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New Allegheny Bridge Project in Final Stage

Mon April 26, 2010 - Northeast Edition
Mary Reed

The Pennsylvania Turnpike Commission (PTC) is heading a $190 million project to replace an aging span across the Allegheny River with the first major post-tensioned segmental bridges in the Commonwealth.

The existing bridge is located 14 mi. northeast of Pittsburgh and the job is part of a project to reconstruct I-76 between mile posts 46.56 and 49.54, improving the highway and increasing traffic safety and capacity.

Chicago-based Walsh Construction Company is handling the $193,837,705 job through its Canonsburg, Pa, branch. Notice To Proceed was issued in May 2007 and work began in July that year with an expected completion date of July 2010.

The project involves two 2,350 ft. (716.3 m) long 56 ft. (17 m) wide bridges, one for eastbound traffic and the other for westbound, between Harmar and Plum in western Pennsylvania.

The dual spans will cross the Allegheny River at a height of 120 ft. (36.6 m) and also traverse a rail line and a highway. The bridges will have three lanes apiece, each 12 ft. (3.6 m) wide, an outside shoulder of the same width, and an inner shoulder 10 ft. (3 m) wide. Provision for pedestrians and cyclists also will be made.

The Allegheny River is a working waterway that must be kept as clear as possible. Tallahassee, Fla.-based Figg Bridge Engineers Inc. therefore designed the spans with twin walled rectangular piers on each side of the commercial channel and a similar pair on each side of the river’s recreational channel.

In addition, the contractors could not obstruct the river, highway or railroad, which in turn meant the distances to be covered were too lengthy for the use of steel girders or precast beams.

“The bridges therefore feature cast-in-place post-tensioned concrete box girders using the balanced cantilever method,” Walsh Project Manager Tom Warren said.

“Rather than having conventional girders erected from the ground and then a conventional deck cast on top, concrete is pumped to the top of the tiers and the box girders and deck are placed together 16 feet at a time, with 16 feet going out in one direction and then 16 feet in the other to maintain balance. This type of construction is typically faster and minimizes disturbances to areas below,” he added.

Walsh’s equipment manager, Art Golembiewski, noted that specialized equipment was necessary for this job, in particular for the traveler segment form system. The system used form travelers fabricated in Asia and supplied by NRS AS of Sandvika, Norway.

Enerpac Corporation in Milwaukee, Wisc., supplied TT series 85-ton (77 t) strand jacks to handle lowering pier table false work as well as traveler formwork.

Several other specialized pieces were used, including IMT 220 caisson drills for the 60-in. (152 cm) radius caissons drilled 50 to 70 ft. (15. 2 to 21.3 m) deep. Two IMT 1200 tieback rigs were used in construction of retaining walls and also drilled the holes for the soil nail placement, which involved installation of 2,532 soil nails 15 to 40 ft. (4.6 to 12.2 m) in length.

At one point 10 cranes were at work on site, including two Manitowoc 999, two Link-Belt 238s and Link-Belt 218 crawler cranes. Such were the requirements of the project that a 999 crane was stationed on a barge platform for almost two years for the erection of travelers and bridge construction in an area not accessible by land.

Walsh Construction self-performed post-tensioning with equipment supplied by Schwager Davis Inc., of San Jose, Calif. The task involved installation of 3.3 million lbs. (1.5 million kg) of post-tensioning system and 16,000 cu. ft. (453 cu m) of grout. In all, the task involved enough post-tensioning strand to stretch from Pittsburgh to Philadelphia and back.

The overall project also includes reconstruction of the overpass on Gulf Lab Road (SR 1088) west of the river, construction of new exit and entrance ramps for the nearby Allegheny Valley interchange with two new bridges to carry I-76 over them, and new toll plaza ramps at the interchanges.

“In addition, two retaining walls are being built along the eastbound side of the Turnpike. One is approximately 1,300 feet long and 30 feet high and the other about 600 feet long and 70 feet high,” Warren said. “Two other retaining walls 12 to 16 feet high, one about 400 feet long and the other approximately 200 feet long, will be built along the realigned interchange ramps and a fifth wall about 1,000 feet long and 60 feet high is being built adjacent to Gulf Lab Road.”

The two bridges are expected to have a life of 100 years. According to the Turnpike Authority should the spans develop cracks, the design will preclude them from spreading. The existing underslung steel truss bridge, constructed in 1950 by the American Bridge Company and opened to traffic on Dec. 26, 1951, will be demolished when the entire project is completed.

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