2016 Issue

Talking Point #10: In parts of California, the MCER is set at the 84th percentile shaking intensity of a nearby fault. In Utah this is not a require- ment. In other words, the code does not require that buildings be designed to resist the higher shaking intensities that could be caused by amagnitude 7 earthquake on the Wasatch fault. BUILDING PERFORMANCE IN A MAGNITUDE 7 EARTHQUAKE You now know that theMCER shaking inten- sity in Utah is based on a very low probability of an earthquake on the Wasatch fault. You also know that the shaking intensitywouldbe almost twice the current code values at the 84th percentile predicted ground motion. Unfortunately, because there is so much un- certainty, the 84th percentile ground motion along the Wasatch fault is very large. An important question to get answered is: “How does the code MCER ground motion relate to the range of ground motions from a magnitude 7 earthquake on the Wasatch fault?” Inmy experience, theMCER ground motion will be about equal to or less than the median ground motion from a magni- tude 7 earthquake. (This will vary depend- ing on the period of interest. It could be as low as 40% or slightly above 50%) The 2/3 MCER or DBE ground motion shaking intensity will be considerably lower than the median shaking intensity from a magnitude 7 earthquake. In other words, if a building is designed to be “Life Safe” after the 2/3 MCER shaking intensity, if a magnitude 7 earthquake happens along the Wasatch Fault, you could only have about a 30%-40% confidence that the shaking intensity would be less than this. You would only have a 50% confidence that the shaking intensity would be less than what would give you “Collapse Prevention.” Saying it a different way – for some building periods, there is a 50% likelihood that a magnitude 7 on the Wasatch fault will push the building beyond “Collapse Prevention.” See Figure 1. Obviously every location is different and you would need to get an expert to help understand how the code MCER and 2/3 MCER shaking intensity relate to various earthquakes at a specific location. The Pacific Earthquake Engineering Research Center (PEER) (http://peer.berkeley.edu/ ngawest2/databases/) has generated a MS Excel spreadsheet that calculates the predicted median and standard deviation ground motion from various magnitude earthquakes. Note that the values from this spread sheet must bemultipliedby 1.2 to 1.3 to get the code-required “maximum direc- tion” (RotD100) groundmotions. The USGS also provides an estimate of the 84th per- centile deterministic shaking intensity at a site. Use “2009NEHRP” as theDesign Code Reference Document at http://earthquake. usgs.gov/designmaps/us/application.php and select the “View Detailed Report.” Talking Point #11: If a magnitude 7 earthquake happened on the Wasatch fault, there is about a 50% likelihood that, for some buildings, the MCER shaking intensity wouldbe exceeded. Remember that the code allows building collapse to begin occurring at the MCER. Talking Point #12: Understand how the MCER and 2/3 MCER Design shaking intensities relate to the shak- ing intensities that can be caused by a mag- nitude 7 on theWasatch fault. Engineers can provide the likelihood that the actual ground motion will be less than the MCER and the likelihood that the actual groundmotion will be less than 2/3 MCER. Talking Point #13: A building owner can choose to consider shaking intensity levels above the code and then decide upon a level of confidence he/ she wants for the “Immediate Occupancy”, “Life Safety”, and “Collapse Prevention” performance states for amagnitude 7 earth- quake on the Wasatch Fault. RISK-TARGETED MCER For the 2012 International Building Code (IBC), the code writers felt that the MCE using a 2% in 50 year ground motion across the country (Uniform Hazard) did not give the same risk of building collapse across the country. They switched from a “Uni- form Hazard” shaking intensity (MCE) to a “Uniform Risk” shaking intensity (MCER). This shaking intensity is set so that across the country there is a 1% chance of build- ing collapse in 50 years (1/5000 per year). This change has the effect of lowering the code-mandated shaking intensity along the Wasatch Front by about 18%. In essence, the code writers said that Utah was not going to experience enough col- lapsed buildings compared to the rest of the country, so they lowered the code- mandated ground motions. This was not because they felt that a magnitude 7 earth- SEISMIC | continued from page 53 54