Seismic Hazard Values

Last updated December 26, 2022
By Ian Story

The codes define seismic hazard based primarily on ground acceleration. For the Equivalent Lateral Force procedure, the key variable is short-period (0.2s) spectral acceleration (Ss), based on a 2% probability of exceedance in 50 years. This corresponds to an earthquake with an expected return interval of 2,475 years.

This website provide an excellent interactive tool illustrating the relevant hazard curves: https://earthquake.usgs.gov/hazards/interactive/

The code-prescribed values are intended to protect life safety and prevent collapse. It is expected that the structure will sustain damage and need repairs after a design level earthquake. For serviceability design, such as deflection and plate crushing, it may be appropriate to consider a reduced seismic load corresponding to a higher probability earthquake. Since serviceability criteria are not life threatening, this is more of an economic decision: after what intensity earthquake should the building require serious repair?

I propose for consideration using a 10% probability of exceedance in 50 years intensity for service criteria design. Per the USGS tool, this corresponds to spectral accelerations approximately half those of the code-prescribed earthquake. Because equivalent lateral forces are directly proportional to Ss, this would result in seismic service design forces roughly half as large as the strength design forces.