Foam Rubber Modeling of the El Centro Terminal Substation Building

Abstract

Results from a layered foam rubber model of soil and the structure (3-D) of the El Centro Terminal Substation building, site of the well known recording of the 1940 El Centro earthquake, indicate that the response of the foundation for frequencies above 4 Hz is as much as a factor of 3 lower than the free-field response amplitude. To validate the modeling technique with sample structure, we use the formulation for calculating theoretical responses of Wong and Luco (1977). Two-dimensional computer model results by Shannon and Wilson, Inc. and Agbabian Associates (1980), gave a response amplitude in the building about a factor of 2 lower than the response amplitude of the free-field for frequencies above 1.5 Hz. Foam rubber modeling shows that at higher frequencies most of the reduction in response is due to the energy being scattered by the shape of the rigid foundation rather than its inertial mass. Under the assumption that most of the energy was vertically incident SH, the free-field peak acceleration during the May 18, 1940 El Centro earthquake was about 50% higher than recorded on the 1940 accelerogram. The transfer function is very similar in shape to the weighting function used by Munguia and Brune (1984), to match synthetic accelerograms with the 1940 El Centro earthquake, suggesting that much of the relative deficit in high frequency energy was a result of soil-structure interaction rather than a source effect.