Effect of vertical motions on seismic response of highway bridges

Results of a parametric study of the effects of vertical motions on the seismic response of typical highway bridges are presented. Six different bridges covering a variety of types and geometry are subjected to a suite of ground motions representing events with magnitudes of 6.5 and 7.5 at both rock and soil sites, and with fault distances of 1, 5, 10, 20, and 40 km. Response spectrum analyses are performed on all six bridges for all ground motion parameters. Response results excluding and including vertical motions are compared, with the effect of the vertical motion expressed as a ratio of the dead load actions. Three of the bridges are also analyzed using linear time history analysis to validate the response spectrum results. Analysis of one bridge is extended to incorporate nonlinear behavior of the piers. Recommendations are made for including the effects of vertical ground motions in a code format, and will be considered for adoption by AASHTO in May 2003.