Influence of Soil Properties in Ground Seismic Response and the Definition of Site Coeffients for Design Provisions

Recent earthquakes have showed the important influence of local site conditions on the characteristics of ground surface motions in the city of Medellin (Colombia). The network of accelerographs installed in this city confirmed this hypothesis, providing information about seismic response of different types of soil profiles, and showed the important role of the impedance ratio rock-soil, due to the high bedrock stiffness. The values and trends of Medellin site coefficients, defined in Provisions of Medellin Seismic Microzonation, are described and compared to those of the 1994 NEHRP, and of the 1997 Uniform Building Code Provisions, These comparisons took into account the influence of soil and rock properties over and under 100 ft (30 m), because it was found that the criterion of the average shear wave velocity of the top 100 ft (30 m) of soil to characterize the site is not appropriate to represent seismic response of Medellin soils. Finally, it was proposed some considerations related to seismic coefficients for the specific case of effective peak acceleration Aa lower than 0.05, considering amplifications registered in Medellin for these rock acceleration levels and the criteria suggested for this condition in 1994 NEHRP Provisions