Site-Specific Spectrum

When designing a new structure or analyzing and evaluating existing structures, the use of site-specific spectrum may be a vitally important tool. If a structural engineer assembles a site-specific spectrum using non-proprietary available data, the spectrum must be constructed from information stored in several databases.

First, preliminary spectral curves must be obtained from the CDMG or USGS for nearby sites. Second, an interpolation on these curves must be done to accurately represent the specified site location and the structural period. Finally, soil conditions at the site must be determined from a CDMG map so that the site spectrum can be adjusted to account for soil amplification. The final step is highly subjective and quite complicated because soil amplification tables are limited in the number of structural periods offered. It is not hard to see why structural engineers are uncomfortable with this process and may decide to use geotechnical engineers to obtain site-specific spectra.

With ST_RISK™, you can have all of this in a single, easy step. In California, ST_RISK™ uses the CDMG soils and liquefaction database, and the USGS earthquake faults and parameters, to calculate a site-specific spectrum. ST_RISK™ uses the soil amplification numbers from NEHRP to maintain a highly qualified spectrum that is consistent with current seismic design codes.

Example Hazard Spectra