Evaluating Code Criteria for Regular Seismic Behavior of Continuous Concrete Box Girder Bridges with Unequal Height Piers

The seismic design and response prediction of irregular bridges supported on piers of unequal heights—a commonly adopted solution when crossing steep-sided valleys—represent a particularly challenging problem that is yet to be effectively addressed by seismic design code provisions worldwide. From a force-based design perspective, shorter piers are subjected to increased ductility demand, and consequently damage tends to localize in these relatively stiff piers at increasing seismic hazard levels. This paper presents an investigation of the seismic response of a few schemes of a three-span case-study continuous bridge (commonly encountered in practice), featuring two unequal piers with relative heights of 0.5, 0.64, 0.75, and 0.86, respectively. Static pushover and time history (under incrementally scaled-up actual records) nonlinear inelastic analyses are performed using OpenSees to check the validity of Eurocode 8 (EC8) and recently proposed AASHTO-LRFD provisions for regular seismic behavior of ductile bridges dimensioned per a force-based design procedure. It has been demonstrated that satisfying the EC8 regularity condition does not necessarily result in a near-simultaneous failure of unequal piers at the extreme hazard level, especially for a low ratio between pier heights ($\le 0.5$). It has been further concluded that regularity criteria in both provisions fall short of providing the absolute regular seismic behavior of the bridge under investigation. Finally, a new design criterion is introduced (and verified) herein and is promoted for bridges with unequal height piers but with the same cross-section dimensions as may be typically dictated by some compelling aesthetical and practical considerations. However, it is worth mentioning that pier behavior (and, hence, failure) is assumed to be governed by flexure, and no shear failure and buckling are considered.