PROVIDENCE — At the urging of a federal agency, transportation officials across the nation have been scurrying to check out hundreds of “steel deck truss bridges,” the kind that collapsed into the Mississippi River in Minneapolis last Wednesday.

The Federal Highway Administration’s tally of 756 of them counts 6 in Connecticut and 19 in Massachusetts, but lists zero for Rhode Island, and the state Department of Transportation said there weren’t any in Rhode Island.

Yesterday, the DOT acknowledged that there is such a bridge in the state, and a prominent one at that: the Sakonnet River Bridge that carries Route 24 from Tiverton to Portsmouth. Kazem Farhoumand, the DOT’s deputy chief engineer and head of the agency’s design section, said that two of that bridge’s three main spans — the ones on either side of the central arch — are steel deck trusses. There are similar spans in other bridges, he said, notably the Newport Bridge.

The DOT yesterday released a list of 37 “fracture critical” bridges. They are to get a closer look, either through a review of inspection reports or, in 14 cases, new inspections. Another seven were inspected either last year or this year, the DOT said.

Farhoumand said, however, that there are two critical differences between the Sakonnet River Bridge and the bridge that fell in Minneapolis. One is the way the bridges’ parts are held together. He said the Sakonnet bridge, built in 1957, is held together with thousands and thousands of rivets, which were used before welding technology “improved” enough to use on bridges.

He said he suspected that the Minneapolis bridge, built in 1964, was welded together. That’s an important difference, he said, because welding is more likely to lead to cracking than the other choices, rivets or the tens of thousands of bolts that hold the new Providence River Bridge together.

Although it’s not clear yet why the Minneapolis bridge collapsed, a recent Minnesota Department of Transportation inspection report bears out Farhoumand’s theory. Many of the major components were welded together, and inspectors found numerous cracks in the welds, according to a June 2006 report.

By contrast, “There are no welds that we know of” in the Sakonnet bridge, according to Anthony Rotondo, an engineer who manages the Federal Highway Administration’s bridge program in Rhode Island.

Another factor, Farhoumand said, is the mass of the bridge. “There’s a lot more substance in ours,” he said, comparing pictures of the Sakonnet and Minneapolis bridges. The Sakonnet is “beefier.”

The Sakonnet bridge’s main structural elements are all rated “poor,” according to the administration. Work is beginning to replace it with a new bridge.

Farhoumand described the DOT’s efforts in the face of more poor inspection findings.

A spring inspection showed significant corrosion in several beams running across the bottom of the truss section closest to Tiverton, he said. The corrosion went far beyond a coating of rust, he said, in some cases causing holes through parts of the beams. The corroded sections of the beams “were all over the place,” he said.

When that amount of damage is found, he said, engineers calculate the loss in strength based on the amount of steel missing. They base their calculations on the weakest beam, the one most likely to fail.

“We immediately lowered the tonnage of the bridge,” he said, to 22 tons on June 29. That put the limit not far above the weight of a loaded school bus.

The DOT also started roughly $500,000 worth of emergency repairs, which involve bolting steel plates onto the beams where they have rusted away, to replace the lost strength, he said.

Rotondo said he suspects that the Sakonnet’s omission from the federal tally was a function of the way the administration keeps track of bridges in its National Bridge Inventory. The agency may have searched only the bridges’ principal design element, which didn’t reflect the bridge’s deck truss spans, he said.

The episode illustrates the complexity of bridge design. Farhoumand pointed to the Newport Bridge, primarily a suspension bridge, which also has several spans supported by trusses.

The Sakonnet bridge actually has three kinds of spans: the arch in the middle, the two deck truss spans on either side, and then several more spans based on girders continuing to shore on each end.

Girders have their own problems, Farhoumand said. Like trusses, they lack “redundancy,” the ability to stay up even when something fails. If a bridge is supported by only two girders, the failure of one will bring it down.

In the end, Rotondo said, the state DOT’s energetic response to the Minneapolis collapse makes the omission of the Sakonnet bridge from his agency’s listing unimportant, because the DOT is going “above and beyond” his agency’s recommendations to state officials in how to respond to the Minneapolis collapse.

blandis@projo.com