2016 Issue

building has been pushed to the limits of its strength and stiffness and is on the verge of collapse. Nonstructural components may be dislodged causing localized deaths from falling debris. Talking Point #5: The code acknowledges that 10% of the buildings that experience the MCER shak- ing intensity could collapse. It is also important to understand what the importance factor does. It modifies the performance state at the DBE and MCER shaking intensities. An importance factor of 1.5 is intended to give a hospital an “Immediate Occupancy” performance state for the 2/3 MCER “Design” ground motion, not the MCER. If the hospital experiences the MCER ground motion it could only be “Life Safe” with the need to be evacuated. I will talk more about hospitals later. Talking Point #6: When discussing building performance, two things must always be discussed: 1) The ground motion shaking intensity, and 2) The expected building performance at that shaking intensity. For a typical house or building, the code performance expec- tations are “Collapse Prevention” at the MCER and, “Life Safety” at the 2/3 MCER shaking intensities. For hospitals, it is “Im- mediate Occupancy” at the 2/3 MCER, and “Life Safety” at the MCER shaking intensities. Note that hospitals may not be operational if they experience the MCER shaking intensity. GROUND MOTIONS FROM A MAGNITUDE 7 EARTHQUAKE Will a magnitude 7 earthquake along the Wasatch fault generate lowgroundmotions, or will it generate very high groundmotions? No one knows. It is like predicting whether a specific kernel in a bag of popcorn will pop before a certain time. There is an element of chance. Seismologists can only predict a median ground motion and the standard deviation from a specific magnitude earth- quake. Talking Point #7: It is impossible to predict the exact ground motion shaking intensity fromamagnitude 7 earthquake on theWasatch fault. Seismolo- gists can only provide a median predicted ground motion and a standard deviation. The median provides a 50% likelihood and themedian + 1 standard deviation provides an 84% likelihood that the actual ground motion will be less than these amounts. WASATCH FRONT VS. CALIFORNIA The Wasatch Front has one very major fault and several other significantly smaller faults. The Wasatch fault is the only source of very large shaking intensities. Because the Wasatch fault only moves about once every 1250 years or so, it greatly lowers the code-mandated ground motion shak- ing intensity. The code uses the smaller of the 84th percentile deterministic ground motion (What happens when the Wasatch fault moves?) and the probabilistic ground motion. (Which takes the deterministic and adds the “What is the likelihood that it will happen?” aspect). If the Wasatch fault moved more often, the code-mandated groundmotion shaking in- tensities along theWasatch Front would be considerably higher than current code. The code assumes that there is about a 1/1250 likelihood in any year that theWasatch fault will move – regardless of how long it has been since the last movement. If the code considered the time since the last move- ment, the code-mandated shaking intensity along portions of the Wasatch Front would be considerably higher. (If a magnitude 7 earthquake occurs on average every 1250 years, then the 1/2500 year ground motion [2% in 50 year] would be the median pre- dicted ground motion for the magnitude 7 earthquake [1/1250 * 50/100 = 1/2500 As- suming only one source of hazard]) California has many faults. Each fault adds to the seismic hazard level. This causes a very high probabilistic ground motion shaking intensity. In these areas, the code allows a lower MCER to be based on the largest 84th percentile deterministic ground motion from a nearby fault. In other words, California considers the scenario of an actual earthquake occurring. In Utah, this is not a requirement. If it was a requirement, and if the MCER were based on the 84th percen- tile deterministic groundmotion then the Ss MCER code values near the Wasatch fault could be in the range of 2.5 to 3.0. This is almost twice the current Ss code values. Talking Point #8: The code ground motion shaking intensity levels along the Wasatch Front are based on a very low probability of a magnitude 7 earthquake on the Wasatch fault (about 1/1250 per year). Talking Point #9: Some segments of the Wasatch fault have built up enough stress to break at any time (tomorrow or many years from now). The code does not consider this increased risk of earthquake. If it did, the code-mandated groundmotion shaking intensities along the Wasatch Front would be considerably larger. Figure 1 SEISMIC | continued on page 54 53