Seismic Analysis of Long Bridges Considering Spatial Variation of Ground Motion

Abstract

Understanding the seismic behavior of long bridges and developing effective measures to mitigate their seismic vulnerability is crucial. One of the key factors that influence the seismic response of long bridges is the spatial variation of ground motion (SVGM). This refers to the variation of ground motion across a bridge site due the geological conditions and the distance from the earthquake source. As the piers of the bridge are located a large distance apart, the variability of ground motion due to local site effects, wave incoherence, wave passage effect, etc., needs to be considered in the analysis. The first part of this dissertation focuses on development of an algorithm to generate asynchronous ground motions conditioned on the recorded ground motion considering the non stationarity of the seismic waves. The developed ground motions are used for further study on seismic analysis of two types of long bridges: extradosed cable-stayed bridge (CSB) and continuous multi-span integral bridge. It also includes the development of numerical model for cable-stayed and continuous bridges. The important design issues in the CSB such as cable modelling and tuning, provision of pre-camber, construction stage analysis, consideration of time dependent effects, etc., are highlighted in the dissertation. The findings reveal that SVGM significantly affects the seismic demand on the bridges compared to a uniform excitation. In the integral bridge, extreme integral piers experience larger seismic demands due to larger relative displacements. Varying soil conditions at the adjacent supports further amplify the seismic forces in the bridge piers. A comparative study on the seismic response of the considered bridges using pseudo-static analysis mentioned in Eurocode 8, and the conditional simulation method, discussed in this dissertation, is also presented. The pseudo-static analysis estimates conservative seismic forces for some of the piers, whereas it underestimates the forces in other piers, as compared to the time-history analysis considering SVGM. Further, in case of CSB, the pseudo-static method yields non conservative forces in all the piers. The study also highlights the influence of SVGM on cable forces in CSBs. To conclude, considering the spatial variation of ground motion is necessary in estimation of seismic demand in the long bridges.