This Star-Tribune story on bridge inspection is very good. It first explains how bridge inspections are done: mostly with a
hammer. Turns out it's more of an art than a science. Which is probably why...
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In a 2001 Federal Highway Administration test, only 4 percent of inspectors detected a hidden flaw on two bridges.
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Yuck. But page two: maybe it's not Iraq funding, but Vietnam funding that caused this one. Can't have guns, butter AND infrastructure? The key stuff:
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Schwartz, who closed a number of bridges in New York City in the late '80s, wasn't surprised to learn that the 35W bridge was built in 1967.
"The worst period of bridge building is after World War II and especially in the 1960s," he said. In the late 1800s and early 1900s, engineers were extremely cautious about bridge design, doubling the strength needed to support the deck, then doubling it again, he said.
"But after World War II, we had much finer calculations," he said. "And we believed we were overbuilding [safety features]. The belief was we could build them sleeker and save money and build them with much lower safety factors."
Many bridges from that era weren't built to ensure that the structure would hold up even if one aspect failed. The flaw was exposed when some of those bridges collapsed in the 1970s.
"Nobody builds bridges like that anymore," Schwartz said.
In the 1960s, Pearson said, bridge builders didn't consider metal fatigue a major threat. He said the concept can be understood by bending a paper clip back and forth until it breaks.
"With steel, you can actually predict and calculate how many bends it will take to do that," Pearson said.
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