The current AASHTO Specifications do not have adequate guidelines for seismic modeling and analysis of steel bridges. Bridge designers in seismic zones have always raised concerns about the adequacy of beam models in performing seismic analysis of steel bridges. This paper discusses the dynamic characteristics of five medium span steel bridges that were modeled using beam models and 3-D finite element models. These bridges consisted of a two span, one column bent with and without skew, a two span two column bents with integral and dropped bent caps, and a twin steel box girder bridge.

The beam models consisted of lumping the moment of inertia of the bridge superstructure into one beam element. The 3-D finite element model, on the other hand, used the 27 node solid element to model the deck, the 8 node shell element to model the plate girder, and the beam element to model the cross frames. The twin box girder bridge was modeled as a one beam element, two beam element, and a 3-D finite element model. The results of the dynamic characteristics showed the beam element models were able to capture the dynamic characteristics of the bridge that can be utilized in design purposes.

Also, this paper presents the results of dynamic analyses of steel bridges with inelasticity allowed to occur in the bridge superstructure. The exterior and interior cross frames in the superstructure were designed as ductile members and were allowed to buckle and yield during severe ground motion. The response of this bridge was compared to the response of a bridge designed according to AASHTO standards, where inelasticity is only limited to the substructure. The results of this investigation showed that the seismic response of a steel bridge with inelasticity occurring in the superstructure is as good as a bridge with inelasticity occurring the substructure.