When a 53-ft long, 60-ton concrete beam came crashing down onto Interstate 70 in South Strabane, Pa., on Dec. 28, 45 years of corrosive road salt draining onto one side of the overpass and trucks frequently hitting its underside were ruled as probable causes after Pennsylvania Department of Transportation (PennDOT) engineers examined the site.
“We believe it was a combination of age, wear and tear in the structure, a history of being hit by trucks and, very recently, another hit” that took its toll, said Gary Hoffman, a PennDOT civil engineer. “The bridge slopes toward the beam and the effect of road salt is one thing we’ll be looking at in a forensic analysis.”
Seven other concrete beams, which sit on piers to create a two-lane overpass for a local road, Lake View Drive, remained intact above the eastbound side of I-70, the Pittsburgh Post-Gazette reported.
In addition to removing the beam, crews also had to patch a big dent left on the middle of the asphalt road surface where it hit, the Post-Gazette reported.
According to the paper, a woman and her two children suffered minor injuries when their minivan crashed into the beam. Two people in a second car damaged by the falling concrete mass suffered minor injuries from broken glass.
The incident closed a three-mile stretch of I-70 in both directions between the I-79/70 South Interchange at Milepost 21 and 84 Interchange at Milepost 24. Because it occurred just east of the I-79 interchange, traffic on I-79 was not affected, the Post-Gazette reported.
“We’re fortunate more people were not injured,” PennDOT District 12 Executive Joe Szczur said. According to Szczur, traffic was somewhat lighter than usual because of the lull between the Christmas and New Year’s weekends.
Heavy equipment was brought from a near-by construction site to clear the beam that snapped near its centerpoint, the paper reported.
PennDOT plans to award an emergency contract to remove the remaining beams sometime next month, and it may be a year before the overpass is rebuilt, the Post-Gazette reported.
The overpass was built in the late 1950s and opened in 1960 as part of an economic development highway, linking Washington, Pa., and Greensburg.
The bridge last underwent a federally mandated, biennial inspection in March 2004 and an interim inspection five months ago, the Post-Gazette reported.
Both inspections noted “some distress and deterioration,” Hoffman said, but nothing to raise a flag. The report indicated some corrosion and indications that the bridge had been hit at times [by trucks],” he said.
On a scale of 1 to 10, with 10 being excellent condition, the overpass received a score of 4, meaning that the bridge was “structurally deficient,” Hoffman said. However, the overpass was judged structurally capable of carrying 40-ton trucks, the maximum legal load in Pennsylvania, the Post-Gazette reported.
The fallen beam, and one of the beams that remained intact, will be taken to a site and put through a series of special tests, including load-bearing capacity, road salt infiltration and the amount of corrosion on the steel cables used for tensioning and the old-style steel reinforcement bars, the newspaper reported.
The concrete beams in the overpass were designed to last 40 to 50 years and are nearing the end of their expected life, according to the Pre-stressed Concrete Association of Pennsylvania.
“That does not mean the bridge is no longer useful. It simply means you need to pay more attention to it,” said Hank Bonstedt, executive director of the Allentown, Pa.-based association.
He said it is not unusual for a tall truck “to bounce and hit something,” like the overpass beam, and set off stress dynamics that could possible bring down a beam whose internal re-bars and cables—not visible to bridge inspectors—have suffered extensive corrosion, the Post-Gazette reported.
When the cause becomes final, PennDOT will examine other old overpasses of similar design and function on I-70 and then elsewhere in the state, the newspaper reported. Most sit perpendicular to highways and compared to the skewed angle and unique features of the Lake View Drive overpass.
