STRENGTHENING OF STEEL STRINGER BRIDGES BY SELECTIVE STIFFENING

When indeterminate structures such as steel stringer bridges are strengthened by the addition of material, they are also stiffened at the locations where material is added. Under elastic conditions, this stiffening has the effect of attracting stress away from other parts of the structure. For steel stringer bridges in need of strengthening, it may be possible to add material selectively in order to strengthen and stiffen some critical locations and at the same time reduce the stresses at other critical locations. Reasonable amounts of stiffening can be achieved by addition of diaphragms, by addition of composite action in noncomposite bridges, and by addition of coverplates or angles to stringers. This stiffening, which can affect live-load moments by as much as 20 percent, can be used to increase the rating of bridges. Strengthening of a stringer bridge by selective longitudinal stiffening under allowable stress design generally increases the flexural strength of the bridge. Because selective stiffening may have positive or negative effects on fatigue-critical details, the effects must be checked.