Investigation of Seismic Site Amplification for Non-NEHRP Site Conditions: Site Response Study of Columbia, SC

Current guidelines for simplified seismic design are mainly based on experience and observations from sites in the Western United States (WUS) and do not account for unique geological and geotechnical conditions prevalent in the Central and Eastern United States (CEUS). For instance, areas where abrupt velocity contrasts caused by very hard rock (shear wave velocity, Vs > 2600 m/s) close to the ground surface are prevalent in the CEUS, especially along the Fall Line. This geologic condition causes significant amplifications at short structural periods that can produce motions well above those typically predicted by current procedures based on the average shear wave velocity of the top 30 m (100 ft) of the profile. The Atlantic coastal plain, another common CEUS geological condition, which consists of a stack of unconsolidated sediments up to 1000 meters thick along the coast, amplifies ground motions at long periods (typically > 2 seconds) that greatly exceed the simplified spectra in the building codes. Current guidelines can be unconservative for both of these geologic settings. This paper presents a site response study of several sites in Columbia, South Carolina. Site amplification factors are computed and compared to those recommended in the building codes.