Owner: PennDOT Engineering District 4-0

Location: IS-81 over Roaring Brook, etc S20909C

Designer: Gannett Fleming

General Contractor: G.A. & F.C. Wagman

Alternate Designer:Patel Chen Associates

Prestress Producer: Schuykill Products, Inc.

Erector: Marikina Engineers and Constructors

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Prestressed Concrete was the Way to Go

With a web of activity below -- the Roaring Brook, a city neighborhood, a concrete arch bridge, high voltage transmission cables, and an active line of the Pocono and Northeast Railroad -- 150 prestressed concrete beams were erected under traffic on Interstate 81. At $48 million, the twin prestressed concrete bridges replaced two 33-year old steel girder spans that were rapidly approaching the end of their useful life. The project also increased the capacity on the interstate by adding shoulder lanes and increased the four lanes to six traffic lanes as part of the region’s massive Lackawanna Valley Industrial Highway project.

The first challenge was to design a southbound bridge that could be built and put into service in half-width. This half-width bridge then carried two lanes of northbound traffic while the old northbound bridge was being replaced. On completion, northbound traffic was returned to the new northbound bridge and the two lanes of southbound traffic were also placed on the now wider (three traffic lanes and two shoulders) northbound bridge. Then the remaining original southbound structure was dismantled. Pier caps were then modified and the second half-width bridge was connected with the first half-width southbound bridge and completed. Because of this staged, half-width construction the southbound structure ended up having an additional line of girders.

The bridges took three years to build. PennDoT’s safety mandate for interstate construction -- maintain full, two lane capacity on each bridge during replacement -- posed significant construction and engineering challenges.

One seasoned construction engineer called it "as tough a job as had ever been done". But in the beginning, the decision to build the Bunker Hill Bridges using prestressed concrete beams was easy.

The project hit the street designed as a steel I-beam bridge. The prestressed concrete beam alternate design provided by PennDoT was sketchy at best, conceptual TS&L (type, size & location) rather than a fully developed set of plans. PennDoT did, however, allow prospective contractors to bid a contractor alternate design of their own. This enlightened contracting method, of allowing value engineering by the contractors as the basis of their bids, won, during 1997, an AASHTO National Value Engineering Award for the Pennsylvania Department of Transportation and for the contractor.

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At that point, the prestressed concrete industry, under the leadership of Schuylkill Products, took the lead. Interested bidders were provided complete plans for a prestressed concrete alternate design that would capitalize on the industry’s ability to deliver its products quickly and efficiently.

The challenging terrain guaranteed that the bridges would be difficult to build regardless of whatever design, in the end, won the job. But the site’s constricted environment gave the prestressed concrete alternate design an opening: Shorter beams would be somewhat easier to maneuver. As it was, field technicians faced the task of erecting prestressed concrete beams that were eight feet deep, 91- to 145-feet long, and weighed between 80 and 104 tons each. Beams were set across nine piers for each structure. Most would be picked from below by massive, 150 and 220 ton-capacity cranes squeezed in and around the site’s many natural and man-made obstacles.

"Sometimes the original design...does not take maximum benefit of the material costs...the site and the circumstances, and how aggressive (the competing industries) want to be," said Leonard C. Bellanca, PE, senior vice president at G.A. & F.C. Wagman, Inc., eventual low bidder on the project. "In this case, steel delivered and erected cost a lot more than prestressed (concrete beams) delivered and erected."

Through this window of opportunity, prestressed concrete demonstrated its economies of fabrication: lower labor costs, readily available and cheaper raw materials (cement, sand and stone, and reinforcing steel) and none of the extra costs required for steel beam fabrication -- welding, testing, x-raying, sandblasting and painting. Wagman’s estimators did the math and found more than one million reasons to embrace the prestressed concrete industry’s alternate design. At the bid opening, the York-based contractor’s $48.8 million estimate was $3.6 million less than the closest steel bridge bid. In spite of the additional design and review fees, the need for four additional piers, and higher field labor costs, prestressed concrete won the race to the bottom line.

"When you add it all up, and we did this very dispassionately, the delivered beam cost for prestressed concrete was significantly less," Bellanca said. "Obviously, prestressed concrete was the way to go."

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