An Examination of Site Response in Columbia, South Carolina: Sensitivity of Site Response to "Rock" Input Motion and the Utility of Vs(30)
This study examines the sensitivity of calculated site response in connection with alternative assumptions regarding input motions and procedures prescribed in the IBC 2000 building code, particularly the use of average shear wave velocity in the upper 30 meters as an index for engineering design response spectra. Site specific subsurface models are developed for four sites in and near Columbia, South Carolina using shear wave velocity measurements from cone penetrometer tests. The four sites are underlain by thin coastal plain sedimentary deposits, overlying high velocity Paleozoic crystalline rock. An equivalent-linear algorithm is used to estimate site response for vertically incident shear waves in a horizontally layered Earth model. Non-linear mechanical behavior of the soils is analyzed using previously published strain-dependent shear modulus and damping degradation models.
Two models for material beneath the investigated near-surface deposits are used: B-C outcrop conditions and hard rock outcrop conditions. The rock outcrop model is considered a geologically realistic model where a velocity gradient, representing a transition zone of partially weathered rock and fractured rock, overlies a rock half-space. Synthetic earthquake input motions are generated using the deaggregations from the 2002 National Seismic Hazard Maps, representing the characteristic Charleston source. The U. S. Geological Survey (2002) uniform hazard spectra are used to develop 2% in 50 year probability of exceedance input ground motions for both B-C boundary and hard rock outcrop conditions. An initial analysis was made for all sites using an 8 meter thick velocity gradient for the rock input model. Sensitivity of the models to uncertainty of the weathered zone thickness was assessed by randomizing the thickness of the velocity gradient. The effect of the velocity gradient representing the weathered rock zone increases site response at high frequencies.
Both models (B-C outcrop conditions and rock outcrop conditions) are compared with the International Building Code (IBC 2000) maximum credible earthquake spectra. The results for both models exceed the IBC 2000 spectra at some frequencies, between 3 and 10 Hz at all four sites. However, site 2, which classifies as a C site and is therefore assumed to be the most competent of the four sites according to IBC 2000 design procedures, has the highest calculated spectral acceleration of the four sites analyzed. Site 2 has the highest response because a low velocity zone exists at the bottom of the geotechnical profile in immediate contact with the higher velocity rock material, producing a very large impedance contrast. An important shortcoming of the IBC 2000 building code results from the fact that it does not account for cases in which there is a strong rock-soil velocity contrast at depth less than 30 meters. It is suggested that other site-specific parameters, specifically, depth to bedrock and near-surface impedance ratio, should be included in the IBC design procedures.