An apparent construction error six decades ago could have caused the fracture discovered Friday in a steel beam that forced the closure of the Delaware River Bridge, an engineering expert who viewed pictures of the cracked truss said Sunday.

An image of the cracked truss - a supporting piece - on the bridge that runs between Bucks County in Pennsylvania and Burlington County in New Jersey shows signs of holes that had been mistakenly drilled into the steel beam and then filled with plug welds, a typical solution in the 1950s when the bridge was built, said Karl Frank, professor emeritus of engineering at the University of Texas at Austin. His areas of study include fractures and fatigues in metal structures and welded and bolted joints, according to the university website.

The image is on the website of the Pennsylvania Turnpike Commission, and Frank viewed it there.

"What we do nowadays, we don't do that," he said. "If you have a misdrilled hole, you would just put a bolt in and tighten it up. The problem is they welded it up."

The plug welds created a weak point, Frank said, and the stress of traffic crossing the bridge continuously since it opened in 1956 could have led to a sudden crack, splitting the beam at the welds.

A spokesman for the commission, which operates the 1.2-mile-long bridge jointly with the New Jersey Turnpike Authority, did not dispute Frank's conclusion, but said turnpike engineers had too little information to confirm anything about the crack's cause.

"Don't want to make premature assumptions without science," said Carl DeFebo Jr., the spokesman.

The commission sent metal samples from the crack to laboratories at Lehigh University to be analyzed and will wait until those tests were completed before commenting on the cause, he said. Cracks like this are uncommon, DeFebo had. He said he had not seen one like it in his 18 years with the commission.

Frank, the engineering professor, said a picture of the crack clearly shows evidence of welds that line up exactly with rows of rivets in the beam. Engineers will likely recommend checking other beams on the bridge for similar plug welds. he said. The beam will likely not be replaced, he said, and a possible fix would be a bolted splice at the crack. He said the engineers working on the problem were among the most capable in the state.

"You have some of the best people in Pennsylvania in the turnpike and PennDot and some of the consultants involved with this," Frank said. "You've got some very good people."

It is unclear when the unrusted crack formed, but the Turnpike Commission and engineering experts agree it shows signs of being very recent. The bridge was inspected in 2014 and received a new paint job in 2016. The crack, beneath a westbound lane on the Pennsylvania side of the bridge, apparently was not noticed during the inspection or the painting. It was discovered Friday as an engineer employed by the New Jersey Turnpike Authority inspected the new paint work. The bridge was closed to traffic by Friday night. Authorities still are uncertain when it will be reopened to the 42,000 cars that typically traverse it each day. Turnpike officials were anticipating a difficult commute Monday as drivers use alternate routes to bypass the bridge. The traffic snarls are likely to be bad enough that DeFebo urged commuters to try car pooling or asking employers for flex time to work from home.

"Anyone who needs to travel over a Delaware River bridge north of Philadelphia, there are limited options there," he said.

It will be at least two weeks before authorities know what kind of repairs are needed, DeFebo said. Completing the work will take longer.

The first step to repairing the bridge will be stabilizing it, he said, which will begin this week. The fracture in the truss caused the bridge to shift, so it will be jacked back up and eight platforms will be placed under it for support. A temporary splice to hold the truss will also be completed, DeFebo said.

Along with displacing commuters, closing the bridge also inconveniences several distribution centers, among them Amazon and Bed, Bath & Beyond, in Florence, near the New Jersey end of the bridge.

The bridge is not likely on the verge of collapse, said Joe Martin, an engineering professor at Drexel University, but the fractured truss beneath an outer lane could have given way if a number of heavy trucks had passed over it in quick succession.

"They're closing it because of truck loads," Martin said. "If you had a convoy, five or six fully loaded trucks, that crack would start to zipper."

Martin agreed with Frank that the high heat and dissimilar metals used in plug welds can cause weak points in beams.

Yet the truss' failure also highlights how well the bridge had been built, Frank said. A severed truss might have caused a collapse in another bridge.

"It's a testimony to the inherent redundancy of this structure and the safety of it," he said.

215-854-4587 @jasmlaughlin