Dynamic Characteristics of Buildings Estimated from Strong Motion Records

Estimation of the dynamic characteristics of buildings is a key issue in seismic design technology. Since 1957, the Building Research Institute (BRI), Japan, has been operating a network to monitor strong motion in buildings. In this study, dynamic characteristics of 25 actual buildings in the BRI network were estimated by using strong motion records. Acceleration records from two sensors respectively placed on the top and at the base of each of these buildings were searched by using the steepest descent method to determine the natural period and damping ratio of a single-degree-of-freedom system that has the best fit for the response displacement. Those two parameters represent the characteristics of the fundamental vibration mode. Changes in the natural period and damping ratio of buildings slightly damaged by an earthquake were examined. The natural period increased from 0.35 seconds to 0.4 seconds during the strong earthquake. Changes in dynamic characteristics depending on the amplitude of response displacement were also investigated. The dependence of the natural period on the displacement amplitude was observed in all the 25 buildings. Finally, relations among the estimated natural period, estimated damping ratio, and building height were determined, and agreed well with existing empirical formulas. Introduction Understanding the dynamic characteristics of buildings is a key issue in evaluating the seismic response of buildings. Strong motion observation is an effective way to gain information about the actual dynamic behavior of buildings. In 1957 the Building Research Institute (BRI), Japan, initiated a network to monitor strong motion in buildings. A large number of strong motion observation records have since been accumulated. In this study, changes in the dynamic characteristics of actual buildings were evaluated using strong motion records from the BRI network. The relations of the dynamic characteristics to the building height and to the structure 1 Senior Research Engineer, Building Research Institute, Tsukuba, Ibaraki 305-0802, Japan 2 Professor, Faculty of Science and Technology, Keio University, Yokohama, Kanagawa 223-8522, Japan Paper No. 1136