Modal parameter variations due to earthquakes of different intensities

Abstract

This article presents the modal parameter variations during 55 low to high intensity earthquakes in a 22 story shear wall building located in Chile. Some of the earthquakes produced moderate damage in the structure. The building has been instrumented since 1997 with a network of 12 uniaxial accelerometers distributed on 4 different levels. Modal parameters are obtained through parametric Multiple Input - Multiple Output identification techniques. For the identified modal frequencies, decreasing values with respect to motion amplitudes are observed, in a range between 4% and 35% for the seismic records studied. For the modal damping ratios, increasing values with increasing motion amplitude are observed, with variation ranging from 50%, for the first translational modes, to over 120% for higher modes. For medium and low level earthquakes this variations disappear after the strong shaking has ended. This article also presents a preliminary analysis of the seismic records on the building during the Mw=8.8 earthquake occurred in Chile in 2010. Structural Damage was observed on the building and modal parameters changed significantly. During this seismic event, modal frequencies decreased up to 35% for the first translational modes. After the earthquake, modal frequencies had an 18% permanent decrease on average from initial values.