As canoeists paddled below, state bridge inspector Eric Evens stood in a cherry picker next to the two-lane steel bridge over the St. Croix River near Scandia on Friday, eyeballing rust, cracks, bolts and rivets.
Computer-aided design and other innovations have changed the way structures are built, but bridge inspections haven't changed much over the years. In this high-tech era, the trained eye and the rap of a hammer to listen for the sound of bad metal are still considered among the best ways to examine bridges.
Many inspectors insist that the great majority of bridges are safe and that their work is thorough. But others say the failure to predict last week's catastrophe in Minneapolis points to flaws in the system and the need for better technology to detect problems.
The aging of the country's infrastructure means inspectors have more aging spans to keep up with than ever. And even inspectors, specially trained and certified, can miss a serious problem.
In a 2001 Federal Highway Administration test, only 4 percent of inspectors detected a hidden flaw on two bridges.
"This accident in Minneapolis shines a light on the one-quarter of U.S. bridges that need major repair or replacement, and I don't think the American people were ready to hear that," said Greg Cohen, president of the American Highway Users Alliance.
About 12 percent of the nation's bridges have been labeled as structurally deficient.
Samuel Schwartz, formerly the chief engineer overseeing bridge safety for the New York City Department of Transportation, said: "I wouldn't want more than 1 percent of bridges being structurally deficient. If you have to keep your eye on 12 to 15 to 20 percent of your inventory that is in that condition, that's hard."
In general, Schwartz said, most bridges are safe, but "that's not good enough when one of your bridges fail. When it comes to things like bridges, especially big bridges, you have to get that probability down to 1 in a million."
The best way to do that is to improve inspections, according to Barry Sweedler, a 31-year safety director with the U.S. National Transportation Safety Board who now runs a California safety consulting firm. "I'm not totally confident that the inspections themselves are as thorough and the reports as meaningful as they could be," he said.
Qualifications vary by state, but inspectors generally are trained engineers or technicians who must have state certification. They also must update their status periodically.
"You don't just put anybody out there," said Leon Pearson, a retired St. Paul bridge engineer.
David O'Longaigh, a structural engineer who oversees 157 bridges in Portland, Ore., said his staff includes 20-year veteran inspectors who "know our bridges intimately like they are our children."
On a typical inspection, the engineer or technician will walk the bridge, looking for cracks, rust and wear and tear. Findings are compared with earlier reports, then documented on a standard federal form. In addition to tapping a hammer against concrete or metal, inspectors sometimes drag chains across decking to listen for "hollow sounds" caused by cracks or weakness, Schwartz said. They also bore samples for microscopic looks at potential flaws.
But largely, the exams are visual and "that's not good enough," he added. "Inspections cost so little. A standard inspection is $4,000 or $5,000 on a simple span."
Schwartz and William Schutt, a corrosion engineer and president of Pennsylvania-based MATCOR Inc., would like to see more technology deployed.
"I think highway engineers are very careful and scrupulous, but I don't know if we're using all the latest technologies and all the best experts to do all this," Schutt said.
He added that inspections should be tailored to different areas instead of a single federal standard. "We're applying one standard and one testing method to all structures, and bridges in Minneapolis operate differently than those in Tampa."
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."
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