For long spans, use of fewer but deeper girders to span the long distance between supports becomes more efficient. With appropriately spaced stringers between the main girders of highway bridges, depth of concrete roadway slab can be kept to the minimum permitted, thus avoiding increase in dead load from the deck. Spans of the longitudinal stringers are kept short by supporting them on transverse floor-beams spanning between the girders. If spacing of the floorbeams is 25 ft or less, additional diaphragms or cross frames between the girders are not required.
This type of construction can be used with deck or through girders. Through girders carry the roadway between them. Their use generally is limited to locations where vertical clearances below the bridge are critical. Deck girders carry the roadway on the top flange. They generally are preferred for highway bridges where vertical clearances are not severely restricted, because the girders, being below the deck, do not obstruct the view from the deck.
Structurally, deck girders have the advantage that the concrete deck is available for bracing the top flange of the girders and for composite action. Bracing of the bottom flange is accomplished with horizontal lateral bracing.
The design procedure for through plate girders with floor beams is described in Art. 12.10.
Article 12.9 presents an example to indicate the design procedure for a deck girder bridge with floorbeams and stringers. In general, design of the stringers is much like that for a stringer bridge (Art. 12.2), and design of the girders is much like that for the girders of a multigirder bridge (Art. 12.4). In the following example, however, the stringers and girders are not designed for composite action. See also Art. 12.3. Therefore, the composite section for G1 is satisfactory. Use for G1 in the region of maximum moment the section shown in Fig. 12.27.
The procedure is the same for design of other sections and for the other stringers. For design of other elements, see Arts. 12.2 and 12.4. Fatigue design is similar to that for straight girders.
