An analytical technique for the seismic response assessment of slender bridge piers

This work proposes an analytical technique for the analysis of the effects of axial loads on the dynamic behaviour and seismic response of tall and slender bridge piers. The pier is modeled as a linear elastic Euler-Bernoulli cantilever beam with uniformly distributed mass, representing the pier mass, and a tip mass at the free end, representing the deck mass. The beam is subjected to the reaction due to deck loads, and to the pier self weight. The seismic problem is described by a linear partial differential equation of seismic motion with variable coefficients. This equation is decoupled by using the exact vibration modes which are obtained by applying the Frobenius method. By this way, the seismic response can be evaluated in terms of superposition of modal contributions. The proposed formulation is applied to the seismic analysis of a realistic slender bridge pier. The results of the study permit to shed light on the effect of axial load on the various vibration modes that contribute to the seismic response and to evaluate the suitability of amplification factors commonly used by seismic codes to account for axial load effects.