Officials said the crash occurred on an on-ramp to Route 22-322, where the ramp curls back over the interstate and runs under another section of Route 22-322. They said the intense heat buckled steel beams on the top section and damaged it so badly there were fears it could collapse and fall onto I-81.
"There really could not have been a worse spot for this to have occurred," state Transportation Secretary Barry Schoch told reporters.
State police said the driver, Thomas Uecker, 52, of Dover, Pa., was treated at a hospital for minor injuries. The truck was registered to Tameric Enterprises LLC in Carlisle.
The truck, carrying 7,500 gallons of diesel fuel, was headed northbound from Carlisle shortly after 6 a.m. when it flipped over on the ramp, just north of where I-81 crosses the Susquehanna River.
Officials announced plans to tear down the heat-damaged top level, and were working to contain and clean up about 2,000 gallons that spilled into nearby Paxton Creek and the lake at Wildwood Park and Olewine Nature Center, a county park that borders the interstate.
Schoch said the heat from the fire was so intense that it forced water out of concrete as steam, generating explosions. A specialist was helping state officials assess damage to the bridges' steel structures.
State police said they were still investigating the cause of the crash, which caused damage estimated at more than $10 million. Uecker did not appear to have a listed home phone number, and a message left for Tameric Enterprises on Thursday was not immediately returned.
"We have no clear timetable ... as to when the repairs will be completed," Gov. Tom Corbett said.
The morning commute turned into a traffic nightmare, as drivers scrambled to find alternate routes around a spot where the interstate, river and local geography funnels drivers. I-81 handles about 100,000 vehicles per day in that area, while Route 22-322 gets about 34,000.
At one point, vehicles were backed up for 10 miles on I-81 in both directions, said state police Commissioner Frank Noonan.
Schoch said he hoped by Monday to convert one of the two unaffected Route 22-322 lanes into a single lane going the other direction, a detour that could last months.
Harrisburg Area Community College shut down its nearby campus and rescheduled final exams from Thursday to Tuesday. The state told many of its workers in the Capitol Complex in downtown Harrisburg to leave work two hours early to ease afternoon congestion.