Assessing the effect of spectral shape on nonlinear response of reinforced concrete moment resisting frames

Abstract

The strength of an earthquake ground motion is usually quantified by an Intensity Measure (IM). This IM is used to predict the response of a structure. Traditionally scalar intensity measures were used for selecting ground motions and exerting them on structure but vector valued intensity measures consist of spectral acceleration and epsilon tend to show more reliable responses for nonlinear analysis. Epsilon as the index of spectral shape is an innovative approach that was developed by Baker and colleagues. This index (defined as the measure of the difference between the spectral acceleration of a record and the mean of a ground motion prediction equation at the given period) tries to refine ground motions in order to obtain comparable responses. The complete dependence of this index in the given period makes some ambiguities in response of the structure in nonlinear region. Therefore the focus of this research is on selecting ground motions in accordance with obtained target epsilon from disaggregation analysis in the first step. Since period elongation alters the period of structures, another disaggregation analysis was performed and an alternate set of ground motions based on new period and target epsilon was selected. Finally a nonlinear time history analysis was performed for new parameters. A set of reinforced concrete four story three bay moment resisting frames that were designed based on ACI 318-05 was used. Nonlinear characteristics of Ibarra model were considered in these frames in order to obtain the most realistic results. These ground motions are selected based on a spectral shape index and exerted on frames using OpenSees. It is shown that period elongation has considerable effect on spectral shape and consequently the response of the structure will alter.