Evaluation of Combination Rules for Orthogonal Seismic Demands in Nonlinear Time History Analysis of Bridges

摘要

One of the parameters that directly affects the seismic demand on a bridge is the excitation angle of the ground motion. Since analyzing a bridge with all possible excitation angles is impractical, combination rules have been used for computation of seismic demands. Provisions and specifications recommend the use of combination rules for computation of displacements and elastic forces, but they do not clearly suggest any combination rules for nonlinear time history analysis of bridges. This paper evaluates the 100/30, 100/40, and SRSS combination rules for nonlinear time history analysis of bridges. The probability of underestimation is computed for each combination rule, assuming a uniform distribution for the excitation angle. Two cases are considered for the combination rules: In the first case, only the major components of the earthquake records are used; the second case consists of using paired acceleration time histories. Results show that in the first case, the probability of underestimation is more than the second case, but using the second case produces conservative results on average, which are not economical to use in the design. Among the combination rules in the first case, 100/40 rule has the smallest probability of underestimation of demands. Therefore, this paper suggests the use of the 100/40 rule with the major component of earthquakes for the nonlinear time history analysis of bridges.