structural members for multiples of the design loads. The moments, shears, and other forces are determined by assuming elastic behavior of the structure. To ensure serviceability and durability, consideration is given to control of permanent deformations under overloads, to fatigue characteristics under service loadings, and to control of live-load deflections under service loadings. To illustrate load-factor design, a simply supported, composite, plate-girder stringer of the two-lane high-way bridge in Art. 12.4 will be designed. The framing plan and the typical cross section are the same as for that bridge (Fig. 12.13). Structural steel is Grade 36, with yield strength f _y = 36 ksi and concrete for the deck slab is Class A, with 28-day strength f= _c = 4,000 psi. Loading is HS25.