Performance of a Linear Solution for Approximating Nonlinear Response of Reinforced Concrete Structures Subjected to Earthquake Shaking
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Nonlinear Time History Analyses (NTHA) are generally regarded as the most accurate way of predicting the dynamic response of a structure to a given seismic ground motion. However, these types of analyses are computationally demanding and require proper software. The aim of this paper is to investigate the feasibility and accuracy of an Equivalent Linear analysis (ELA) where material nonlinearity is accounted for through an iterative procedure with the use of secant stiffnesses. The method is applied to one of the pylons of a major suspension bridge recently designed in Chile. For comparative purposes, two models are created in the open-source framework OpenSees: 1) a full nonlinear fiber model where nonlinear material behaviour is accounted for through distributed plasticity, and 2) an elastic model where the element flexural stiffnesses are updated using the ELA method. The analyses are carried out for seven earthquake time histories. The results show that the ELA is able to reproduce the maximum forces in the structure with a satisfactory accuracy. However, the method is not able to capture regaining of stiffness once cracks are closed due to cyclic responses. For further verification, several pushover analyses are also conducted on both models with the inertial forces extracted from the most unfavourable time-steps in the nonlinear time domain analysis. The pushover analysis verifies that the ELA method is capable of predicting the structural response up to the point of yielding of the steel reinforcement or crushing of the concrete. Nevertheless, the method is fairly accurate in identifying possible plastic hinges, and to some degree, assessing the ductility of the structure.