Response of an unreinforced masonry building during the Loma Prieta earthquake

This report presents a case history of how an unreinforced masonry building sustained the 1989 Lorna Prieta Earthquake. The subject structure is a two-story historic building, and former firehouse located in downtown Gilroy California. The building was constructed in the last decade of the last century. Horizontal ground accelerations were as large as O.29g, and were amplified through the vibration of the floor and roof diaphragms. Although horizontal accelerations at the roof were as high as O. 79g, only minor cracking of the masonry was observed. At the turn of the century, unreinforced masonry construction was similar across the nation. Thus, response of the firehouse in Gilroy can help foretell the earthquake hazard in the eastern United States where similar URM buildings exist and moderate earthquakes are anticipated within a reasonable probability over the next fifty years. Though the ground motions recorded at Gilroy were but of moderate intensity, they represent an upper bound for assessing possible hazards associated with similarly constructed buildings in the eastern United States. Since the firehouse was not damaged appreciably, there is some hope that similar historic buildings across the nation may survive future seismic events. The purpose of this study was to examine dynamic response of the two-story URM structure in detail so that it may be better understood how the remaining stock of URM buildings across the nation may respond to future earthquakes. Specific objectives were: ./ 1. To investigate reasons why the firehouse withstood the ground shaking with little damage. 2. To estimate wall stresses using both simplified and state-of-the-art analytical methods, and correlate with current building codes and patterns of observed damage. 3. To develop a simple, discrete multi-degree-of-freedom analytical model to compute dynamic response of URM building systems with flexible diaphragms, and calibrate the model with measured response of the firehouse. 4. To run sensitivity studies using the analytical model so that the influences of masonry· wall stiffness, floor and roof diaphragm stiffness, and soil characteristics could be assessed. 5. To evaluate the use of finite element methods for estimating wall stresses and dynamic characteristics of a icomplete building system.