Seismic assessment of an overpass bridge with a torsional fundamental mode

Abstract eng:
The study focuses on the seismic response of a straight, overpass bridge with two equal spans, whose fundamental mode, unlike most bridges of this type, is purely torsional. The particular structure is supported on a two-column pier of cylindrical cross-section that is monolithically connected to the deck, while it rests on its two abutments through elastomeric bearings. The seismic assessment of the bridge was performed in the longitudinal and transverse directions using non-linear static analysis. An additional pushover analysis was carried out using the fundamental (torsional) mode loading. Parametric analyses were carried out involving consideration of foundation compliance, and various scenarios of accidental eccentricity that would trigger the torsional mode. The pushover curves derived in the longitudinal and transverse directions highlight the satisfactory performance of the bridge, even for motions twice as strong as the design earthquake. On the other hand, if one focuses on the fundamental torsional mode, the corresponding load pattern is antisymmetric and the resulting base shear zero, hence a ‘standard’ pushover curve cannot be drawn; an alternative pushover curve in terms of abutment shear vs. deck maximum displacement (that occurs at the abutment) is found to be a meaningful measure of the overall inelastic response of the bridge. Furthermore, assessment based on the torsional fundamental mode leads to failure of the elastomeric bearings, but the related displacements are irrelevant since they are about an order of magnitude higher than the design displacement. Clearly, proper assessment of the bridge should be based on the longitudinal and transverse loading distributions, wherein the critical pier section failure precedes the bearing failure.

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