Bridging Engineering Compromises

August 4, 2007 | By | Reply More

In recent news, another highway bridge collapsed. Every 20 years or so, a major bridge fails unexpectedly. The nature of civil engineering is to understand how things fail, and design the next generation to avoid that failure. Each failure leads to better designs. Each of the major bridges that collapsed had been certified as less-than-acceptable, but no imminent danger. As are a significant fraction of bridges built decades ago. So why do they fail?

The compromise part of the equation is that a perfectly safe bridge can be built, for about the cost of a few days of our occupation of Iraq. This is a huge number, compared to the cost of a normal bridge with a known, small statistical chance of failure.

The current cost estimate to bring all of America’s thousands of sub-standard bridges up to spec is still less than a month of our expensive venture over there. Prohibitive. But this recent collapse has people in power considering the option.

The basic cause of bridge failures is a combination of simply wearing out and changing conditions, like the riverbed, the weather, and increased demands on it. Failures happen.
A more complex underlying cause is politics. How can a bridge get built, and who will pay how much for it? How can congress allocate so much public money to benefit apparently a small area? How much safety can builders put in and still win a bid? How much money can be allocated to upkeep? This last, critical and unglamorous, number is where bridges fail. If every bridge had a full time engineer and maintenance crew with an uncapped budget, then no bridge would ever fail. But how can so much money be spent, be justified, if the end result is…nothing?

Bridges have to fail occasionally to remind people that maintenance must be paid for. How often is optimum? Well, history shows that about once a generation is about right.

This time, fewer lives were lost in the collapse than can be expected to be saved by having the corridor closed. (Multiply fatalities per interstate mile in urban areas by the number of interstate miles that will be closed for how many months while the bridge is replaced). Had it been winter, the death toll would have been higher.

The shocking part is that it could have been any of us on the bridge at the time. I’ve driven across it, myself. But I was more impressed by the 1983 rural Connecticut bridge that failed in the middle of the night, and travelers kept sailing into the abyss until someone managed to stop in time and report it.

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Category: Current Events, Economy, Politics, Science, War

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A convoluted mind behind a curly face. A regular traveler, a science buff, and first generation American. Graying of hair, yet still verdant of mind. Lives in South St. Louis City. See his personal website for (too much) more.

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