2016 Issue

D on’t ever tell a client along theWasatch Front that you designed his/her build- ing to resist amagnitude 7 earthquake. I know that is an easy answer, but it is not accurate and it is misleading. One of the reasons that the public is so confused about seismic design, is that we as engineers don’t communicate the same message. There are many times when we communicate the wrong message. The goal of this article is to help us as practicing engineers in Utah to understand some basic seismic issues and to propose a common message that we all use to communicate with clients. EARTHQUAKES VS GROUND MOTION The code writers didn’t do us any favors by using terms such as “Maximum Con- sidered Earthquake” (MCE) and “Design Earthquake” (Sometimes referred to as the Design Basis Earthquake or “DBE”). Using these terms implies that we are designing for a specific magnitude earthquake and that this earthquake will generate a specific level of ground motion. This is not accu- rate. Both of these terms do not refer to a specific earthquake; they refer to a specific ground motion shaking intensity, which is given as an acceleration. The code writers should have used the terms, “Maximum Considered Ground Motion” and “Design Ground Motion.” Talking Point #1: Engineers design for a specific ground motion shaking intensity, not a specific earthquake. (Avoid the use of “earthquake” in your discussions unless it is “the ground motion caused by an earthquake.”) Talking Point #2: Engineers design to a code-mandated ground motion shaking intensity level that is not caused by a specific magnitude earth- quake, but could be caused by different magnitudes of earthquakes. For example, the MCER ground motion shaking intensity along the Wasatch Fault could be caused by a magnitude 6 earth- quake. If we are lucky, amagnitude 7.5 earth- quake may not cause the MCER ground motion. In a magnitude 7 earthquake, the MCER could be exceeded in some areas and not in others. Do not tell a client that their building is designed to resist a specific magnitude of earthquake. Talking Point #3: The “Maximum Considered Earthquake” (MCER) ground motion shaking intensity is not the largest possible shaking intensity. It is the largest shaking intensity that the code requires to be considered. SEISMIC COMMUNICATION Let’s Get on the Same Page By Brent Maxfield , SE SEISMIC | continued on page 52 51