SEISMIC FRAGILITY ANALYSIS OF BRIDGE ABUTMENTS

Abstract

Bridges have always been one of the most vulnerable structures during a seismic event. Design philosophy of bridges being completely different from that of buildings, these are faced with completely different challenges. One of the major bridge component susceptible to damage during a seismic event is abutment. Its performance is affected due to ignoring of some crucial factors, ultimately resulting in failure of the whole bridge, as has been seen in the past earthquakes. The main drawback in the present design of bridge abutment is the ignoring of the soil-structure-interaction between the abutment and the retained backfill. In the past, many researches have contributed to the development of a design philosophy of abutments by simulating the static and seismic earth pressure distribution. The principle aim of the present study is to extend this method to a coupled abutment-backfill system. In this dissertation, finite element (FE) models have been developed to study seismic response and vulnerability of abutments through development of fragility curves. To start with, a classic earth pressure problem of a vertical wall retaining a horizontal backfill is considered to understand the behavior of the system. A 2D finite element analysis is performed using ABAQUS with rigid wall retaining frictional soil. Backfill soil is modelled using plane-strain quadratic quadrilateral (CPE8R) elements using Mohr-Coulomb failure criteria. Pseudo-static methodology is used to account for the horizontal inertial component of the system during a seismic event. Further, the key parameters effecting the earth pressure coefficients, including backfill soil friction angle, ϕ, with different backfill-wall interface friction angle, δ, and horizontal seismic coefficient, αh, are explored in detail for both active, ka, and passive, kp, earth pressure coefficients. The study shows an increase in both active and passive seismic coefficients with increase in horizontal seismic coefficient applied in opposite direction of backfill and reduction if the horizontal acceleration is applied in the direction of backfill. Finally, seismic vulnerability of an abutment is investigated through extensive numerical simulation using incremental dynamic analysis (IDA) with 2D plane-strain finite element models of abutment-backfill system using ABAQUS subjected to a near field ground motion suite. This helps to incorporate the stochastic response of the abutment due to uncertainties in ground motions. As a final outcome of this study, iv fragility curves are developed as a function of Peak Ground Acceleration at rock horizon. Fragility curves for different backfill-foundation combinations and with different friction angles and isotropic parameters are compared.