Washington State School Seismic Safety Project: Soil Seismic and Structural Assessments at 220 K–12 School Buildings

Date: 4/25/2019

Time: 04:30 PM

Room: Pine

The Pacific Northwest ranks second in the nation in earthquake risk. Close to 70% of Washington schools are situated in high seismic zones and over a third of permanent school buildings were constructed with building codes that did not incorporate current estimates of shaking from Cascadia and Seattle fault earthquakes. School facilities have therefore become a top priority for seismic risk assessment in the State. To identify potential seismic vulnerability at schools, the Washington Geological Survey (WGS), in coordination with the Office of the Superintendent of Public Instruction, began implementing a comprehensive two-phase strategy for seismic assessment based on early pilot studies completed in 2010–2017. This project couples detailed engineering screenings by structural engineers with a site-specific site-class assessment. In 2018, WGS expanded its work from two school districts to 74. The Survey coordinated tier-1 screenings, performed by licensed structural engineers per American Society of Civil Engineers 41-17, and conducted site-class assessments at 95 school sites and five fire stations. The engineers screened 220 buildings for tier-1 and produced 15 concept-level seismic upgrade designs. Site-class assessments used multi-channel analysis of surface waves and the microtremor array method to determine shear wave velocity profiles. Refraction data and horizontal-to-vertical spectral ratios of microtremor were also incorporated into the assessment to identify horizontal changes in soil layers and determine site dominant period, respectively. Of the 95 sites assessed, 29 have measured site-classes that differ from those previously predicted by a statewide site-class map based mainly on 1:100,000-scale geologic maps too general for site-specific assessments. Changes in site-class can significantly impact cost of retrofit by altering design requirements, further demonstrating the value of a two-phase comprehensive seismic assessment for Washington schools.

Presenting Author: Loyd T. West

Authors