PITTSBURGH -- A girder slipped off a rocker bearing that was connected to a support pier, causing one deck on the Birmingham Bridge to drop several inches and the entire bridge to be shut down, according to the Pennsylvania Department of Transportation.

The pier that supports the affected part of the bridge has shifted since Friday morning.

The bridge rockers are little blocks that are wedged between the bridge's concrete piers and metal spans, all the way across the bridge. In this case, the span fell off the rockers, said Dan Cessna, of PennDOT.

"What happen is, over the course of the night, the span dropped and came down off of that rocker," said Jim Struzzi, of PennDOT. "All the rockers."

Struzzi said the rockers support beams and allow for some movement so the bridge can flex with the traffic and the wind.

But the span is now sitting flush on the pier.

"What concerns us is we don't know if the pier shifted and caused the span to slip or if the span fell and caused the pier to move," said Struzzi.

Equipment is being brought in to lift the span off the pier, but that process is expected to take several days.

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But PennDOT inspected part of the bridge last month, so why wasn't the problem found?

PennDOT said it's been almost two years since the Birmingham Bridge last got a full inspection. Inspectors found no critical problems. But then, about a month ago, PennDOT sent out inspectors for an unscheduled look at the bridge.

And their boss said there's a good reason why they didn't find any warning signs of Friday's deck shift.

"This bridge is currently due for its two-year inspection this June," said Cessna. "June of 2008."

He said the area that was looked at a month ago had nothing to do with the area in question that caused the bridge to shift.

PennDOT considers the Birmingham to be two separate bridges, with two separate inspection schedules. The steel arch bridge over the Monongahela River is one span. The second is the section that traverses the Parkway, Second and Forbes avenues.

That's where the deck shift happened.

"This area was not re-inspected at that time," said Cessna.

Team 4 discovered that the number of bridges in Allegheny County listed by PennDOT as structurally deficient has increased from 238 in August to 346 bridges in the latest report.

But neither section of the Birmingham Bridge is on the list.

But how close of a call was Friday's shift?

"It's certainly a severe issue," said Cessna. "We took immediate action to close the structure. It's still standing. If traffic would have continue to move over it, I don't know."

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The bridge, which crosses the Monongahela River to link Oakland with the South Side, is closed indefinitely while repair crews work on the problem.

The on-ramps from Fifth Avenue and Forbes Avenue are also closed.

Drivers are advised to use the Hot Metal or Glenwood bridges as alternate routes to enter the South Side.

Port Authority bus routes are also affected by the bridge closure. Go to www.portauthority.org for the latest information.

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