Utah Engineers Journal 2021 Issue

48 The Wasatch Front has long been known as earthquake country to scientists and engineers. The grand mountain ranges frame our cities and towns and create the vibrant outdoor recreation that often draws people to our state. They also stand as evidence of the thousands-to-millions of years of uplifting geologic activity that created them. As a community, however, we all seem to be lulled into complacency by the very fact that we have never had catastrophic earthquake damage here. There has never been a building collapse resulting in extreme casualties or a clear “near miss” similar to the 1933 Long Beach, California earthquake. Looking back on the Long Beach event, it is easy to see why it was such a catalyst for changes to building standards in California. More than 230 unreinforced masonry (URM) school buildings were destroyed, suffered major damage, or were judged unsafe to occupy following the earthquake. 1 The magnitude M W 6.3 event shook the ground with a maximum Modified Mercalli Intensity (MMI) of VIII (severe) at 5:54 p.m. If the event had occurred mere hours before, while school was in session, Long Beach would have experienced a devastating loss of school-age children. California prohibited the construction of URM schools the following month, and the Riley and Field Acts followed within the year. The Riley Act required all California local governments to have a building department and inspect new construction, mandating that all state structures be designed to withstand a horizontal acceleration of 0.02 times the building mass. The Field Act mandated that schools be designed to 0.03 times the building mass by registered architects and engineers. URM building failures were among the leading causes of casualties in many California seismic events. The failures led to widespread retrofitting ordinances. 2 Casualties and economic losses in California’s significant earthquakes (M W 5.3 to M W 7.8) have formed the foundation of modern seismic provisions in building codes in the United States. By contrast, Utah’s largest modern earthquakes occurred in rural parts of the state. Both Hansel Valley (1909) and Elsinore (1921) experienced events estimated at magnitude 6+ with no casualties. 3 Given the relatively modest community impacts of these events, it becomes more understandable that unreinforced masonry buildings were constructed in Utah through the 1970s. It is common to estimate that prohibition of this construction type was enforced by 1975 with statewide adoption of the Uniform Building Code (UBC). However, some URM construction may have continued for a few years where permitting may have already occurred. Utah would go over 20 more years before building codes would fully recognize the significant earthquake risk and raise the seismic demand for which buildings would be designed by adopting the 2000 International Building Code. The previous seismic provisions of the 1997 UBC categorized the Wasatch Front and surrounding areas as Zone 3. In contrast, the updated provisions would be approximately equivalent to UBC Zone 4, roughly a 1/3 increase for parts of the Wasatch Front. It bears restating that buildings constructed under the old provisions (anything permitted before 2001) were likely designed at roughly 75% of the seismic design forces in place now. A map of the 2018 national seismic hazard in Figure 3 shows the highest seismic hazards along the urban Wasatch Corridor. Figure 3. 2018 Long-Term National Hazard Seismic Map Showing Peak Ground Accelerations Having a 2% Probability of Being Exceeded in 5 0 Years, for a Firm Rock Site. https://www.usgs.gov/media/ images/2018-long-term-national-seismic-hazard-map Continued from the previous page With knowledge of Utah’s seismic risk, various communities and organizations have been retrofitting critical structures. 4 In 1987, the Salt Lake City and County Building was one of the world’s first buildings to be retrofitted with base isolation. 5 FEMA programs and grants such as the 1997 Project Impact initiative have been critical to addressing URM parapets and retrofitting schools in Utah. 6 Other notable seismic retrofits in the state include the Utah State Capitol, the Wallace F. Bennett Federal Building, the Salt Lake Tabernacle, the Marriott Library and, currently underway, the Salt Lake Temple. While the voluntary retrofitting projects are significant, especially by health care organizations, they do not come close to eliminating the risk to all Utahns. In 2008, at the Utah legislature’s urging, the Utah Seismic Safety Commission (USSC) compiled an inventory of unreinforced masonry buildings. The commission found that in the Salt Lake Valley alone, there are over 185,000 such structures. 7 Most URMs are single-family residences ranging from historic bungalows to midcentury multilevel tract homes. Other common URM building types include apartment buildings up to three stories above grade and commercial buildings up to five or six stories tall. Additionally, many historic midrise steel structures have unreinforced masonry infill that may fail in a moderate-to-large magnitude earthquake.