12.2 EXAMPLE-ALLOWABLE-STRESS DESIGN OF COMPOSITE, ROLLED-BEAM STRINGER BRIDGE

To illustrate the design procedure, a two-lane highway bridge with simply supported, composite, rolled-beam stringers will be designed. As indicated in the framing plan in Fig. 12.1a, the stringers span 74 ft center to center (c to c) of bearings. The typical cross section in Fig. 12.1b shows a 26-ft-wide roadway flanked by 1-ft 9-in parapets. Structural steel to be used is Grade 36. Loading is HS25. Appropriate design criteria given in Sec. 11 will be used for this structure. Concrete to be used for the deck is Class A, with 28-day compressive strength f ? _c = 4,000 psi and allowable compressive strength f _c = 1,400 psi. Modulus of elasticity E _c = 33 w ^1.5 ? f ? _c = 33(145) ^1.5 ?4,000 = 3,644,000 psi, say 3,600,000 psi.

Assume that the deck will be supported on four rolled-beam stringers, spaced 8 ft c to c, as shown in Fig. 12.1.

Concrete Slab. The slab is designed to span transversely between stringers, as in noncomposite design. The effective span S is the distance between flange edges plus half the flange width, ft. In this case, if the flange width is assumed as 1 ft, S = 8 ?1 + = 7.5 ft. For computation of dead load, assume a 9-in-thick slab, weight 112 lb/ft ² plus 5 lb/ft ² for the additional thickness of deck concrete in...