Erecting an 825-ft-long bridge is always a challenge. Doing it with just 17 days of road closure is a record-setting achievement, made possible with an innovative construction technique!
The project team on the Interstate 376 Commercial Street bridge replacement in Pittsburgh erected a 3,700-ton steel structure adjacent to the existing bridge, demolished the existing bridge, and slid the new bridge 102 ft onto the existing highway alignment. The demolition and slide required I-376’s closure for only 17 days, eight fewer than the Pennsylvania Department of Transportation (PennDOT) originally scheduled—and much less than the four years that a more traditional replacement might take. It is one of the largest-ever bridge slide projects in the U.S. by weight and distance slid. The choice of a structural steel for the bridge made it lighter and easier to slide.
“This is the most impressive bridge project in the history of PennDOT, and I’m going to be so bold as to say it’s the most impressive bridge project of this size in the United States,” PennDOT Secretary Mike Carroll said upon I-376’s reopening.
Horizontal sliding is an Accelerated Bridge Construction (ABC) technique that, in this case, eliminated the need for about four years of long-term lane closures and restrictions. The Commercial Street bridge carries about 100,000 vehicles per day, making long-lasting repairs an undesirable solution. It’s a crucial part of I-376, known locally as Parkway East, and sits between a busy urban exit and a 0.8-mile-long tunnel.
The new bridge is 825 ft long and has a 70-ft-deep steel delta frame, a rigid triangular steel assembly that can handle high load distribution across wide spans. It has seven girder lines and zero nonredundant steel tension members (NSTMs). It’s a three-span structure, with a main span of 380 ft and two other spans at 241 ft, 6 in. long and 203 ft, 9 in. long. It is 86 ft, 4 in. wide out to out, and it also includes 2,500 cubic yards of concrete deck and three full-length barriers. The steel structure made it lighter and easier to slide.
The arched delta frame gives the bridge a graceful arched appearance and is a nod to the old arch concrete bridge, but minimizes thrust forces to make sliding more practical. It also provides the appearance of an arch but has inspection and maintenance requirements similar to those of a multi-girder bridge.
I-376 closed July 10, and demolition began immediately. Sliding began on July 19 and continued into the next day. The new bridge—built on four temporary sliding support towers located at temporary abutments and piers—was lifted using hydraulic jacks and moved about 3 ft at a time. The new bridge’s foundations were constructed under the existing bridge using micropiles due to the low clearance. When the slide was finished, the bridge was lowered onto the substructure and secured. Traffic reopened on July 27, about one week ahead of the initially planned Aug. 3 completion.
HDR designed the bridge, and AISC full member Veritas Steel fabricated the steel members. Fay S&B USA was the project’s general contractor and Mammoet conducted the slide.
