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 306 years ago.) 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 about 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 are weaker than California’s “zone 4” standards, and even “zone 4” standards are not intended to withstand a sustained M9+ event (like Cascadia). 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 than the 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 earthquake.
But there is a way to construct buildings which can be expected to survive such tremendous forces. 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.
Recently Portland City Council gave the green light to the fire station 18 relocation project (off Barbur Blvd.) as part of the fire station seismic upgrade program. Commissioner Saltzman had the foresight to ask that the Station Advisory Committee consider a dome design for this station. A letter or phone call thanking him for his comments is a simple yet powerful action you could take to help encourage and promote the building of a disaster-resistant infrastructure in our city. His address is: 1221 SW 4th Ave. Rm. 230, Portland, Oregon 97204