The necessity for framed structures to be capable of dissipating significant amounts of energy inelastically under severe earthquake excitation is generally acknowledged. Bridge structures differ from buildings in their seismic lateral resistance mechanism in so far as they generally possess a small number of clearly identifiable potential zones in which plastic yield can occur and consequently tend to be amenable to postelastic studies. This paper presents the application of an analysis technique in which the response time history of a bridge structure, treated as a three-dimensional frame, is determined by direct integration of the equations of motion with allowances incorporated for inelastic member behavior. Aspects studied include the differences in responses predicted using a nonlinear three-dimensional model rather than a planar frame idealization and the effect of unequal span lengths or torsional vibrations.

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