Influence of axial loads and higher order modes on the seismic response of slender bridge piers

A significant number of bridges is built on tall and slender piers in zones exposed to seismic hazard. The seismic behaviour of slender piers can be strongly influenced by higher modes contribution, related to the distributed pier mass, and also by second order effects, related to the interaction between axial loads and transverse seismic displacements. Although these latter effects may induce an increase or decrease of seismic demand, in current design practice they are usually taken into account by introducing amplification factors for the pier base moments evaluated by first-order analysis. This paper presents an analytical approach that permits to investigate efficiently and accurately the influence of both axial loads and higher order modes on the seismic behaviour of slender bridge piers. The proposed approach is based on a continuous pier model and on an analytical solution of the eigenvalue and seismic problem. The proposed analysis technique is applied to three case studies with different pier properties. The results show that the effects of axial loads and higher order modes can play different roles on the seismic response of slender bridge piers.