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Disaster-Resistant Fire Station

May 15, 2006 - 9:27am — Michael Kisor

Not long ago an article ran in a local paper, citing a PSU professor of urban studies, which suggested Portland was pretty disaster-resistant because we’re retrofitting fire stations to meet seismic code. Another article quoted an engineer saying that the fault lines running through Portland only produced large earthquakes every 6,500 years or so. Both sounded reassuring, and both had a kernel of truth. However neither told the whole truth, and the devil, as they say, is in the details.

The fault lines running through Portland are not the ones which are of greatest threat to Portland. The Cascadia subduction zone a hundred or so miles from Portland is the one we need to worry about the most. It regularly produces M9+ megathrust earthquakes every 300 years or so. (The last one occurred more than 300 years ago, on January 26, 1700.) And when it slips, the entire land mass between the Cascade mountains and the Pacific Ocean, from southern Canada to northern California, will be in motion for up to five minutes. And the shaking will be more than a thousand times more powerful than the M7.1 Loma Prieta (aka “World Series”) earthquake which – a hundred miles from its epicenter – collapsed a freeway, a span of the Bay Bridge, and wreaked havoc in San Francisco’s Marina District.

Okay, so maybe we can expect a really big earthquake soon, but at least we’re retrofitting fire stations and bridges to meet seismic code, right? Technically, yes… however that is not the whole story. Portland’s zone 3 seismic code standards anticipate an earthquake about the size of the Loma Prieta quake. They are weaker than California’s zone 4 standards. Even zone 4 standards aren’t intended to withstand a sustained M9+ event (like Cascadia). Only Japan and Chile are building to withstand such great quakes. Furthermore, seismic standards are intended only to prevent total collapse, thus protecting lives; the building may not be safe or usable afterward. But for earthquakes larger or longer than the seismic code zone anticipates, total collapse may occur. So a school or fire station built to meet Portland’s seismic code has no assurances it will remain standing nor protect life, much less remain functional and safe, after a Cascadia subduction zone megathrust earthquake… an earthquake at least 1000 times more energetic than what we're building to withstand.

So why are our seismic codes so lax? Two reasons: 1) It is only in recent years we have come to understand the severity of the earthquake threat we face here in the Pacific Northwest, and 2) Money. In general, stronger seismic standards mean higher building costs. And that often leads to political push-back from developers. However there are economical ways to build stronger, if you think outside the box… literally.

Conventional buildings contain many joints between walls, roof & foundation. During an earthquake these joints, called “moment connections,” are subjected to enormous stresses. If the stress is great enough or prolonged enough, the joint fails and the building collapses. Creating stronger joints can rapidly escalate construction costs, yet failure of the joint is still possible. A better and more economical approach is to reduce the number of moment connections in a building. A building without any would distribute seismic loads evenly across the entire structure. A thin-shelled one-piece (i.e. monolithic) dome made of steel- & fiber-reinforced concrete would have few, if any, consequential moment connections an earthquake could damage. This type of building has by far the best chance of surviving Portland’s greatest seismic threat. Amazingly, a large monolithic dome structure is less costly to build, heat or maintain than similar sized conventional structures.

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