Beam and Girder Spacing for Steel Beam and Plate Girder Bridge

In regard to efficiency in the number of lines of girders in bridges consisting of multiple girders connected by cross frames, cursory cost comparisons almost always conclude that the widest spacing of girders is the most economical. Savings result not only from the reduced number of main members but also from the reduced number of secondary elements (shear connectors, cross frames, stiffeners, and bearings). However, other costs must be considered. Wide girder spacing will generally be accompanied by a wide slab overhang over the outside girders, for a balance of load on interior and exterior girders. This may necessitate extra reinforcing steel in the top of the deck slab beyond the amount required for the slab span between girders, and may require a thicker slab. Use of three lines rather than four or more puts the bridge closer to a nonredundant condi­tion. In some cases, a greater number of girders than the optimum for minimum material cost may be necessary or desirable to permit the bridge to be built in stages, or to have the deck replaced while maintaining traffic on a portion of the deck width. In general, beam and girder spacings of up to 10.0 or 12.5 ft (3 or 4 m) should be investigated for typical bridges. For economy, the size of interior and exterior stringers should be the same.