The overall rating of the bridge was almost identical to that of the Minnesota expressway bridge that collapsed in August, killing 13 people.

The I-95 findings were not considered serious enough at the time to require immediate repairs. Engineers for the Pennsylvania Department of Transportation believed they could wait for planned repairs this summer and reconstruction by 2012.

But an unscheduled re-examination by an inspector Monday showed that things had quickly changed. The bridge was not scheduled to be inspected again until October 2009.

"It was obvious that things were happening and things were changing quickly," said Charles Davies, assistant district engineer for design in PennDot's Southeastern District. He reviewed photos of the column about 5 p.m. Monday, and within an hour, PennDot decided I-95 had to be closed.

The decision to close the highway immediately was prompted by the "significant change from October to today," Davies said.

The steel-and-concrete viaduct that carries I-95 over Richmond Street is one of at least 15 "structurally deficient" bridges on I-95 through the Philadelphia area and among 57 such bridges that carry at least 25,000 vehicles daily in the eight-county region.

Engineers grade bridges according to the condition of their primary structures. Those in poor or serious condition are of less concern to engineers than those graded as "critical" or "imminent failure." The last designation requires a bridge to be closed.

The substructure of I-95 in the closed area was given a "4" (poor) on a 1-9 scale at its last biennial inspection on Oct. 2, PennDot officials said.

They declined to release the inspection report, citing "risk management" and concern about lawsuits.

The deck of the viaduct and the superstructure were each rated "5" (fair) and the overall sufficiency rating was 51 on a scale of 100. A bridge is deemed structurally deficient if one of its three components is rated below 5. A sufficiency rating below 50 qualifies a bridge for federal replacement funding.

The I-35 bridge that collapsed in Minnesota on Aug. 1 had a sufficiency rating of 50 and a deck score of 5, a superstructure score of 4, and a substructure rating of 6.

"Structurally deficient" does not necessarily mean unsafe, PennDot engineers say.

Pennsylvania has the most structurally deficient bridges in the nation, more than 6,000.

And that tally is likely to rise as the many bridges built in the highway boom of the late 1950s and 1960s reach the end of their expected lives.

PennDot engineers say they are balancing costs and urgency as they try to calculate which bridges must be fixed and which can wait. But it's an inexact science, they acknowlegde.

"We'd be lying if we said this was the first close call we've had on 95," Davies said yesterday.

"No doubt, there are other locations on 95 that would rate poor condition, and yes, there are a lot of structurally deficient bridges on 95," he said. "It's a constant challenge to keep up with reconstruction."

Davies said the worsening condition of bridges like the I-95 viaduct was a result of a "lack of overall resources put into infrastructure. We're making the best calls we can. But we may not always be right."

The stretch of I-95 between Spring Garden Street and Cottman Avenue is scheduled to be rebuilt by 2016, as it contains the largest number of structurally deficient bridges and the highest traffic loads.

But south of that, between the Girard Point Bridge and Washington Avenue, and north of that, in Bucks County, the bridges are not scheduled to be replaced.

To fully repair and rebuild the 51 miles of I-95 in Pennsylvania would cost about $5.9 billion, PennDot officials estimate.