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St. Paul city officials say the bridge was originally slated to be replaced in 2004. A MnDOT plan calls for replacement between 2015 and 2023, the same period during which the I-35W bridge was to be replaced.

Dorgan said that, unlike the I-35W bridge, Cayuga is not a fracture-critical bridge. It has redundant beams, meaning that the failure of one part would not necessarily lead to complete collapse. The main concern with Cayuga, he said, is the bridge's deck.

"The worst that can happen is you punch a hole in the deck and you have to shut a lane down," Dorgan said.

Why has the Cayuga not yet been replaced?

In a word, money. Five years ago Elwyn Tinklenberg, then MnDOT's commissioner, announced that 163 projects during the next decade would be deferred because of a lack of funds. One of those projects was the Cayuga bridge.

Across the country, the public is scrutinizing bridges' sufficiency ratings -- a federal highway administration measure for funding bridge projects. Not just a measure of safety, the ratings take into consideration factors such as the amount of traffic a bridge carries and the length that a detour would require.

"It's a composite of all kinds of different issues," Dorgan said, adding that a bridge with a lower sufficiency rating could actually be in better structural condition than one with a higher rating.

There's no point on the sufficiency scale that warns of imminent collapse, Dorgan said. If engineers determine that a bridge is unsafe, MnDOT would close it, he said.

Bridge inspectors look for deterioration by measuring metal thicknesses after corrosion, cracks and chips in concrete and cracks caused by metal fatigue. Inspectors who find cracks in key locations would call an inspection engineer and possibly shut the bridge down, Dorgan said.

As a bridge deteriorates, highway officials will restrict heavy loads from crossing it, he said. That may mean keeping extremely heavy trucks off first, and possibly all traffic later. In the meantime, transportation officials often plan for the bridge's repair or replacement.

Fracture-critical bridges are inspected more often and are watched more closely, Dorgan said.

Facing the heat

Closing a bridge can be an unpopular call.

Samuel Schwartz faced scorn from merchants, commuters and the mayor when, as chief engineer overseeing bridges in New York City in 1988, he closed the Williamsburg Bridge between Manhattan and Brooklyn.

"You have to have a lot of backbone," he said. "You're gonna catch both political heat and public heat."

It got so bad, he said, that he spent part of a day watching the scene in "To Kill a Mockingbird," where Gregory Peck wipes spit off his face instead of fighting back.

In Minnesota, when Richard Braun was transportation commissioner in 1984, he shut down the nearly century-old St. Paul High Bridge because nobody knew "when it might collapse into the Mississippi River," according to a newspaper report. The bridge was later replaced.

"To this day I have no idea how I had the guts to do this all by myself," Braun said.