Contents lists available at ScienceDirect Soil Dynamics and Earthquake Engineering journal homepage: www.elsevier.com/locate/soildyn Seismic behavior of irregular reinforced-concrete structures under multiple earthquake excitations Resat Oyguc a, ⁎ , Cagatay Toros a , Adel E. Abdelnaby b a Istanbul Technical University, Institute of Earthquake Engineering and Disaster Management, Maslak 34469, Istanbul, Turkey b Department of Civil Engineering, The University of Memphis, 106C Engineering Science Building, 3815 Central Avenue, Memphis, TN 38152, United States ARTICLE INFO Keywords: Multiple earthquake Irregular RC structures Degrading material models Nonlinear time history analysis N2 method Extended N2 method Residual displacements Aftershocks ABSTRACT Reconnaissance studies on the recent Tohoku earthquake have reported collapse of structures due to multiple earthquake excitations in the earthquake-affected region. Strength and stiffness degradation is shown to be the primary reason for the observed damage. The present study aims to investigate the degrading behavior of ir- regularly built reinforced concrete structures subjected to the Tohoku ground motion sequences. Three-di- mensional numerical models of three irregular reinforced concrete structures are developed. The structural characteristics of these buildings are then altered to achieve a regular case. The models contain appropriate damage features that can capture both the irregularity and material deterioration effects. The capacities of both cases are evaluated using the N2 and extended N2 procedures. The degrading models are then used for ground motion sequences measured at 23 selected stations. The results indicate that multiple earthquake effects are significant, and irregularity effects increase the dispersed damage under these excitation sequences. 1. Introduction Reinforced concrete (RC) structures during the recent Tohoku and Christchurch earthquakes experienced excessive loss of stiffness and strength due to repeated shaking. Many RC buildings that were not heavily damaged immediately after the main excitations have collapsed because of aftershocks. Correspondingly, many previous in situ ex- aminations have reported the unfavorable effects of multiple ground excitations on structural systems. In literature, to determine the response of structures by modeling their structural behavior, single-degree-of-freedom (SDOF) systems were extensively used because of their simplicity. Degrading systems were first introduced by Aschheim and Black [1], who used a modified Takeda hysteretic model. Their model was able to capture both the pinching and strength degradation effects. Base on their conclusions, the displacement response of an initially damaged SDOF system was approximately the same as that of its undamaged counterpart after the peak displacement was reached. Amadio et al. [2] investigated the nonlinear behavior of SDOF structures under multiple excitations using three different hysteretic models: non-degrading stiffness and strength, degrading stiffness and non-degrading strength, degrading stiffness and strength. They concluded that elastoplastic systems can be classified as the most vulnerable SDOF systems. Hatzigeorgiou and Beskos [3] conducted an extensive parametric study to obtain an appropriate inelastic displacement ratio while examining the period of vibration, viscous damping ratio, strain-hardening ratio, force reduction factor and soil class. They revealed that the repeated earthquakes have sig- nificant effect on both the inelastic displacement ratios and maximum inelastic displacement values of SDOF systems. In order to consider degrading behavior of moment resisting frame systems in structural analyses, component-level-based degrading models (multi-degree-of-freedom systems) have been developed and widely used in the literature. These models utilize nonlinear moment- rotation relationships at locations of possible plastic hinges (beam and column ends) that consider both stiffness and strength degradation. The idealization of assuming concentrated inelasticity at predefined plastic hinge locations lacks the consideration of localized failure modes and therefore can lead to inaccurate assessment of degrading response under earthquake sequences. Hatzigeorgiou and Liolios [4] investigated the effectiveness of component-level-based models under multiple ex- citations, assuming bilinear moment–rotation relationships at beam–- column connections. Moreover, beam and column elements are as- sumed to behave elastically. These developed models can also consider second-order effects; however, they exclude material deterioration ef- fects. The mentioned studies highlighted the fact that residual dis- placements play a major role on stiffness degradation. To the best of the authors’ knowledge, Abdelnaby and Elnashai [5] are the only researchers who have studied the effects of multiple http://dx.doi.org/10.1016/j.soildyn.2017.10.002 Received 19 February 2017; Received in revised form 15 June 2017; Accepted 1 October 2017 ⁎ Corresponding author. E-mail address: oyguc@itu.edu.tr (R. Oyguc). Soil Dynamics and Earthquake Engineering 104 (2018) 15–32 0267-7261/ © 2017 Elsevier Ltd. All rights reserved. MARK