2016 Issue

quake would cause lower ground motions. It was to increase the number of collapsed buildings to be consistent with the rest of the country. Think about this new code objective of 1/5000 risk of collapse in any year. If any specific segment of the Wasatch fault moves on average about every 1250 years, when theWasatch fault moves, there could be about ¼ (1/5000*1250) of the new code-designed buildings collapse. That is not likely, but it gives a sense of the code expectation. Talking Point #14: The 2012 International Building Code (IBC) lowered ground motions by about 18%. This results in weaker buildings and will result in more collapsed buildings when the Wasatch fault moves. MY OPINION The Wasatch Front is in a unique position. The long recurrence interval puts the code design groundmotions very low compared to what could happen if the Wasatch fault moved. The code assumes that the move- ment of the Wasatch fault is a random (time independent) event. The movement of the Wasatch fault is not a random event. Seismologists have observed a regular occurrence of earthquakes on each of the major segments of the Wasatch fault. The Brigham City segment is past due and the Salt Lake City segment is approaching its recurrence interval. (See Putting Down Roots in Earthquake Country, page 7.) There are numerous efforts both private and government to prepare for the “Big One.” We are constantly being told to prepare. If we as a community along the Wasatch Front want to be prepared, why are new buildings not being designed to safely resist the potential ground motions from a magnitude 7 earthquake on the Wasatch fault? Shouldn’t we be like parts of California, where they must design to “Col- lapse Prevention” for the 84th percentile shaking intensity of the controlling fault? It is my opinion that the current IBC MCER shaking intensity level is significantly too low. The switch to the Risk-Targeted ground motions has made the situation even worse (by reducing shaking intensi- ties by 18%). Some new buildings could be a collapse risk if they experience shaking intensities above the median predicted shaking intensities from a magnitude 7 on the Wasatch fault. I believe that ground motions caused from a magnitude 7 earthquake on the Wasatch fault must be explicitly considered and the code shaking intensities should be increased. Exactly where to set these shak- ing intensities must be a matter of healthy public debate, weighing the associated costs and the risks. Building codes have focused solely on preventing loss of life. If you consider the staggering economic loss that will occur in amagnitude 7 earthquake, the debatemust also consider limiting eco- nomic loss and the cost to society if build- ings are allowed to be heavily damaged. The debate must consider how quickly the State of Utah can recover and return to (a new) normal. This is sometimes referred to as resiliency. A small investment in stronger buildings will pay big dividends in repair and recovery time and cost. I recommend that there be dual design cri- teria. One for the “planned-for” event that will limit economic damage, and one for the extreme event, that will focus on preventing building collapse. Are we expecting hospitals to be operational after a magnitude 7 earthquake? I believe that the community expectation is, “yes”. The current code does not require this. The code objective is “Immediate Occupancy” at 2/3 MCE shaking inten- sity. There is only about a 30% - 40% likelihood that the actual ground motions will be less than this level. I believe that essential fa- cilities should be designed to consider the higher ground motions that a magnitude 7 on the Wasatch fault could cause. I challenge the structural engineers of SEAU to becomemore conversant in these topics. We have a responsibility to our clients and building owners to clearly present to them what the code writers have deemed an ac- ceptable risk. We then need to present them with the information that will allow them to accept the code risk or to make other informed risk decisions. We need to help the communities along theWasatch Front be better prepared for the “Big One” by giving them structures that are better able to resist the potentially higher groundmotions that can be caused by it. I w e l c ome y o u r thoughts. 55