Investigators have begun sifting for clues to the cause of a bizarre fatal bridge accident in Colorado in a process expected to take six to eight months.

DANGER AHEAD Girder temporarily braced by adjacent overpass flipped over and sagged into active lanes below. (Photo by Marc Williams)

On May 15, a bridge girder that had been erected over the eastbound lanes of Interstate 70 west of Denver fell on its side and sagged into the path of a sport-utility vehicle, killing its three occupants, all members of one family. Construction work was not in progress at the time of the accident. The girder had been erected four days earlier, but subsequent scheduled work had been suspended because of bad weather.

Asphalt Specialties Co. Inc., Henderson, Colo., is the prime contractor for the $12-million project to improve the interchange between Colo. 470 and I-70 in Jefferson County. The project is adding three girder lines fabricated by AFCO Steel, Little Rock, to widen the C-470 overpass, built in 1988, from between 50 ft and 62 ft to 80 ft to accommodate one additional lane of traffic and extend a merge lane.

Subcontractor Ridge Erections, Arvada, Colo., on May 11 set the first girder, consisting of two segments totaling 28 tons. It picked each piece with separate cranes and spliced the steel in the air with bolted plates to create a 153-ft span. Subsequent construction was to have added a third segment across the westbound lanes of I-70 to the other two girders. The sequence was left to the contractor’s option, but Mark Leonard, state bridge engineer, does not know what it was.

Five angle brackets held the girder in place, with one bolt in the girder and one to three bolts in the top of the deck of the adjacent overpass, Leonard says. But he doesn’t know details. “The contract documents do not specify the means or methods of temporary support,” he says.

Rainy weather that threatened to become snow caused the suspension of work scheduled for the rest of the week, but winds were not high. “To observers, it was...