Pushover analysis of a reinforced concrete structure

Abstract

Engineers are constantly searching for new and more realistic methods to account for the structural behavior. Performance based strategies need to estimate the inelastic deformation and the associated damage in structures but elastic analysis cannot provide this information. Nonlinear dynamic response history analysis can provide this information, but it is a tedious procedure based on uncertainties coming from the excitation. So the scientists proposed some new design methods and rehabilitation strategies that incorporate performance based engineering concepts. It is clear that damage control should be considered as a more explicit design consideration. This goal can be reached only by consideration of some kind of non linear analysis into the seismic design methods. The most logical approach seems to be a mixture of the nonlinear static analysis (pushover analysis) and the response spectrum method. The static pushover analysis procedure is becoming the dominant method implemented in the computer to evaluate the seismic performance of structures. The method assumes that the response of the structure can be checked by considering its first mode, and this mode during a monotonic increase of loading governs the motion constantly. There are some methods that are based on this methodology such as the capacity spectrum method (in ATC 40) and the nonlinear static procedure (in FEMA 273). The second procedure is used in ATC 40 by “displacement coefficient method” as an alternative method. The Capacity Spectrum Method (CSM) approach is used to compare the structure’s capacity with the demands of earthquake ground motion on the structure. A nonlinear force displacement curve is used to represent the capacity of the structure (pushover curve). Using the coefficients that represent effective modal masses and modal participation factors, the base shear forces should be converted to equivalent spectral accelerations and the roof displacements should be converted to equivalent spectral displacements. These spectral values define the capacity spectrum. The earthquake ground motion demands can be represented by response spectra that correspond to the level of equivalent viscous damping representing the dissipated hysteretic energy. Finally both of the curves are drawn within a same graph to determine the intersection point of the two curves that expresses the performance of the structure to the design earthquake incorporated in the particular spectrum. In this work a simplified method for nonlinear static analysis of building structures subjected to monotonically increasing horizontal loading (pushover analysis) is presented using the SAP 2000 software. Following a step by step analysis an approximate relationship between the global base shear and top displacement of the structure is determined. During the analysis the development of plastic hinges, at different stages, throughout the building are monitored. The mathematical model, the base shear- top displacement relationships and the step by step computational procedure are described. The method is applied for the analysis of an existing seven-story reinforced concrete building. The results are presented and the evaluation of the building performance is discussed. Finally an appropriate type of intervention is proposed to improve further the seismic behavior of the building.