SYNTHETIC SEISMOGRAMS ADJUSTED TO CONSTANT-DISPLACEMENT SPECTRA FOR DESIGN OF UTAH STATE CAPITOL BUILDING SEISMIC RENOVATION

摘要

The Utah State Capitol Building in Salt Lake City is located on the footwall block less than 1 km east of the Salt Lake City segment of the Wasatch fault. Few normal-faulting strong-motion recordings are available for use in engineering design, so the Composite Source Model was used to develop faultparallel, fault-normal, and vertical seismograms for earthquakes of Mw 7.0 and 7.3, taken to be equivalent to the BSE-1 and BSE-2 motions. Probabilistic uniform-hazard acceleration response spectra for exceedance probabilities of 50, 10, and 2 percent in 50 years were developed by others for design of the seismic renovation of the Utah State Capitol Building. The acceleration response spectra were converted to displacement response spectra based on a singledegree- of-freedom oscillator in circular motion. At a period of approximately 1 s the spectral displacements plotted on nearly straight lines with slopes of 4.8 and 9.2 cm/s for the fault-parallel and fault-normal components of the BSE-1 motion, and slopes of 13.1 and 25.1 cm/s for the BSE-2 motion. The spectral displacements at a period of 4 s were 27 and 41 cm for the fault-parallel and fault-normal BSE-1 motion and 73 and 112 cm for the BSE-2 motion. Adjustments were made in the displacement spectra so that the spectral displacements became constant at periods of 2.1 to 2.3 s, and the acceleration response was recalculated. The maximum spectral displacements were 20 and 25 cm for the fault-parallel and fault-normal components of the BSE-1 motion, and 52 and 65 cm for the BSE-2 motion. The Composite Source Model produced very high short-period motions in part because the model treats all geologic materials as linear. Therefore, adjustments were made to the Fourier amplitude spectra of the acceleration time history records to bring the acceleration response close to the constant-displacement response spectra.