Assessment of AASHTO LRFD guidelines for analysis of regular bridges subjected to transverse earthquake ground motions

Abstract

This paper evaluates the two procedures given in AASHTO LRFD specifications for the transverse analysis of regular bridges subjected to transverse earthquake ground motions. The Single Mode Spectral Analysis (SMSA) method and the Uniform Load (UL) method are evaluated for different bridge geometries, rigidities, and ground motion levels and were also compared to the sine shape function approximation and the computer methods. Two-span, three-span and four span continuous units were investigated. The effect of reduced column stiffness was also evaluated. The results from this study showed that for some bridges in certain seismic zones, the differences in column elastic shears and deformations from the UL method can be about 10% lower than those from the SMSA method and the other methods. Abutment elastic forces from the UL method were about 25% to 300% higher than those from the SMSA and other methods. The UL method, permitted by AASHTO LRFD may not be a conservative analysis method for predicting pier forces and deflections for certain bridges. The method also significantly over estimates abutment forces for longer bridges thus making them overdesigned and uneconomical.