NASCC Technical Highlight #4: "How to Talk Seismic in Language We Can All Understand" by Brent Maxfield

At some point in your career, you will likely receive a question from a client along the lines of "What magnitude earthquake is my building designed to resist?"  This questions is tricky to answer because the general public typically thinks in terms of Richter scale magnitude, but this is not the basis for seismic design.  So how do you answer it?  Here's Brent Maxfield's advice:

The building code does not address a specific magnitude, but rather it deals with "ground motion intensity".  Earthquake magnitude refers to the amount of energy that is released, which is a function of the area along the fault that ruptures.  However, for a given earthquake magnitude, the intensity of the ground shaking can vary based on a number of factors, e.g. fault type, soil conditions, distance from the rupture, and random variability.  The relationship between magnitude and ground motion intensity is like making a bag of microwave popcorn.  Each bag represents an earthquake magnitude and each popped kernel is a specific earthquake.  For the given magnitude, some kernels pop early (representing smaller intensity ground motions), most pop somewhere near the middle (representing ground motion intensities near the median), and some straggling kernels pop late (representing larger intensity ground motions).  Because a small magnitude earthquake can cause the same intensity of ground shaking as a larger magnitude earthquake (albeit with different probabilities), magnitude is not a reliable value for design.

Now that it's been established that the level of ground motion is the pertinent value to discuss, what intensity does our code use?  First, the code establishes what we call the risk-targeted Maximum Considered Earthquake (MCE_R).  This is the ground motion intensity at which a structure would have a 1/5000 risk of collapse in a given year, i.e. a 1% probability of collapse in 50 years (the cost assumed life expectancy of buildings).  We then design the building for 2/3 of this value, which results in no more than 10% probability of collapse when a MCE_R level ground motion occurs.

So it appears the answer to the client's question is that their building is designed using forces caused by a ground motion intensity (not an earthquake magnitude) for which their is a 10% probability of collapse during the MCE_R shaking intensity.  It is important to note that designing per the code does not completely eliminate risk of collapse.  Furthermore, designing per the code says little about the level of damage that may occur.  Just because a building doesn't collapse, doesn't mean it is able to be occupied or functional.  This kind of knowledge can be obtained through Performance Based Design, but that is a topic for another day.

To see Brent Maxfield's talk, you can do so here:  https://www.aisc.org/education/continuingeducation/education-archives/lets-talk-seismic---in-language-we-can-all-understand-n31/#.W1I32C0-KL4