Structural Seismic Demands Considering the Uncertainty Associated with the Direction of Earthquake Excitation

摘要

The earthquake may attack the structural building from any angle, but in current seismic design codes, this type of uncertainty is seldom accounted for. Current design practice calculates the maximum structural responses subjected to earthquake excitations from two perpendicular directions which are parallel to the two axes of the building plan, and then the maximum structural response is obtained by some sort of superposition rules. However, the maximum structural response may be produced by the earthquake excitation from the direction unparallel to any axis of the building plan; as a result, the actual structural response may be underestimated by current seismic design codes. The uncertainty associated with the direction of earthquake excitation was considered in this paper, and its effect on structural responses was investigated. For this purpose, a simple 3-dimensional model with symmetric plan was established, which had fundamental periods that ranged from 0.1 s to 5.0 s, and was subjected to a set of 30 ground motion pairs for which both linear and nonlinear time history analyses were performed. If the structural behavior is elastic, the Real Acceleration Response (RAR), accounting for the variation of imposing directions of earthquakes, was compared with the Bilateral Acceleration Response (BAR) which is specified by current codes using the combination rule of SRSS, to evaluate the effect of directional uncertainty. If the structural behavior is inelastic, the Real Displacement Response (RDR) was compared with the Bilateral Displacement Response (BDR). Analyzing results showed that, on average, RAR is 0.68 times smaller than BAR; RDR is 0.86 times smaller than BDR. Recognizing the conservativeness of current codes, a modification factor for the seismic demand was proposed thorough a statistical analysis, which guarantees a probability of 95% design safety.