This project developed a rating methodology for existing bridges that incorporates some of the inelastic capacity present in most multigirder bridges. Composite and noncomposite multigirder bridges possess substantial load redistribution capacity because of the stiffness provided by the concrete deck, diaphragms, and cross-bracing. Current rating procedures differ little from design provisions for new bridges, and do not recognize the structure's ability to redistribute loads once local yielding has begun. In selecting the limit states for this new rating procedure, care was taken to consider both analytical and practical issues. Because of the cyclic nature of loads applied to bridges, an ultimate strength limit state is unconservative and should not be used to rate them. Rather shakedown, or that load causing a set of residual moments throughout the structure such that the bridge responds to subsequent loads of the same magnitude or smaller in an elastic fashion, is the recommended limit state to be used when cyclic loads are present. Although the concept of shakedown is not familiar to most rating engineers, it is easy to grasp from the conceptual point of view. In addition, the calculation of the shakedown loads follows directly from the elastic calculations made for the analysis of the bridge, and represents very little, if any, additional effort on the part of the rating engineer. Research conducted clearly indicates that the use of inelastic action in rating straight, composite, and noncomposite multigirder bridges is justified. The permanent deformations expected under the factored rating vehicles are very small, less than what is visually evident, and the members possess, in general, more than adequate ductility to allow for required rotations. Rating factors using inelastic action not only yield a more realistic assessment of the structure's capacity but also provide the means to determine economical and reliable strengthening and repair methods. (Author/publisher).
Abstract