GSA Connects 2021 in Portland, Oregon

Paper No. 208-10
Presentation Time: 10:35 AM


WEST, Loyd, WERNLE, Alex, COOLEY, Jade and ALLEN, Corina, Department of Natural Resources, Washington Geological Survey, 1111 washington St SE, olympia, WA 98501

Washington State ranks second in the nation in earthquake risk and almost 70% of Washington schools are in high seismic risk zones. Furthermore, over a third of schools were constructed using now-outdated building codes that underestimated ground shaking from Cascadia and Seattle fault earthquakes. Schools are arguably the backbone of a community, responsible for educating the next generation, hosting community events, and providing shelter post-disaster. The purpose of this study was to assess the seismic risk of permanent, public, K–12 school buildings in Washington State. Each assessment took into account the local geology, engineering, and construction. In total, 561 school buildings (274 schools at 245 campuses) across the state were assessed. The project was conducted in two phases, over two biennia of work, funded by the 2017– 2019 and 2019–2021 WA capital budgets. The Washington Geological Survey worked with the Office of the Superintendent of Public Instruction and a team of engineers led by Reid Middleton Inc. to complete the seismic risk assessments in two parts: (1) a geologic assessment to determine site class per the National Earthquake Hazard Reduction Program guidelines, and (2) a tier-1 seismic screening evaluation, per American Society of Civil Engineers 41-17. The geologic assessments of site class used multi-channel analysis of surface waves and the microtremor array method to model shear wave velocity profiles. WGS scientists also incorporated refraction analysis and horizontal-to-vertical spectral ratios of microtremor to help characterize lateral changes in the subsurface. Measured site classes were compared to predicted site classes that were based primarily on 1:100,000-scale surficial geologic maps. Of the 245 campuses assessed for site class, 59 measured site classes differed from those that were predicted. Additionally, nearly half of the predicted site classes ranged between two site classes due to high variance. The geologic assessments resolved this uncertainty, stressing the necessity for site-specific studies over proxy maps. When site-class measurements were incorporated into the engineering assessments, more efficient seismic retrofit designs and more accurate cost estimates were developed. Our results can be used to prioritize school retrofit efforts.