Effect of Rayleigh waves on seismic response of bridge pylons via Incremental Dynamic Analyses

Seismic ground motions associated to basin-induced surface waves can be particularly damaging for long-period large-scale infrastructures such as bridge pylons. In the present work, we perform a parametric study within an Incremental Dynamic Analysis (IDA) framework with a twofold aim: a) to quantify the additional distress induced by Rayleigh waves on bridge pylons and identify potential correlations with the frequency content of these waves and b) to examine whether the acceptance of nonlinearity at foundation level can render the overall design more vulnerable to the incidence of surface waves. The effects of Rayleigh waves are studied by extracting the Rayleigh wave component from signals recorded in sedimentary basins and by performing structural analyses with and without this component. The effect of the rotational seismic component, often neglected in engineering practice, is also considered. Two types of foundation design are studied: a) "conventional" design (large foundation dimension) with the aim of developing limited nonlinearity at foundation level and b) "innovative" design (reduced foundation dimension) inviting for development of high foundation nonlinearity but offering an isolation effect for the superstructure. In this context, we develop a simplified model using a nonlinear soilfoundation macroelement and a multifiber model for the bridge pylon. We show that Rayleigh waves can increase up to approximately an order of magnitude the severity of relevant engineering demand parameters according to the studied configuration. Amplification factors due to Rayleigh waves are quantitatively similar for both types of foundation design (conventional - innovative) but they need to refer to the most relevant engineering demand parameters for each case.