Friday, January 16, 2015

What kind of house base absorbs the most shock?

There are real, practical consequences from geology that affect every single one of us. Here's another, though you have to think like an engineer to understand all the issues involved.

Q: What kind of house base absorbs the most shock during an earthquake? - Maddie D-N

A: There are two different aspects of the same issue here: a "walking" building, vs a shock-absorbing building.

1. A building foundation that is anchored in bedrock will SHIFT the least. The foundations of my house in Washington State are built (excavated) down into bedrock. In addition, all parts of the foundation are tied together with reinforced concrete. This will keep my house from "doing the splits" when the next Cascadia earthquake hits. Being anchored in bedrock means I have a better chance that my house won't take a ride - walk - over to my neighbor's property, either.  During the Loma Prieta earthquake of 1989 in northern California, some houses that "walked" and some that "did the splits." They were build on landfill in North Beach, landfill made up largely of debris from the 1906 Earthquake dumped there nearly a century earlier. Their foundations failed - sagged, did the splits - because they were not tied together, nor were they anchored in bedrock. That landfill turned partially liquid with the shock waves passing through it. In geology-ese, this is "liquifaction."

2. There are some (generally rare) structures designed with shock-absorbing materials between the bedrock-grounded base and the structure itself. Some examples include the Trans-Alaska pipeline, and the underground facilities hosting NORAD (North American Air Defense Command) in Colorado Springs, CO. Designing a structure to ABSORB the most shock is a very expensive thing to do, however, and when NORAD was built during the early Cold War, cost was not an issue. 
   However, no matter what the structure is, engineers must decide on how BIG an event to design for.(i.e., how much displacement can they anticipate). There is not enough money to design everything in the country to survive a magnitude 8 or 9 earthquake, as we sadly learned with the Great Tohoku earthquake of 2011. The Fukushima Dai-Ichi nuclear plant had been built to survive a Magnitude 7+ event. It was unable to withstand the consequences of a magnitude 9 event (the initial shaking and the 15-meter tsunami that followed). To build it for this, the facility would have cost one or two orders of magnitude more than it did, and no one had ever experienced a M = 9 event in Japan before. 

People around the Pacific Rim will live for many years with the consequences of that structural inadequacy. 

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