Turnpike officials say the fracture is on a 14-inch truss below the westbound right lane of I-276.
The bridge connects the Pennsylvania Turnpike in Bucks County to the New Jersey Turnpike in Mercer County.
Tens of thousands of drivers are being told to expect "extreme" delays for weeks because of the closure.
Officials say it will be two weeks before engineers fully understand how badly damaged the bridge is and that there is no estimate about when a permanent repair can be made to allow reopening of the bridge until that initial assessment is made.
The Pennsylvania Turnpike Commission and the New Jersey Turnpike Authority say commuters should carpool or consider working from home, changing their work hours or using public transit.
The nearly 61-year-old bridge carries more than 42,000 vehicles per day. Severe traffic is expected at all other area river crossings.
A travel advisory was issued due to the bridge closure, and it also applies to the Commission's five southerly crossings:
- Trenton-Morrisville (Route 1) Toll Bridge (both directions)
- Scudder Falls (I-95) Bridge (both directions)
- Lower Trenton ("Trenton Makes") Toll-Supported Bridge (eastbound)
- Calhoun Street Toll-Supported Bridge (eastbound)
- Washington Crossing Toll-Supported Bridge (eastbound)
Since Friday, motorists have not been able to use the Pennsylvania Turnpike (I-276) to travel between Pennsylvania and New Jersey.
The Pennsylvania Turnpike Commission said work was being done over the weekend to stabilize the Interstate 276 bridge, which links Bristol in Bucks County to Burlington Township in New Jersey.
"This was a unique and complete fracture, which may have happened quickly due to changing weather conditions," Turnpike commission chairman Sean Logan said in a statement. "Out of an abundance of caution and to protect traveler safety, the bridge must remain closed until a full-scale analysis and repair plan have been completed."
The fracture was spotted during a routine check of a bridge-painting project by the New Jersey Turnpike Authority, and a sample of the fractured truss has been taken for forensic analysis. The bridge will be under constant watch and monitored around the clock utilizing a high definition video survey, officials said.
Transportation officials have suggested possible detours in advising drivers to prepare for slow-moving or stopped traffic, especially during the afternoon and evening hours.
ALTERNATIVE ROUTES AND MORE INFORMATION:
Pennsylvania motorists heading eastbound to New Jersey should use the following alternative route: Exit at the Bensalem Interchange, #351 in Bucks County. Follow U.S. Route 1 north, to I-95 north, to I-295, south, to I-195 east. Take Exit 6 on I-195, and reenter the NJ Turnpike.
Alternatives routes for New Jersey motorists headed to Pennsylvania include Interstate 78, Interstate 295 and the Burlington-Bristol Bridge.
For more information, Pennsylvania motorists can call 866-976-TRIP or 800-331-3414, or download the TRIPTalk app.
New Jersey motorists are encouraged to download the SafeTripNJ app or call 511 for more information.
The Associated Press contributed to this report.