Utah Engineers Journal 2021 Issue

61 year). (This article does not discuss the exceptions.) The code sets the level of shaking for which a building must have “Life Safety” performance at 2/3 the value of the MCE R shaking. The code value of MCE R shaking along the Wasatch Front is heavily influenced by the long recurrence intervals (average time between large earthquakes) of the different segments of the central Wasatch fault, which are about 1200+/-100 years. The code does not consider the elapsed time since the last major earthquake. If the fault ruptured more often, reducing the recurrence interval, the code required MCE R shaking level would be significantly higher along the Wasatch Front than it currently is, and buildings would be required to be stronger. Let us now summarize and tie this discussion together. We know that a magnitude 7.0 earthquake on the Wasatch fault will cause a wide range of shaking intensities, some of which will be very large. We compared the predicted range of shaking to a short girl (16th percentile) and a tall girl (84th percentile). We introduced the code MCE R shaking level and said that a building could have a 10% chance of collapse if it experiences this shaking level. We now need to compare where the code MCE R shaking level fits along the range of potential shaking levels that could happen in a large Wasatch fault earthquake. Understanding this relationship will show that the building code does not protect the Wasatch Front from a Wasatch fault earthquake to the level that most people expect. Because of the methodology used to calculate the MCE R value, as discussed above, MCE R falls at about the median predicted shaking from a large Wasatch fault earthquake. The consequence of this is that there is a 50/50 chance that a site will experience a shaking level that exceeds the MCER. The 84th percentile shaking is about twice the value of MCE R in most areas of the Wasatch Front (2.70 versus 1.48 for Ss and 1.16 versus 0.541 for S1 for a site in downtown Salt Lake City). The scary thing about this factor-of-two difference is that the probability of collapse could jump from 10% to 45% if a building experiences the 84th percentile shaking intensity. Luckily, most locations will not experience an 84th percentile shaking intensity in a Wasatch fault earthquake (about double MCE R ), but some areas could be hit with this shaking. Not all buildings that experience shaking levels twice the MCE R will collapse, but almost all of them that do experience it will be severely damaged and near collapse. Many newly constructed buildings in areas that experience the median predicted shaking intensity will be severely damaged, and a few could collapse. The 2/3*MCE R shaking level is in the range of the 25th to 35th percentile, which means that there is a high likelihood that most buildings will experience a shaking that exceeds the 2/3*MCE R “Life Safety” shaking level. Since most buildings will experience damage that could exceed “Life Safety,” there could be many deaths from falling hazards. Why would the building code allow for so much damage and risk of collapse from a Wasatch fault earthquake? The code uses a specific methodology across the United States to calculate the MCE R shaking level. The Wasatch Front is in a somewhat unique situation because there is one very large fault with a long recurrence interval and many smaller faults. This combination tends to lower the value of MCE R compared to some other areas of the country that have multiple large faults with shorter recurrence intervals. The SEAU Technical committee wants to study the impact that a Wasatch fault earthquake could have on the Wasatch Front. We want to quantify and compare the economic and life safety impact from staying with the current building code methodology, as described above, compared to enhancing the code to ensure that buildings that receive the 84th percentile shaking intensity have no more than a 10% chance of collapse. (The 84th percentile shaking level is required by the building code to be used in areas of the country where large earthquakes happen frequently. It is also often used when designing for a specific earthquake scenario.) We are seeking funding for this study. The building code is based on an acceptable risk set by the code writers; however, the Wasatch Front’s unique aspects may create higher potential consequences from a Wasatch fault earthquake than what many people expect or desire from the build ing code. Once the risks are quantified, we can have informed discussions about what to do about it. The code allows concerned owners to design and construct new buildings beyond the risk provided by the code. Please speak to your structural engineer about your desired building performance levels at specific levels of shaking, especially as it relates to shaking from a Wasatch fault earthquake. Brent Maxfield, S.E. Brent Maxfield, S.E., is a member of the SEAU Technical Committee and the Seismic Subcommittee Co-Chair. He has been employed as a structural engineer in the Special Projects Department of The Church of Jesus Christ of Latter-day Saints for almost 29 years. In 2012, he was selected the Utah Engineer of the Year by the Utah Engineers Council. He loves spending time with his wife, five sons, and their families exploring Utah’s great outdoors. Eric Hoffman, S.E., Eric Hoffman, S.E., is SEAU’s Technical Committee Chair and is a project manager with Ensign Engineering, Inc. He received his B.S. and M.S. degrees in Civil Engineering from Brigham Young University; is a licensed professional engineer in Arizona, Utah and California; and has Structural licenses in Utah and California. Eric lives with his wife and four children in Springville, Utah.