Higher Mode Effects in the Pushover Analysis of RC Buildings Subjected to the Near-Fault Ground Motions

Accurate identification of seismic hazard and the estimation of associated seismic demands are the key components in the performance-based seismic design (PBSD). Although nonlinear time history (NTH) analysis is the most precise procedure to evaluate the seismic demands of buildings structures، it is relatively complex and time-consuming for routine applications. Nonlinear static procedures (NSPs) are now prevalently used in engineering practice instead of nonlinear time history analysis for the prediction of seismic demands in building structures. Current Instruction for Seismic Rehabilitation of Existing Buildings (No. 360) uses invariant load distributions. While those invariant load distributions (such as inverted triangle، uniform، or mass proportional) are based on the assumption that the response is primarily in its fundamental mode of vibration، it can lead to incorrect estimates for structures with significant higher mode contributions. The objective of this paper is evaluation of the assumptions، approximations and limitations of pushover analysis and the differences between static and dynamic methods to estimate seismic demands in building structures subjected to both far- and near-fault ground motions. Analytical results show that lateral load pattern used to estimate demands can have a significant influence on the computed demands. When compared to nonlinear time history estimates، pushover methods tend to underestimate demands at the upper levels signifying the relevance of high mode participation in mid to high rise structures.