Seismic Site Classification for Structural Engineers

December 30, 2006
Publication: Structure Magazine p 21-24
Author(s): Dominic Kelly

Abstract: Many states and municipalities have adopted the International Building Code (IBC) and, by reference, the seismic provisions in Minimum Design Loads for Buildings and Other Structures (ASCE 7-02 and ASCE 7-05). As engineers use these documents, they are beginning to realize how dependent the magnitude of the design earthquake force is on the site class. In seismic provisions of previous model cuilding codes other than the 1997 UBC, the soil type impacted the force level for mid-rise and high-rise buildings, but generally did not affect the seismic design force for low-rise buildings. The site classes in the IBC, ASCE 7-02, and ASCE 7-05 directly impact the seismic design force for all buildings, whether a low-rise or high-rise building. In regions of low or moderate seismicity, a difference in site class may change the seismic design category (SDC), resulting in a difference in design and detailing requirements.