The Delaware River Bridge between New Jersey and Pennsylvania has reopened to traffic after being closed for seven weeks for the repair of a fractured truss member. The 2km-long bridge, which carries traffic between the two states, was unexpectedly closed to traffic on 20 January when a complete fracture was discovered in the superstructure beneath the westbound lanes on the Pennsylvania side.

The joint owners of the bridge, the Pennsylvania Turnpike Commission and the New Jersey Turnpike Authority worked with at least two dozen contractors, consultants and other entities to fix the fractured truss member.

PA Turnpike chief engineer Brad Heigel confirmed that two mis-drilled holes — each approximately 25mm in diameter and filled with weld material — were identified as the primary factors that contributed to the fracture. But bridge experts agree it was certainly not the only trigger. After an exhaustive study of the entire bridge — which included weeks of hands-on inspections, ultrasonic testing and metallurgical analysis from three different, independent laboratories — no other such weld-filled holes were found to be present. “We continue to look at other factors such as air temperature and loads like heavy trucks or high winds that may have had an impact when the fracture occurred,” Heigel said. “But getting a definitive answer as to why it happened is akin to solving a 60-year-old mystery perpetuated by unseen offenders. We may never know for sure what all the factors were that caused the fracture,” Heigel said.

The bridge, which carries about 42,000 vehicles per day, first opened to traffic in 1956. “Given the number of experts who have inspected, tested and studied it over the last 49 days, it’s safe to say this bridge is perhaps the most scrutinised structure in the nation and maybe the world,” said Heigel. “We have a high level of confidence on both shores of the river regarding the reliability of our repair and the safety of the bridge for travellers and for nearby residents.”

The repair team began the effort by halting any further movement of the truss with a temporary splice. Then, they deployed hydraulic jacks — each capable of lifting 600 tons — held by eight, 24m-high towers, to realign the truss, which was displaced by approximately 50mm when the fracture occurred.

Once the bridge was realigned, the repair team initiated a post-tensioning technique to bring the damaged truss back into alignment and to reintroduce loads into the fractured truss member. Then, the team built a permanent splice to rejoin and repair the fractured truss member.

“The repaired truss member is now much sturdier than the original because of the splice,” Heigel explained.  Before the bridge was reopened, the team ran a series of load tests to study the effectiveness of the permanent splice. Eight, three-axle trucks carrying 40 tons of aggregate were driven across the bridge while sensors installed on the truss and splice detected the structure’s reaction. Load testing results showed that the bridge can support the weight of live traffic.

“We will continue to monitor loads and stresses on the bridge,” said NJTA chief engineer Robert Fischer. “The temporary sensors will remain for now. In the coming months, we expect to install a system for monitoring the health of the bridge over the long term.”