Problems in Pushover Analysis of Bridges Sensitive to Torsion

Abstract

The study addresses the issue of pushover analysis of bridges sensitive to torsion, using as a case-study a straight, overpass bridge with two equal spans, whose fundamental mode is purely torsional. The deck is supported on a single pier consisting of two columns of cylindrical cross-section, monolithically connected to the deck, while it rests on its two abutments through elastomeric bearings. The seismic performance of the bridge was assessed in the longitudinal and transverse directions using non-linear static (pushover) and time-history analysis. An additional pushover analysis was carried out using the fundamental (torsional) mode loading. Parametric analyses were performed 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, one of the key points of this study is that, if one focuses on the fundamental torsional mode, the corresponding load pattern is anti-symmetric and the resulting base shear inevitably equal to zero, hence a ‘standard’ pushover curve cannot be drawn. Along these lines, an alternative pushover curve in terms of abutment shear vs. deck maximum displacement (that occurs at the abutment) was found to be a meaningful measure of the overall inelastic response of the bridge. Another interesting aspect is that assessment based on the torsional fundamental mode leads to failure of the elastomeric bearings, although the related displacements are not relevant since they are about an order of magnitude higher than the design displacement. Overall, it can be concluded that for bridges with a fundamental torsional mode, assessment of their seismic response relies on a number of justified important decisions that have to be made regarding the selection and reliable application of the analysis method adopted, the estimation of the foundation and abutment stiffnesses, and the appropriate numerical simulation of the pertinent failure mechanism of the elastomeric bearings used.