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 REFRACTIONS

BY HENRY PETROSKI
HENRY PETROSKI

Skimping on Repairs

Risky shortcuts on public infrastructure are inexcusable.


The embarrassing problems with a cracked eye-bar and a failed fix that plagued the San Francisco-Oakland Bay Bridge last fall reminded me of some analogous problems I once had with my Volkswagen Beetle.

A bridge is not unlike a machine, in that each is an assemblage of parts that move relative to one another. As large as the bridge parts might be, their movements are typically relatively slight and not easily noticeable to the untrained, unaided eye. Nevertheless, every bridge does flex whenever a vehicle passes over it. For the Bay Bridge, this happens about 270,000 times a day.

A bridge is also moved by larger forces. Gusts of wind can lift and drop it as turbulence can an airplane. In 1989, the Loma Prieta earthquake shook the Bay Bridge so violently that a section of its upper deck fell onto the lower roadway, killing a young woman motorist. That incident ultimately led to the design of a replacement for the East Bay spans of the bridge, a project that is ongoing.

Movements repeated over time can cause cracks to grow in steel components. If the progressive damage to the bridge is not detected and arrested before the cracks become too large, collapse attributable to metal fatigue can occur. This is why inspections and proper preventive maintenance are so important.

My VW was certainly nowhere near as important as the Bay Bridge, but, like all machines, the small automobile was made up of parts that moved and vibrated. In that car, I could feel the effects of wind gusts and potholes much more than I could in our heavier family car.

One morning, sensing much greater vibration than normal, I stopped to see what was going on. Opening the engine compartment, I saw that the generator was no longer firmly attached; the steel strap that clamped it to the engine block was cracked almost in two. It was a classic example of metal fatigue, and the strap had to be replaced.

Not being near an auto supply store, I went into a hardware store to get something to make a quick fix. I bought some straps for securing an exhaust duct to a clothes dryer and used one on my car’s generator and forgot about it.

Soon the noise returned, with a vengeance. The makeshift clamp had broken after just two weeks of use. I replaced the broken clamp with a new one and vowed to visit an auto parts store that weekend to get a proper replacement.

When a fatigue crack was found in a part of the Bay Bridge over Labor Day weekend, a quick fix was devised and installed so that the bridge could reopen as soon as possible. The makeshift clamp was designed to supplement the cracked part, sort of like providing a pair of suspenders to help a steel belt hold up the bridge.

Just two months after the fix, the 5-ton pair of suspenders broke free and fell onto rush-hour traffic. Some vehicles were damaged, but fortunately no one was injured. The bridge remained closed for almost a week, while a more substantial repair was made. Plans were to inspect the new bridge fix daily.

We sometimes take shortcuts maintaining, operating, and repairing our own machines, risking our lives and the lives of those who rely upon us. But we do not expect those in charge of large public machines to take shortcuts that place at risk the people who trust those machines to be in good working order. We expect and we deserve more from the custodians of our infrastructure.

Henry Petroski is the Aleksandar S. Vesic Professor of Civil Engineering and a professor of history at Duke University. His newest book, The Essential Engineer, deals with how scientists and engineers differ in their approach to solving global problems.

 

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