Utah Engineers Journal 2021 Issue

60 Figure 4 • 2020 Magna, UT M5.7: https://earthquake.usgs.gov/earthquakes/eventpage/uu60363602/map • SLC Scenario M7.0: https://earthquake.usgs.gov/scenarios/eventpage/uulegacyshakeoutff_se/map Both accessed March 29, 2020, annotated. Figure 2 Modified Mercalli Intensity Scale: https://www.usgs.gov/media/images/modified-mercalli-intensity-scale, accessed April 1, 2021. Well-engineered modern buildings were not affected other than some minor cracks and broken windows. As the shaking intensity increases to red (violent) or dark red (extreme) shaking, even well- engineered modern buildings can be heavily damaged and might even collapse. We will discuss how buildings could perform from a Wasatch fault earthquake, but first, we want to introduce the concept of code building performance. We all want our buildings to be life-safe during and after “an earthquake.” We also want our hospitals, fire stations and police stations to be functioning after “an earthquake.” But what is “an earthquake,” and what does life-safe mean? First, “an earthquake” is not specifically addressed in the building code and is not useful for engineers designing buildings. Instead, engineers use a shaking level at the building’s site. The building code is very specific about what “Life Safety” means and even more specific about the level of shaking that engineers should use to achieve “Life Safety” for a building’s design. We will first discuss two building performance levels discussed in the commentary to ASCE 7-16 (the portion of the building code that specifies earthquake forces). Per Section C11.5., “Collapse Prevention” means that a building will have a “suitably low likelihood of collapse,” which is defined by the code as a 10% chance of collapse. “Life Safety” means that “life-threatening damage, primarily from a failure of nonstructural components, is unlikely.” Buildings with damage in either one of these performance states could be heavily damaged, and it could be months before repairs are made to allow them to be reoccupied. In some cases, the cost of repairs will exceed the replacement cost of the building. Building performance must be associated with a level of shaking for it to have any meaning. One cannot just say, “I want my building to be ‘life safe,’” because there is always a level of shaking where the building will not be “life-safe.” As previously stated, a “Life Safety” performance or a “Collapse Prevention” performance must be associated with a specific level of shaking. The code sets the shaking level at which a new building must have “Collapse Prevention” performance and “Life Safety” performance. The shaking level for “Collapse Prevention” is defined as the Maximum Considered Earthquake shaking (MCE R ). It is not the highest level of shaking that could ever happen at a location, but it is the highest level of shaking that the code requires engineers to consider. The value of MCE R shaking for each location in the U.S. generally has a 1% probability of building collapse in 50 years (1/5000 chance per