Influence of higher-order modes of slender tall pier bridge columns on the seismic performance of pile foundations

Abstract

Numerous highway bridges have been constructed with slender tall piers over 40 m in Southwest China, due to the mountainous topography. Current researches on this type of bridges mainly focus on the higher mode effects on the seismic behavior of pier columns, while the responses of group pile foundations are generally neglected. The objective of this paper is to investigate how the dynamic demand of group piles are influenced by the higher-order modes of slender tall pier columns. To better understand the influence of higher order modes, two analysis scenarios, representing (1) the equivalent single-degree-of-freedom (SDOF) system neglecting higher mode effects of piers, and (2) the multi-degree-of-freedom (MDOF) system of the original bridge, are developed and analyzed. Fiber elements are utilized considering the potential nonlinear behavior of pier columns and piles, while nonlinear p-y, t-z and q-z springs are employed simulating the properties of soil layers in these numerical models. Using displacement of pile cap as damage index, the effects of higher-order modes on pile foundations are investigated through both incremental dynamic analysis (IDA) and fragility analysis procedures. The results show that the seismic demands of pile foundations could be significantly underestimated when the contribution of higher-order modes of columns are neglected. The damage of piles is observed prior to that of the pier columns, indicating that damage state of pile foundations should be carefully considered during seismic design, and only focusing on the pier columns would lead to unconservative results. Furthermore, p-y springs are shown to perform nonlinearly under strong excitations, which implies that modeling the soil layers with linear springs is insufficient and should be better considered with these nonlinear springs.