Probabilistic Collapse Behavior Evaluation of Low-Rise In-Plan Irregular Buildings

The present paper aims at evaluating the post-peak and collapse behavior of low-rise irregular buildings. Irregularity -in this study- is defined as the unidirectional mass irregularity in plan to produce torsional models. In previous earthquake events, most of torsional buildings have suffered from extensive damages and even total collapse. To investigate the performance and collapse behavior of the considered buildings from the probabilistic point of view, three-dimensional three and six-story reinforced concrete models with unidirectional mass eccentricities ranging from 0% to 30% (of the building overall plan dimension) were subjected to nonlinear static (pushover) as well as extensive nonlinear incremental dynamic analysis (IDA) under 21 two-component ground motion records. Currently, FEMA P-695 is the reference document to evaluate the collapse behavior of common structural systems in a completely probabilistic framework that contains a step-by-step procedure to examine the seismic design parameters including the response modification (R), the structural over-strength (W) and the structural ductility (m) factors. All models were built and analyzed using the OpenSees simulation platform. The SP version of the software, which is able to efficiently solve large systems of equations using the capacity of multi-processors was utilized in this study. For performing nonlinear analyses, the structural system was modeled using concentrated plasticity nonlinear modeling approach in which concentrated hinges are modeled and defined at the ends of each frame element. All degradation sources including the loading and reloading stiffness, peak-strength and hardening zone stiffness degradation effects in each cycle of response have been taken into account in the modeling process. The hysteretic model known as "peak-oriented hysteretic model" which is based on kinematic hardening rules were used for the modeling of the structures to assess their dynamic behavior. All models were created in the OpenSees platform by using CECARC-3D; a graphical pre- and post-processor for OpenSees designed by the authors for modeling and analyzing nonlinear static and dynamic response of 3D reinforced concrete structural systems. Geometric nonlinearities including the global P-∆ as well as the local p-delta effects were also considered in the model utilizing the co-rotational formulation. Performance of each model was then examined via the calculation of conventional seismic design parameters including the response modification (R), structural overstrength (W) and structural ductility (m) factors; the calculation of probability distribution of maximum inter-story drift responses in two orthogonal directions (and their combination); and also by the calculation of the collapse margin ratio (CMR) defined as the ratio of the median of all collapse-level spectral intensities (determined by the IDA results) to the MCE-level spectral intensity of the building at the fundamental period of vibration in the direction of interest. Basically, all performance checks in the procedure of FEMA-P695 is based on CMR. Results of this study demonstrate that substantial differences exist between the behavior of regular and irregular buildings in terms of the lateral load capacity and collapse margin ratio. Besides, results indicate that current seismic design parameters (including R, W and m) are non-conservative for buildings with high degrees of plan eccentricity, and such structures cannot satisfy the target “life safety” performance level based on the calculated safety margin against collapse. It appears that design codes need to address more precisely the torsional effects on seismic design parameters as well as on analysis and design procedures for irregular structures to provide the required safety margin against collapse under severe seismic loading conditions.