Rocker bearings, which allow movement of a bridge superstructure as it sits on piers, were not functioning when part of the Birmingham Bridge dislodged and dropped about eight inches two weeks ago.

The devices, which function in a way similar to an ankle and foot, were "frozen" on the southbound span instead of rotating normally, creating enormous stresses that contributed to the failure.

Pennsylvania Department of Transportation officials have indicated the condition was a probable cause of the 2.1 million-pound span literally going off its rockers and falling onto Pier No. 10 along Second Avenue.

The T-shaped pier also has shifted and will have to be reinforced or replaced this year.

PennDOT District 11 engineer Dan Cessna refused to speculate whether the quick sequence of events involving a mass of more than 1,000 tons of concrete and steel could have toppled the pier and sent the span crashing to the ground.

"I don't know if there was enough force, so I don't want to offer judgment," he said. "[But] these spans are not tied together, so there's no chance the whole bridge could have been come down" like the Interstate 35W bridge that collapsed in Minnesota last summer.

When bearings freeze and bridge superstructures expand, contract and otherwise flex to account for temperature changes and varying traffic loads, something eventually gives.

The rocker bearings are supposed to move or "rock" in one direction when the weather is warm and in the opposite direction when the weather is cold.

PennDOT has hired a special bridge consultant, Harrisburg-headquartered Modjeski and Masters Inc., to determine whether the frozen rocker bearings failed and caused the pier to shift when the span dropped, the likely scenario, or whether the pier shifted first.

"I won't venture a guess at this time, but what happened is extremely unusual," Mr. Cessna said. "It could have been a combination of events."

PennDOT has scheduled a news conference today to announce that March 3 is the target for restoring two lanes of traffic -- one in each direction -- on the southbound span, which currently is being repaired by crews working two 10-hour shifts, seven days a week.

The independent but parallel northbound span, which is intact, nonetheless will remain closed for several months for modifications and retrofitting with a different style of bearing. Rocker bearings are seldom used in modern bridge construction.

At six lanes wide, the tied-arch, 32-year-old Birmingham Bridge over the Monongahela River is one of the biggest in Western Pennsylvania. It was the only project ever built as part of what was supposed to have been an inner-city beltway between the Mon Valley and Route 28.

The bridge now connects East Carson Street on the South Side with Forbes and Fifth avenues in the vicinity of Uptown, Soho and South Oakland.

An average of 23,000 vehicles a day have been diverted to the 10th Street Bridge and other routes during the closure. Six Port Authority bus routes have been detoured.

PennDOT has advanced $1.6 million under an emergency contract with West Mifflin-based Trumbull Corp. to make repairs, although Mr. Cessna indicated the final cost could grow several times higher if Pier No. 10 must be replaced. The land pier is 50 feet high, 5 feet deep and 72 feet wide at the top.

Mr. Cessna said rust and corrosion of the rocker bearings were noted in a July 2006 bridge inspection. Because the condition was considered "moderate" at the time, the bearings were not lubricated or reset as part of a $3 million rehabilitation of the bridge last year.

He said lubricating the rocker bearings "would not have been out of the question, considering these were rusted," but that's not a normal maintenance procedure.

"The project was a preservation project, to stop the expansion dams from leaking and to patch the potholes in the bridge deck," Mr. Cessna said. "Had this been a full rehabilitation project, similar to what we performed on the 31st Street Bridge, these bearings would likely have been included."

Structural ratings for the bridge superstructure also weren't cause for alarm although, since the incident, PennDOT hurried to inspect similar bearings on the Interstate 79 tied-arch bridge over the Ohio River.

A similar bridge failure occurred July 27, 2005, on an expressway ramp in Albany, N.Y. A subsequent investigation blamed frozen bearings -- "loss of function leading to unintended forces on substructures" -- for causing the devices to over-rotate until they tipped and the girders that they supported collapsed onto a pier.

Mr. Cessna said findings by Modjeski and Masters could be used to revise future bridge inspection procedures and will be shared with the Federal Highway Administration.

The southern approach span that dropped is approximately 150 feet long and 70 feet wide, one of 19 spans that make up the Birmingham Bridge.

Its nine primary I-beams sit on nine rocker bearings. Counting adjoining spans, a total of 21 rocker bearings are being replaced. Workers are building temporary shoring on both sides of Pier No. 10 to support the span for traffic after March 3 and while engineers draw up a permanent solution.

On the adjacent northbound span, a total of 19 rocker bearings will be replaced as a precaution because they, too, are rusted and corroded.

"A lot of these type bridges from yesteryear are functioning relatively well and were built with a lot of redundancy," Mr. Cessna said. "If there's something different we should know or do, we want to know and do it."

He praised engineers and trades workers toiling in adverse weather and suppliers providing materials on short notice for PennDOT to expedite repairs, calling it "an amazing effort."

The emergency closure of the Birmingham Bridge began about 3 a.m. Feb. 8 with a call to 911 by an unidentified driver who noticed a problem.