Seismic response of a bridge–soil–foundation system under the combined effect of vertical and horizontal ground motions Zhenghua Wang* ,† , Leonardo Dueñas-Osorio and Jamie E. Padgett Department of Civil and Environmental Engineering, Rice University, Houston, TX, USA SUMMARY Different levels of model sophistication have recently emerged to support seismic risk assessment of bridges, but mostly at the expense of neglecting the inﬂuence of vertical ground motions (VGMs). In this paper, the inﬂuence of VGMs on bridge seismic response is presented and the results are compared with the case of horizontal-only excitations. An advanced ﬁnite element model that accounts for VGMs is ﬁrst developed. Then, to investigate the effect of soil–structure interaction (SSI) including liquefaction potential, the same bridge with soil-foundation and ﬁxed boundary conditions is also analyzed. Results show that the inclusion of the VGMs has a signiﬁcant inﬂuence on the seismic response, especially for the axial force in columns, normal force of bearings, and the vertical deck bending moments. However, VGMs do not have as much inﬂuence on the seismic demand of the pile cap displacements or pile maximum axial forces. Also, the signiﬁcant ﬂuctuation of the column axial force can reduce its shear and ﬂexural capacity, and a heightened reversal of ﬂexural effects may induce damage in the deck. In addition, relative to the ﬁxed base case, SSI effects tend to reduce response quantities for certain ground motions while increasing demands for others. This phenomenon is explained as a function of the frequency content of the ground motions, the shift in natural vertical periods, and the VGM spectral accelerations at higher modes. Moreover, the mechanisms of liquefaction are isolated relative to SSI effects in nonliqueﬁable soils, revealing the inﬂuence of liquefaction on bridge response under VGMs. Copyright © 2012 John Wiley & Sons, Ltd. Received 11 September 2011; Revised 21 May 2012; Accepted 25 May 2012 KEY WORDS: bridge–soil–foundation system; soil–structure interaction; vertical ground motions; liquefaction; axial force; shear demand; bending moment; higher modes 1. INTRODUCTION In practice, the effects of vertical ground motions are typically represented by a response spectrum with two-thirds of the horizontal response spectrum. This procedure was originally proposed by Newmark et al. [1]. However, measurements of ground motions during past earthquakes such as the 1989 Loma Prieta, 1994 Northridge, 1999 Chi-Chi, and 2011 Japan Tōhoku earthquakes, indicate that the vertical acceleration can reach values comparable to horizontal accelerations or may even exceed these accelerations [2, 3]. Several studies [2–5] have clearly demonstrated that the ratio of peak vertical ground acceleration (PGAV) to peak horizontal ground acceleration (PGAH) has been underestimated in near-fault regions and the spectral acceleration ratio of vertical ground motions to horizontal ground motions has also been underestimated at short periods. Furthermore, the frequency content of the vertical ground motion is quite different from the horizontal ground motion. When considering soil– structure interaction (SSI) with liquefaction, the inﬂuence of vertical ground motions (VGMs) on structural responses is even more complex. Hence, few studies have investigated the effect of SSI with *Correspondence to: Zhenghua Wang, Department of Civil and Environmental Engineering, Rice University, Houston, TX, USA. † E-mail: zw8@rice.edu Copyright © 2012 John Wiley & Sons, Ltd. EARTHQUAKE ENGINEERING & STRUCTURAL DYNAMICS Earthquake Engng Struct. Dyn. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/eqe.2226