Effects of Multiple Earthquakes on Reinforced Concrete Structures

Abstract

Multiple earthquakes have the potential to cause rapid strength degradation in reinforced concrete structures. Researchers mainly focused on seismic vulnerability of reinforced concrete buildings under the single earthquake, and consequently neglected the effects of multiple seismic sequences. Building may fail in aftershocks due to the stiffness loss which is resulted from accumulated damage due to main shocks. In this study, natural ground motion records from four earthquakes have been selected. Numerical model for a six storey and three bays Reinforced Concrete Frame from an existing commercial building in Islamabad, Pakistan was established in OpenSees. Mander’s Model (1984) for confined concrete was used to model Concrete of Columns and Beams. Menegotto and Pinto (1973) relationship was used to calculate Steel-Stress relationship during each excursion. Park and Ang Model was used to develop Damage Index during each Analysis. Damage Index was used to quantify the total damage in the structure at any time. Displacement based Nonlinear Static Analysis was conducted to study the weak structural elements in the Frame. Frame was subjected to 4.5% top roof drift during Static Analysis. Acceleration Time History for October 8, 2005 Kashmir Earthquake with PGA 0.236 was applied on the Frame to conduct Nonlinear Dynamic Analysis. Incremental Dynamic Analysis (IDA) Technique was used to estimate the collapse limit of the Frame during Nonlinear Dynamic Analysis. A comparison of results of Nonlinear Static Analysis and Nonlinear Dynamic Analysis shows that Nonlinear Static Analysis over-estimates the results. Time History Analysis was performed using back-to-back approach for multiple earthquakes. Residual Displacement was observed by comparing Displacement during Sequential Earthquake and Single Earthquake. Main Shocks and After Shocks of each Earthquake were scaled to reach the Collapse Limit of the Frame. Modification with diagonal beams was introduced in the mid-bay of the Frame. Modified Frame was also subjected to scaled Mani Shocks and After Shocks Sequences. Response Curves of Original Frame and Modified Frame for Scaled Main-After Shock Sequences were compared. It was concluded that Damage accumulation in Main Shock event has a significant effect on time period and stiffness of structure in subsequent seismic events.