Initially, only the southbound lanes of the Birmingham Bridge were closed to vehicles yesterday morning as inspectors tried to determine why a portion of an approach ramp had dropped by nearly 8 inches.

To be on the safe side, the Pennsylvania Department of Transportation several hours later closed the northbound lanes until a complete inspection could determine that the independent inbound span hadn't incurred any collateral damage.

And then, because of growing concerns about what had caused such an unusual occurrence, even pedestrians and bicyclists were no longer allowed to use the span connecting Fifth and Forbes avenues, Uptown, with East Carson Street on the South Side.

Officials weren't sure how long the City of Bridges would have to get along without use of a span that carries an average of 23,000 vehicles daily.

But PennDOT said the entire span would be closed to vehicles and pedestrians through the weekend and into next week, at least.

"We don't know what is occurring with that bridge and until we know that it's isolated, we're not going to allow anyone to use it," said PennDOT spokesman Jim Struzzi.

The emergency closure of the 32-year-old bridge over the Monongahela River began about 3 a.m. with a call to 911 by an unidentified person who apparently noticed a problem. Officials speculated it was a motorist who either felt a bump where the roadway had dropped or saw how the Jersey barrier in the center of the southbound lanes was 7³/₄ inches below that of the northbound lanes. On the other side, the drop was much smaller, but still significant -- 2¹/₂ inches.

At a news conference on the bridge before it was closed to pedestrians, PennDOT District 11 Executive Dan Cessna said a preliminary inspection indicated a girder supporting the approach span had slipped off a "rocker bearing" and slid down onto the pier cap.

A rocker bearing is a brick-sized steel bridge support placed between a pier and girders to accommodate expansion and contraction of the superstructure through a rocking action. The bearing is supposed to move one way when the weather is warm and another when it is cold.

Additionally, Mr. Cessna said, it appeared that the pier cap had shifted.

"Certainly we've had bearing shifts on many bridges. That's a normal occurrence. As folks know, bridges do move. It's important as temperatures change that bridges shift," he said.

But what occurred on the Birmingham Bridge hadn't been seen here before.

"This is not typical," Mr. Struzzi said. "You might have a little slippage but not this type of complete drop."

Mr. Cessna said engineers are trying to determine if significant temperature fluctuations earlier in the week caused the problem. But, he added, it is far too early to say just what happened, or why.

PennDOT began a two-pronged response yesterday. Inspectors began the laborious task of checking the entire span for failures or potential failures anywhere else. Should the northbound span be deemed structurally sound, PennDOT likely will institute a two-way traffic pattern in what is normally the northbound lanes.

PennDOT instituted two-way traffic like that last summer during routine work replacing expansion dams on the span. That work did not occur near yesterday's incident, Mr. Cessna said.

Also yesterday, a contractor began working round-the-clock to shore up the span where the incident occurred, one of 19 independent sections, including the steel tied arch over the river.

Once the cause of the slippage is determined, a plan will be formulated to repair it.

Previous inspections of the bridge by PennDOT showed it not in dire need of replacement. In August, when PennDOT released the ratings of all of the 25,000 bridges it maintains statewide, the Birmingham Bridge was above average with the approach needing more work, but not in desperate need of repair.

The bridge and its approaches were opened in 1976. Of the 1,159 bridges in Allegheny County, 184 are newer.

Though the ratings of the bridge and its approaches technically qualify it for state and federal money for renovations, it just barely makes the list. The Birmingham Bridge received sufficiency ratings of 78.1 for the bridge and 65.3 for the approach to the steel truss arch. A rating below 50 qualifies for federal money to replace the bridge and between 50 and 79 means the bridge qualifies for rehabilitation money.

The sufficiency ratings are on a weighted scale, with 55 percent of the rating based on structural condition, 30 percent on the bridge's ability to meet current traffic conditions and 15 percent for how essential it is for public use.

While the approach to the bridge had ratings of 5 for the deck, superstructure and substructure on a scale of 0 to 9 with 9 being best, it would take a rating of 4 or under to call for a weight limit or special speed limits on the bridge. The main bridge deck rated a 7, with the steel arch superstructure and substructure and the piers that hold up the steel beams all earning a rating of 6.

Because of the closure, several bus routes that travel through Oakland and the South Side were detoured, including 51E Camp Hollow, 54C North Side-Oakland-South Side, 59U South Side Works-Oakland-Waterfront, 84A Herron Hill-Oakland, 84B Oakland Loop and 84C Hill Loop.

The following bus stops for those routes are eliminated until further notice:

• 51E inbound Fifth Avenue stops at Kirkpatrick, at Moultrie, opposite Seneca, at Wyandotte, opposite Gist, at Dinwiddie, opposite Van Braam, at Pride, at Stevenson, at Magee, at Washington Place, and at Diamond opposite Boyd.

• 51E outbound Forbes Avenue stops at McAnulty, at Magee, at Stephenson, at Pride (Mercy Hospital), at Van Braam, at Miltenberger, at Gist, at Jumonville, at Seneca, and at Moultrie.

• 54C inbound East Carson Street stops at South 19th, at South 20th, at South 21st, and at South 22nd streets, along with outbound East Carson Street stops at South 22nd, at South 21st, at South 20th, and at South 19th streets.

• 84A East Carson Street inbound stops at South 22nd and at South 21st. Outbound East Carson Street stops at South 20th, at South 21st, and at South 22nd.

• 84C inbound stops include Forbes Avenue at Jumonville, at Seneca, and at Moultrie, and East Carson Street stops at South 22nd and at South 21st. 84C outbound stops include Fifth Avenue at Kirkpatrick, at Moultrie, at Seneca, and at Wyandotte, and East Carson Street at South 20th, at South 21st, and at South 22nd.

The Port Authority said riders of routes traveling through Oakland and the South Side should expect delays in departures, arrivals and travel times.

Because of the bridge closing, PennDOT is delaying until next weekend the closure of Forbes Avenue in Oakland as part of the Boulevard of the Allies bridge replacement project.