State highway inspectors will begin top-to-bottom examinations of the half-dozen Colorado bridges with designs similar to that of the Minneapolis bridge that collapsed last week.

Lacking any real conclusions yet about how the steel deck truss structure of the Interstate 35W bridge over the Mississippi River failed in Minnesota, engineers will go over the six bridges - including four owned by local governments - without benefit of knowing any particular flaws.

The Colorado bridges that use the steel deck truss design are considered to be in good shape.

"These things don't fail very often, if ever," said Jeff Anderson, chief bridge inspector for the Colorado Department of Transportation, who plans to be in southern Pueblo County this morning to start on a bridge on County Road 273 over Greenwood Creek, south of Colorado City. "I can't even think of an example of one failing before this."

So far, CDOT sees no cause to alter any of its bridge design procedures based on the sketchy information out of Minnesota. The state hasn't built a steel deck truss bridge in ages - only two of the six belong to CDOT, out of more than 3,700 bridges in the system.

But whatever caused the failure in Minnesota could be a factor that applies to other bridge designs, whether things such as metal fatigue, corrosion - especially from salt-based deicing solutions used on the highways - or effects of vehicles.

"Information could come out of the Minneapolis incident that could lend itself to other structure types, but to this point no information has come out of that incident," said Mark Leonard, CDOT's chief bridge engineer.

Steel deck trusses, which have the steel support assembly below the deck of the roadway rather than above, are rare in Colorado because they are generally used to span long distances. Colorado's smaller streams and rivers don't call for such designs.

Anderson said that his crew will inspect the Union Avenue structure Thursday, and another in nearby Penrose by Friday. Bridges in Red Cliff and near Buena Vista will be inspected next week, and the remaining one in Mineral County will be after that.

Neither of the state-owned bridges, in Penrose and Red Cliff, is considered structurally deficient. The Penrose structure rates 50.5 on a scale of 100, with 50 or below being deficient. The Red Cliff bridge is rated 80 or above, the highest classification.

Richard M. Gutkowski, professor of structural engineering at Colorado State University, said that truss bridge designs are generally reliable because they have redundancy built into them. If one part of the truss fails, the load it bears can be picked up by the rest of the structure.

But that cuts both ways. An increasing load on other parts of the structure can help lead to more failure. The Minneapolis bridge is an example of a progressive collapse in which the failure of one section helped to bring down the next, he said.

"The saving grace is the redundancy in this bridge was, overall, a positive thing," Gutkowski said. "The presence of the deck with steel girders under it meant that it did not fully go down everywhere. There were vehicles that did not go into the water."

Colorado spans

Six steel deck truss bridges in Colorado

Marsh Bridge over Rio Grande, Mineral County

Pueblo County Road 273 over Greenhorn Creek, Colorado City

Water Street Bridge over Eagle River, Red Cliff

CO 120 bridge over Arkansas River, Penrose

U.S. 24 -bridge over Arkansas River, south of Buena Vista

Union Avenue bridge over Arkansas River, Pueblo