Performance-Based Seismic Assessment of RC SMRF Compliant and Noncompliant Structures

Abstract

The Pacific Earthquake Engineering Research Center’s (PEER) probabilistic performance based earthquake engineering (PBEE) assessment and design framework FEMA P-58 2012 has been employed in the current research, with the application to reinforced concrete (RC) special moment resisting frames (SMRFs), to assess the seismic performance of code compliant and non-code compliant RC frames (having non-seismic provisions and construction defects/deficiencies), taking into consideration uncertainties in the definition of seismic hazard and structural response. The present research included uni-directional shake-table testing of 1:3 reduced scale RC frame models designed to Building Code of Pakistan – Seismic Provisions (BCP-SP 2007), which were detailed as per the ACI 318 (2005). The frames were built complying with the code requirements and having construction defects: low-strength concrete, lacking ties in joint panels, beam-column stirrups being provided at larger spacing, practicing reduced longitudinal reinforcements, along with non-seismic hooks. Five 1:3 reduced scale, two-story representative models were subjected to a linearly scaled accelerogram of the 1994 Northridge Earthquake for multiple excitations test. The observed damage mechanism and lateral force-deformation behavior of tested frames models were obtained, which were analyzed to calculate the frames seismic response parameters. The response modification factor (R-Factor) calculated for the code complaint frame is approximately 7.50, which is reduced by 40 to 60% for models with construction defects. One of the major findings reveals the minimum ACI 318 (2005) requirement for exterior column depth of 15 times the diameter of longitudinal steel bars is inadequate to avoid joint panel damage, specifically for structures built with lowstrength concrete and/or joint panels lacking confining ties under design-level earthquake. For numerical studies, a simple finite element based numerical model was prepared in SeismoStruct (2016) for nonlinear inelastic seismic analysis, which was calibrated with the experimental observed behavior (mechanism) and global displacement response. The modeling technique was found to capture the frame response (peak displacement and peak base shear) reasonably well. The calibrated numerical models were extended for the incremental dynamic analysis using a suite of ten spectrum compatible acceleration time histories, to derive the frames’ seismic response curve (roof drift demand versus peak input acceleration). Response ii modification factor R-Factors was calculated using both the experimental response data and numerical investigation (i.e. incremental dynamic analysis), which is critically compared with the BCP-SP (2007)/ACI 318 (2005) building code specified R factor for RC SMRF. Furthermore, the seismic performance of the considered code complaint and non-code compliant prototype structures were assessed using the PEER’s/FEMA P-58 2012 second generation PBEE methodology. This included the definition of seismic hazard, which was obtained from other existing studies. The development of frames’ fragility functions derived herein using a fully probabilistic Nonlinear Dynamic Reliability Based Method (NDRM). The considered frames performance assessment was evaluated for seismic hazard of various return periods (42, 75, 250, 475 and 2475 years) in order to compare the seismic performance of non-complaint structures relative to the code compliant structure. This research study will help provide guidance on the revision and updating of the Building Code of Pakistan – Seismic Provisions BCP-SP (2007) that can in turn help improve seismic design practices in Pakistan and contribution towards seismic risk mitigation.