The Pennsylvania Department of Transportation may have inadvertently collapsed one of its own bridges, a 116-year-old structure in a rural part of Cambria County.

The one-lane, 103-foot-long steel truss structure began buckling and falling into Clearfield Creek behind a PennDOT truck equipped with a snow plow as it was headed toward the northwestern end at 9:20 p.m. Thursday.

Movement of the bridge caused an edge of the plow to strike an abutment, break loose and be pushed to the side by the truck's front bumper as the unidentified driver safely made it to state Route 1012, known locally as School House Road in Dean Township, near the village of Dysart.

PennDOT District 9 Executive Thomas Prestash acknowledged that the truck, plow and load of salt and cinders weighed "very near" a total of 12 tons, the legal load limit for the bridge. Whether the gross vehicle weight of the truck exceeded the posted limit and caused the collapse is part of an internal investigation and "something I don't want to comment on at this time."

The truck driver has not been suspended, but was not available to explain his experience on the bridge.

Mr. Prestash said the southeastern end of the span near Route 53 remained on its abutment but the rest was "kinked up in the middle of the creek." The snow plow that became detached from the truck fell to a creek bank.

PennDOT dispatched a special team of bridge inspectors to the site at 3 a.m. yesterday. In addition to looking at the weight of the state truck that was patrolling roads in search of icy spots, the engineers are investigating whether cold temperatures may have made the old steel more brittle and incapable of bearing the load.

The bridge carried an average of 270 vehicles a day, but no school buses. Local fire companies and other emergency responders mapped out other routes some time ago.

PennDOT has posted a nine-mile detour, but closer alternatives are available in Asheville and Condron.

The incident took place 12 hours after PennDOT Secretary Al Biehler and other local and state officials broke ground for a new Point Marion Bridge over the Monongahela River between Fayette and Greene counties. In a news release, Mr. Biehler said, "Replacement of this 77-year-old bridge is another example of how we are focusing on bridge repairs and replacements in an effort to reduce the number of structurally deficient bridges."

Mr. Prestash said the collapse of the Dysart Bridge is typical of the challenge that PennDOT faces with 6,000 of its 25,000 bridges classified as structurally deficient and/or functionally obsolete. Fifty-four are closed and 800 have weight or lane restrictions.

The Dysart Bridge, the oldest in Cambria County, fell into both categories.

"A while back, they lowered the weight limit to 3 tons and couldn't drive a snow-plow across it," Dean Township Supervisor Ted Rudalski said. "One PennDOT truck plowed and ashed one side and a different truck plowed and ashed on the other side."

After repairs were made, mainly stringing steel cables to add support, the 3-ton weight limit was lifted and the current 12-ton limit was imposed. The last bridge inspection was made in July; inspectors found no evidence to recommend lowering the weight limit again.

Recently, PennDOT has been drawing up plans to tear down the Dysart Bridge and replace it with a new, concrete structure.

"We were anticipating construction starting in 2009," Mr. Prestash said. "Obviously, this has changed our plans and the time frame."