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Thursday, May 30, 2013

The significance (or not) of Skagit

A truck strikes a bridge carrying Interstate 5 over the Skagit River in Washington state, and the bridge collapses. No one is killed, but the press rapidly fills with stories about tens of thousands of obsolete or structurally deficient bridges across this country. Further, the press criticizes bridges that have a single point of failure. What does one make of this?

The proximate cause of this accident is human error. The truck carried a "high and wide" load. The route for this load had been meticulously checked, and the truck was operated by professionals experienced in the movement of high and wide loads. Nevertheless, as this article explains, the truck driver inexplicably did not cross the bridge in the center of the road -- perhaps because someone saved money by not providing a rear escort car, which is not required.

Every bridge is a design tradeoff; positives such as capacity, resilience, and duration of service are tempered by negatives such as cost to construct, cost to maintain, and impact of construction and maintenance on the environment. This article provides a basic introduction to a truss bridge, which was the most common design for medium-sized bridges until advances in concrete technology supplanted it. Bottom line: if a "member" (as engineers would say) of a truss is taken out, the bridge could be in peril.

Full redundancy is expensive. I fly over the Atlantic several times a month, and I wouldn't want to do that in a single-engine aircraft. In that circumstance, I willingly pay for 100% redundancy; but in most scenarios, either implicitly or explicitly we accept less than full redundancy. Instead we rely on safety margins. Many of our bridges are indeed vulnerable to a single failure, but what the press doesn't mention is that such a failure is extremely unlikely. As this article explains, the bridge in question had been struck many times -- as bridges often are. Safety margins handle almost all such accidents. Besides, as we see in aviation, full redundancy is not a panacea.

Instead of spending a trillion dollars annually on defense, I want our nation to divert tax dollars to civil construction projects like bridge replacements and high-speed rail. Even so, we cannot afford to replace tens of thousands of bridges. A triage system is already in place for the worst ones, but the I-5 bridge was nowhere near the top of that list. The question here should be: what measures do we take to reduce the probability that human error will bring down a bridge that, although it has no structural redundancy, does have a margin of safety adequate for all but the most ridiculous accidents?