Aplicación del Método Equivalente Lineal para el Análisis Sísmico de Estructuras de Hormigón Armado con Comportamiento No Lineal Moderado

Abstract

The equivalent linear method is widely used in Geotechnical Earthquake Engineering to calculate the accelerations at the surface of soil deposits subject to earthquakes of moderate intensity. The method approximately takes into account the nonlinear behavior of soils by using equivalent shear moduli and damping ratios that are function of effective shear strains. A series of linear analysis are performed each time using equivalent soil properties until their values at two consecutive steps are approximately equal. The effective strain is defined by reducing the peak strain retrieved from the time response. The reduction factor, usually 0.65, accounts for the fact that the peak strain only occurs at a single instant of time. This paper investigates the application of this technique to calculate the seismic response of reinforced concrete buildings with moderate nonlinear behavior. The method is tested using a 3-D finite element model of a building created in the program ANSYS. Nonlinear constitute relations in the form of stress vs. strain are used to calculate the full nonlinear response and also to define the equivalent modulus of elasticity and damping ratio by means of the Masing’s rule. It was found that a key parameter affecting the accuracy of the results is the reduction factor. Considering a number of seismic records with different frequency contents an optimal reduction factor was defined that is a function of six parameters related to the intensity of the earthquake. The accuracy of the results obtained with the equivalent linear method depends on the response of interest sought (displacement, shear, moment, acceleration) but it proved to be quite acceptable for all the cases considered. The floor response spectra for the building with nonlinear behavior were also obtained and they compare very well with those computed with the exact nonlinear analysis.