Widening of existing bridges on pile groups exploiting nonlinear soil-structure interaction

The paper presents a comparative assessment of two different approaches in the retrofit design of bridge pile groups. The first approach is based on the conventional concept of elastic design which prevents plastic deformations of the soil-foundation system during the earthquake, guiding any potential plastic mechanism to the superstructure. The second approach allows non-linear soil-foundation behavior under seismic loading. In this case, the dissipation mechanisms mobilized through soil yielding act as a fuse for the superstructure. A case study of a bridge widening is presented to compare the two approaches. According to elastic design widely applied in the current codes, the existing foundation of the widened bridge needs to be retrofitted to support the new loading conditions. Initially, the study focuses on the nonretrofitted pile group, quantifying the actual need for retrofit on the basis of horizontal pushover tests. Subsequently, the two foundation alternatives (retrofitted and non-retrofitted) are compared under nonlinear dynamic time history analysis. It is shown that the performance of the non-retrofitted pile group is proven at least equally efficient, reducing the structural damage of the bridge, experiencing however increased residual settlements and rotation.