Current seismic design criteria for highway bridges generally require that the effects of earthquake loading be evaluated using either an equivalent static load approach for simple bridges or a dynamic analysis for more complex bridges. These provisions usually provide detailed explanations and commentaries on techniques which are judged to be suitable for static and dynamic modeling of the bridge superstructure and supporting columns or piers. There is, however, a significant lack of guidance on exactly how the boundary conditions and soil-structure-interaction should be incorporated into the model.
The purpose of this study is to present a simple analytical model of pile and pile group foundations for use as boundary conditions in a numerical model for seismic analysis of highway bridges. Both the axial and lateral response are considered. This simple model consists of a set of springs, dashpots, and masses for each degree-of-freedom on a pile, and it is based upon the Winkler hypothesis. The spring behavior is established by using the finite element method for static load conditions. The lumped dashpot constants and masses are based on realistic approximations. The effect of a sliding interface, nonlinearity of the soil and geometric, hysteresis, and viscous damping of the soil have been considered.
The p-y curves for lateral and axial vibration of single piles of 0.457 m (18") and 0.610 m (24") diameter based on plane analysis for different depths have been presented. Similar curves for direct-lateral, shear-lateral, and axial vibrations have also been presented for two-pile groups with three different spacings.
Using these p-y curves, pile responses have been obtained which have been compared with those obtained from a rigorous analysis. Good agreement has been observed for a single pile response. The comparison justifies the use of this simple model.