Combination rule for the prediction of the seismic demand on columns of regular bridges under bidirectional earthquake components

A percentage rule for the prediction of interacting seismic responses of bridge columns under multi-directional earthquake components is derived for eastern and western Canada seismic regions. A total of 14 regular bridges with differ- ent geometrical properties were examined for the two sites. The new rule was developed by comparing the column longitu- dinal steel ratio required from response spectrum analysis with combination rules to the ratio determined from the results of linear dynamic time history analysis of the bridge structures under bidirectional ground motions. The required steel ratios were found to vary with the weighted percentages used in the combination rules, the ground motion characteristics, and bridge properties. It was found that a 100%-20% combination rule is applicable for most bridges studied in eastern Canada. A 100%-40% rule is more appropriate for bridges located in western Canada. The results show that these rules need not be applied for regular straight bridges without skew but further investigation is needed before such a relaxation can be applied. Nonlinear dynamic time histories analysis of a two-span skewed bridge designed with the proposed percentage rule for east- ern and western Canada showed that the columns would not experience excessive damage.

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