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Bad era for construction
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.
"Bridges were so massive and the actual bounce and flex back and forth is so slight, it wasn't considered a severe issue. And then we had some failures on the national level and said: 'Oh, my God, this is an issue.' "
Three people were killed in 1982, nine months after an inspector failed to find fatigue on the Mianus River Bridge in Greenwich, Conn. Five years later, a bridge near Amsterdam, N.Y., with a clean safety report fell and killed 10. Inspectors hadn't checked the concrete footings, which had been scoured in a flood.
Loads have doubled
Schwartz said increases in traffic flows, particularly heavy trucks, have produced stresses that were never envisioned.
The number of cars and trucks passing over interstate bridges has doubled since the system was completed in the early 1970s. The I-35W bridge, at 141,000 vehicles a day, was Minnesota's busiest.
"Part of our problem as a country is that we've substituted trucks for a lot of freight rail lines and then made our trucks into trains," Schwartz said. "You'll see these tandems going down the road and they are enormously heavy and they are bouncing and moving at high speed. And each time they are doing that they may be adding a tiny bit to that microcrack [in a bridge]. Our interstate system was never designed to handle these kind of loads."
Said Pearson, the former St. Paul bridge engineer: "If you take steel, no matter how thick, and change the number of vehicles and trucks that give you the most flex ... that increases those flexes tremendously and decreases the life of the bridge's critical members."
Those threats suddenly have a new urgency.
"We regulate by counting tombstones," said Sweedler, the ex-NTSB safety director. "Now that we have this terrible tragedy, everybody is coming out of the woodwork to fix the problem people have known about for years."