New versus Previous ASCE 7-16 Earthquake Loads for 500 Overseas Locations

Abstract

The American Society of Civil Engineers (ASCE) Earthquake Loads Overseas project is an ongoing collaboration between the U.S. Geological Survey (USGS) and the Global Earthquake Model (GEM) Foundation to compute Risk-targeted Maximum Considered Earthquake (MCER) and other design ground motions for locations outside of the U.S. The ground motions are derived from the GEM Global Mosaic of Seismic Hazard Models in the same way that those inside of the U.S. are based on the USGS National Seismic Hazard Model. They enable overseas application of the ASCE 7 Standard for design of new buildings and other structures, as well as the ASCE 41 Standard for evaluation and retrofit of existing buildings. This paper compares the ASCE 7-16 (2016) ground motions derived by the USGS and GEM for approximately 500 overseas sites with previous design values provided by the U.S. Department of State (DoS) and U.S. Department of Defense (DoD).

Whereas all the new design values are based on the regional probabilistic seismic hazard analysis (PSHA) results from GEM, the previous DoS and DoD values were either (i) roughly approximated from Uniform Building Code (UBC) seismic zones; (ii) roughly approximated from 10%-in-50yr peak ground accelerations (PGAs) from the Global Seismic Hazard Assessment Program (GSHAP); (iii) based on site-specific PSHA results for PGA but approximate factors for 0.2- and 1-second spectral accelerations; or (iii) directly based on site-specific or regional (prior to the GEM Mosaic) PSHA results for PGA and the two spectral accelerations.

Comparing the newly computed design values to those provided by DoD and DoS, approximately two-thirds are more than +/-20% different from the previous values, although corresponding seismicity regions defined by FEMA P-154 (Federal Emergency Management Agency) are only different for approximately half of the values. Most of the changes are at locations where the previous values were not directly based on site-specific or regional PSHA, but there are impactful differences at almost one-tenth of the other locations, as well. On average, the new design values are less than the previous ones, in part because of differences in the risk-targeted calculations. The global distribution of the 500 locations reveals the extent to which the effect of risk targeting varies by region, and the extent to which deterministic capping of ASCE 7-16 and ASCE 41-17 MCER ground motions increases the collapse risk.