Evaluation of response modification factor, strength and stiffness deterioration effects by endurance time method

Abstract

Most recent seismic codes include response modification factor in the definition of equivalent lateral forces that are used for the design of earthquake resistant buildings. The response modification factor, used to reduce the linear elastic design spectrum to account for energy dissipation capacity of structure, is the central feature of force-based seismic design. The main goal of this paper is the assessment of Endurance Time (ET) method results accuracy in the evaluation of response modification factors and estimation of strength and stiffness deterioration effects on them according to frame performance evaluation. To reach this goal a sample set of steel moment-resisting frames (3, 7 and 12-story) are designed and ET results are compared to Incremental Dynamic Analysis (IDA) ones as an evaluation base. In order to model deteriorating connections, frames are also modeled using concentrated plastic springs. ET analysis is a new dynamic pushover procedure in which structures are subjected to gradually intensifying acceleration functions and their performances are assessed based on their responses. It is observed that although the results of ET analysis are not exactly consistent with the results of IDA, the ET analysis can reasonably assess the response modification factors and strength and stiffness effects in most cases. Also, considering the great amount of computational efforts that should be done for evaluating response modification factors and the effects of strength and stiffness on them by IDA analysis, ET analysis can evaluate them and the performances of structures with less analysis.