Officials from the Pennsylvania Turnpike Commission (PTC) (Harrisburg, Pennsylvania) and the New Jersey Turnpike Authority (NJTA) (Woodbridge, New Jersey) recently reopened the jointly owned Delaware River Bridge, which connects the two states just north of Philadelphia.
The 1.25-mi (2.0-km) bridge, which carries traffic between the two states, first closed on January 20 when a complete fracture was discovered in the superstructure beneath the westbound lanes on the Pennsylvania side in Bristol Township, Bucks County.
“We pledged to reopen the bridge as soon as we could do so safely,” says Richard T. Hammer, transportation commissioner for the state of New Jersey and chairman of the NJTA board. “Thanks to a round-the-clock effort involving dozens of engineers, contractors, scientists, and other workers, we are able to get that done much sooner than we expected with every confidence that it will be safe for the motorists who depend on it.”
The PTC, NJTA and at least two dozen contractors, consultants, and other entities that make up the joint repair team have been working to fix the fractured truss member, or girder.
“We were able to bring in some of the very best minds in the field to work with the staffs of our agencies to evaluate the fracture and then design and execute the repair,” says Joseph W. Mrozek, the NJTA’s executive director. “Because of hard work by a lot of talented people, we’ll be reopening the bridge weeks ahead of the date we projected when it was first shut down.”
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 scrutinized structure in the nation and maybe the world,” says Brad Heigel, the PTC’s chief engineer. “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 travelers and for nearby residents.”
The repair team began the effort by immediately bolstering the truss with a temporary splice. Then, they deployed hydraulic jacks—each capable of lifting 600 tons—held by eight 80-ft (24.4-m) towers to realign the truss, which was displaced by ~2 in (5.1 cm) 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 explains.
The team ran a series of load tests to study the effectiveness of the permanent splice. Eight tri-axle dump 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 could support the weight of live traffic.
“We will continue to monitor loads and stresses on the bridge,” says Robert Fischer, the NJTA’s chief engineer. “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.”
Heigel confirms that two mis-drilled holes—each ~1 in (2.5 cm) in diameter and filled with weld material—were identified as primary factors that contributed to the fracture. But bridge experts say it was 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 weld-filled holes were found.
“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 says. “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.”
Source: PTC, paturnpike.com.