Fallen section closed traffic on busy interstate highway outside Pittsburgh. (Photo courtesy of AP/Wideworld)

Pennsylvania Department of Transportation inspectors are evaluating scrapes on the underside of a highway overpass that partially collapsed Tuesday night outside Pittsburgh, dropping an 80-ton section onto Interstate 70 and injuring three motorists. The scrapes may be evidence that trucks contacting the underside of the aging bridge contributed to the failure.

The bridge section, consisting mostly of concrete guardrail and part of the deck, weighed about 80 tons, says PennDOT District 12 Spokesman Jeff Ofsanik.

The place where the deck section failed is its lowest point above Interstate 70, 14 ft 11 in., says another PennDOT spokes person.

Trucks in the area are limited to 13 ft 6 in. in height and anything higher would require a special permit.

PennDOT is preparing specifications for an emergency replacement of the overpass.

Two vehicles struck the collapsed section. A woman and her two small children suffered minor injuries when their minivan crashed into the section Tuesday evening. A second car was damaged and two people in it were cut by broken glass when pieces of the falling concrete hit their car, say state police.

State officials closed a three-mile section of I-70 in both directions for most of Wednesday. About 33,000 vehicles travel that section each day, according to PennDOT officials. Traffic resumed in both directions Wednesday evening after crews removed the debris and patched the highway.

The overpass served mostly local traffic, about 500 vehicles a day. It will be dismantled and replaced “for safety’s sake,” Ofsanik says.

Ofsanik says the collapse is highly unusual and that so far no cause could be determined. “We are concerned about the bridge. Just looking at it, [inspectors] couldn’t tell what happened,” he says.

At 45 years of age, the overpass had reached the end of its service life, state officials say. Among the causes being evaluated are

weather and age-related deterioration and the possibility that trucks had made repeated contact on the underside of the bridge. “They did see signs of scraping on the underdeck,” Ofsanik says.