A Simplified Spectral Method for the Evaluation of Seismic Forces in Highway Bridges With Massive Piers

Abstract eng:
Seismic isolation, using nonlinear isolators, is often used to lengthen the fundamental period of vibration and to reduce inertia forces of highway bridges. Nevertheless, engineers often use elastic equivalent seismic analysis methods to determine the displacements and internal forces. The simplified single-mode spectral analysis (SMSA) is a quick and simple analysis to be conducted for preliminary design or evaluation of seismically isolated bridges. However, SMSA neglects the effects of inertia in the substructure elements located under the isolators, which can lead to an underestimation of piers internal forces, particularly if their masses are important. In this paper, a new simplified approach is proposed for the seismic design and evaluation of isolated highway bridges with massive piers. A decoupled SMSA is proposed wherein massive piers are first analyzed as free standing cantilevers using their fundamental modes of vibration without the presence of the superstructure. The isolated superstructure is analysed separately as a rigid body supported by the isolators. The design seismic forces are computed from a Square Root of the Sum of the Square (SRSS) of the contributions from the piers and the superstructure. To validate the proposed simplified method, 11 highway bridges of different geometries, stiffnesses, and deck to pier mass ratios are modeled and analyzed in the longitudinal direction. The results of these analyses indicate that pier inertia forces must be taken into account when the mass of the piers reaches 10% to 15% of the mass of the superstructure. The proposed method is simple to implement and is shown to be accurate as compared to multimodal spectral analyses.

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