Seismic response of long span cable-stayed bridge to near-fault vertical ground motions

Abstract

In design and analysis of seismic resistant structures and particularly the bridge structures vertical ground motion tends, in general, to be ignored or underestimated. However, during recent earthquakes high amplitude of motions were recorded in vertical direction near to the fault, invalidating such design assumptions of neglecting the vertical motion. This paper presents an analytical investigation on the effect of the near fault vertical ground motions on seismic response of a long span cable-stayed bridge. Responses of the bridge subjected to ground motions with and without vertical ground motion is carried using near fault ground motions on three dimensional bridge model. A suite of five near field ground motion with varying V/H (Vertical to Horizontal) ratio of peak ground acceleration is used. Influence of vertical motion on global and local structural response is presented. The study also takes into consideration the arrival time of peak vertical motion which has received little attention previously. Effects of coincidence of peak vertical motion with the peak horizontal motion in time domain are also analyzed. The study reveals that influence of vertical motion on the seismic response of the Karnali Bridge is slight and coinciding peak vertical motion with peak horizontal motion also have slighter effects compared to the motion without such coincidence.