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."