Ambient and forced vibration testing of a reinforced concrete building

Abstract

The seismic performance of reinforced concrete buildings depends on their lateral stiffness and mass distribution, as well as their lateral load capacity and ductility characteristics. In recent decades, seismic strengthening of existing buildings has been commonly employed, for improving their performance under severe earthquake excitation. The results obtained from vibration tests and system identification methods specifically provide valuable information on the influence of seismic strengthening on the lateral stiffness attributes of the building, and help evaluate the effectiveness of the strengthing technique applied, in terms of improving the building‟s vibration characteristics and seismic performance. In this study, ambient and forced vibration tests were conducted to evaluate the influence of the seismic strengthening on the vibration characteristics of the six-story reinforced concrete ET-B building, located on the Boğaziçi University Campus in Istanbul, Turkey. Modal vibration characteristics of the building were identified at various stages of strengthening. Ambient vibration tests were conducted on the building before and after its seismic strengthening. Forced vibration tests were conducted after seismic retrofit. Results of these tests enabled quantifing the changes, due to strengthening, in the modal characteristics (modal frequencies and mode shapes) of the building, and variation in the modal characteristics of the strenghened building with the intensity of shaking. In addition, simple finite element models of the building were generated, representing its structural configuration before and after seismic strengthening, also considering the impact of the brick-infill partition walls on its stiffness characteristics. The experimentally-identified modal vibration parameters of the building were compared with the results of the finite element model. The experimental and analytical results were found to be consistent in representing the pronounced influence of the structural walls on the lateral stiffness and thus the vibration characteristics of the building, as well as the relatively minor, yet noticeable influence of the brick-infill partition walls on its vibration characteristics.