Integrating the foundation component into seismic fragility analysis of bridges

A component-based seismic fragility analysis method for bridges recently developed by the authors is extended here to include the foundation component. In line with the treatment of other components like the piers or abutments, the foundation system is analysed separately, and its capacity and limit state thresholds are described in terms of the displacement of a monitoring point, which in the usual case of pile foundations is the pile cap; in this way all sources of strength and stiffness degradation are accounted for. The influence of the way the pile cap is loaded in the foundation component analysis is estimated by exploring various 'scenarios' of increasing complexity. Components of two typical overpass bridges are analysed; one with single column piers and one with wall-type piers, to evaluate the effect of foundation on bridge fragility in the case that energy dissipation occurs primarily in the plastic hinges of the piers and the case that part of the dissipation takes place in the foundation system. It is concluded that in the case of hinging piers the pile foundation is not critical for the bridge fragility and may be ignored for simplicity, whereas this is no more the case when piers remain elastic and damage in the piles and/or the soil occurs.