July 14, 2008
Allegheny River Bridge First in State Built From Top Down
By Michael Aubele, The Valley News-Dispatch, Tarentum, Pa.
Jul. 13--Call it a piece of history.Or, if you want to get technical, call it a cast-in-place, balanced-cantilever, post-tension segmental bridge.
Either way, you're talking about the Pennsylvania Turnpike bridge being constructed between Harmar and Plum.
The span, which is officially called the Allegheny River Bridge, will be the first of its kind in the state. Two aspects of its construction will make it unique:
--The twin concrete bridge decks -- one carrying traffic east and the other west -- are being built literally in the air.
"On this project, the deck is being poured at its permanent location, atop the piers, using the balanced cantilever method," turnpike spokesman Joseph Agnello said. "Actually, it's 16 feet out one direction, from whatever pier, then 16 feet out the other direction -- until everything meets and it's all done.
"They do an average of two pours per week."
Craig White, project manager with the engineering firm McTish, Kunkel and Associates of Pittsburgh, said of the system, "That's the key that makes this different than what anybody else is doing."
--After the concrete is poured, crews will run steel cables through the bridge decks and tighten them at each end to keep the segments together.
Gary Graham, turnpike assistant chief of engineering, likened the system to wrapping a rubber band around a group of wooden blocks set end-to-end.
"The construction is probably the unique end of this project," he said. "It's unique for Pennsylvania. There are no other major post-tension segmental bridges (in the state)."
The turnpike's Susquehanna River Bridge near Harrisburg is similar in design in that it's a concrete segmental bridge -- the first of its kind in the state.
However, it was precast, meaning crews manufactured its piers and bridge deck in sections on the ground and then hoisted them into place.
Turnpike officials said there are several benefits to segmental construction. Segmental bridges, they said, are often cheaper to build and maintain and can be built faster than many other bridge types.
Graham said the Susquehanna River Bridge, which is more than a mile long, was the cheapest per square foot of deck area to build. The project, which includes construction leading to the bridge, cost about $95 million.
Construction of the Allegheny River Bridge, which includes road improvements, will cost about $190 million.
Graham said the cost of the Allegheny River Bridge is comparable to other designs that could have been used, such as the current underslung steel truss span, which opened on Dec. 26, 1951.
What makes the new design suitable for this project is the landscape.
"There are certain bridge types that fit certain geometric layouts," Graham said. "Here, you either have to go with a truss span or a post-tension bridge."
He said the project demanded fewer piers be erected in the Allegheny River. Consequently, longer bridge deck spans are required.
"On the Allegheny River, you're dealing with a commercial waterway, and you can't put (a lot of) piers in a commercial waterway," Graham said. "This river crossing demands what we would say is a longer-span bridge. And you can go much greater spans with this design."
Each span is long enough that you can't use conventional precast beams or steel girders, he said.
The bridge crosses the Allegheny River, Freeport Road and railroad tracks.
"These things would preclude us from putting up any type of barges in the river or (temporary structures) over the railroad for a long period of time," Graham said. "This (design type) allows us to build from up above. We can get across these openings and busy travel ways from above. If we start to impact the (Freeport) road or railroad or waterway, it starts to cost more or become inconveniencing for a lot of people.
"It just really lends itself well to such restrictions."
Graham said another benefit of the design is its durability. The span will have a life expectancy of 100 years, he said. Even if the bridge develops cracks, the design precludes them from spreading.
In terms of design, there really are no downsides, Graham said. The only obstacle turnpike officials ran into, he said, was preventing the scope of work from precluding state contractors from bidding on the job because none had experience with this type of construction.
Graham said officials worked around that by requiring only that the bidder have at least one team member who had worked on a similar design.
In the end, however, the job was given to the Walsh Construction Co. of Chicago, the low bidder.
Figg Bridge Engineers, which is part of Tallahassee-based Figg Engineering Group, designed the span. Figg is best known for designing the 4.2-mile Sunshine Skyway Bridge connecting Tampa and St. Petersburg.
A company official did not respond to phone calls or e-mails for comment.
Bridge construction began in July 2007 and is expected to be complete by July 1, 2010. The project should be completed by Oct. 29, 2010.
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Copyright (c) 2008, The Valley News-Dispatch, Tarentum, Pa.
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