Seismic Response Analysis of Bongo Bridge Subjected to Multiple Support Excitation due to Spatial Variation of Ground Motion

Abstract

The spatial variation of ground motion (SVGM) may cause detrimental effects on extended structures such as bridges. The SVGM is mainly caused by three phenomena: the incoherence effect, wave passage effect, and the local site effect, which leads the structure to undergo multiple support excitation. In most cases, in the Philippines, the conventional response spectrum analysis per DPWH Bridge Seismic Design Specifications (BSDS) 2013 is used to perform seismic analysis of bridges. However, the method assumes that the ground motion is spatially uniform, thus ignoring the potential effect of SVGM. The study focuses on the seismic response analysis of a conventional existing highway bridge in the Philippines subjected to multiple support excitation accounting for the effect of SVGM. The researcher used the Bongo Bridge located in Ilocos Norte, originally designed using the conventional response spectrum. A method for generating a modified response spectrum that accounts for the effect of SVGM based on the fundamental principle of random vibration theory was also presented to provide guidelines for bridge engineers in the Philippines. Three cases were performed where each seismic response of the existing bridge was obtained and evaluated to determine if the application of multiple support excitation due to SVGM will cause a significant effect on the existing bridge. Based on the results, the effect of multiple support excitation to Bongo Bridge amplified the seismic response of some pier columns and de-amplified some. The study also shows that the effect of multiple support excitation decreased the displacement demand significantly on the Bongo Bridge. Since the study is limited to Bongo Bridge and the seismic response can be affected by many factors, the result may vary to other bridges. Therefore, it is still recommended to perform a parametric analysis accounting for multiple support excitation especially if the structure is situated on a varying ground type or bridges with multiple and longer spans, to determine if this phenomenon will cause a significant effect on the bridge.