Development of seismic ground-motion criteria for the ASCE 7 standard

The ground-motion section in the ASCE 7-05 standard has been revised to incorporate (1) maps of a new parameter, TL, the period on the design response spectrum separating the constant-velocity and constant-displacement segments, (2) new Maximum Considered Earthquake (MCE) ground-motion maps for the conterminous U.S., Puerto Rico (PR) and the Virgin Islands (VI), and (3) criteria for conducting site-specific ground-motion analysis including lower limits on ground motions for Site Class F soils. Values of TL are designated within irregularly shaped regions within the U.S. and are based on 2-sec deaggregation results from probabilistic seismic hazard analysis (PSHA) conducted by the U.S. Geological Survey (USGS). Although acknowledged to be imperfect, the maps are intended to provide improved estimates of ground motions for the design of long period structures. The values of short period MCE spectral acceleration, SS, and 1-sec MCE spectral acceleration, S1, for PR and VI are generally smaller than the default values of SS and S1 in the previous ASCE 7-02 standard. The new values are based on the USGS PSHA, whereas the old values were based on the judgment that PR and VI were equivalent in hazard to that represented by Zone 3 and Zone 4, respectively, of the Uniform Building Code. The new values of SS and S1 for the conterminous U.S. are generally within + 10% of the values in ASCE 7-02.