Seismic performance evaluation of an axially loaded reinforced concrete footing using pseudo-dynamic experimentation

Abstract

Southern Africa is characterised as a region of moderate seismicity with several instances of both natural seismicity and mining-related seismicity occurring within the last century. Evaluating the performance of a structure due to increasing seismic intensity is traditionally calculated post-earthquake using statistical means. However, the limited network of accelerometers in South Africa has prevented a detailed statistical analysis to be undertaken to determine the resultant structural damage to South African designed structures with increasing earthquake intensity. Therefore, this research investigates a method to relate damage to a structure with earthquake intensity by performing numerical analysis in combination with physical experimentation. The pseudo-dynamic experimentation technique was utilised to evaluate the damage occurring in a reinforced concrete footing due to the overall response of a linear elastic two-storey, twobay moment resisting steel frame structure that is subjected to earthquake excitation. The implicit Newmark’s method with static condensation was utilized in the present study to solve the governing equation of motion of the multi-degree of freedom system. Five pseudo-dynamic experiments were performed by scaling the El Centro ground motion record, which occurred in California on May 18, 1940, to produce peak ground accelerations that ranged between 0.34 g and 2 g. To supplement the pseudo-dynamic tests, two cyclic load tests were also undertaken. All the laboratory experiments were undertaken under a constant axial load for the duration of the applied earthquake excitation and utilised Rayleigh damping to model the energy loss with the overall linear elastic frame structure. Utilising the results produced during the experiments, an analytical hysteretic model and a damage index was formulated for the analysed reinforced concrete footing with the aim of interpolating damage at peak ground accelerations and overall structural fundamental period of vibration that were not evaluated during the laboratory test. The Park and Ang damage index was used in combination with the results to formulate damage curves and fragility curves for the reinforced concrete footing. The pseudo-dynamic method provides a reliable method to relate damage suffered by the footing due to the overall structure’s response to the applied earthquake excitation. The method enables the structural capacity and failure mechanisms of the reinforced concrete footing to be observed in relation to the seismic demand. The hysteretic response of the footings and energy dissipation characteristics were determined and was shown that the yield strength of the longitudinal reinforcement within the footing has a significant impact on the maximum shear capacity and damage incurred by the footing. The reinforced concrete footing could only sustain a maximum PGA before failure, which is related to the structure’s natural frequency and overall energy loss within the structure. Five damage states can be determined for the reinforced concrete and are related to the design of the footing and material properties that comprise the footing. The damage is more pronounced with an increase in the number of cycles of vibration, particularly at displacements that exceed the yield strength of the reinforcement. An increase in the hysteretic energy dissipated by the reinforced concrete footing results in a concomitant increase in the observed damage to the footing in the form of concrete cracking, reinforcement yielding and spalling of the concrete. The investigation shows that the resultant damage to an individual structural component is complex and is dependent on several characteristics that define the structure.