Feature Story - November 2005

Bridge Redecking

Bronx Whitestone Slims Down in a Major Overhaul

The 66-year-old bridge connecting the Bronx to Queens is midway through the last phase of a weight reduction project that will reduce stress on its suspension cables and extend its useful life.

The Bronx Whitestone Bridge is taking off the pounds - nearly a quarter of its dead weight - and keeping them off, thanks to the final push of a two-phase project that wraps up next year.

Project crews began replacing the suspension bridge's original roadway deck in June with lighter steel orthotropic plates - the central part of a $136.7 million, 35-month contract issued by Metropolitan Transportation Authority Bridges and Tunnels. The effort follows a $32.7 million contract completed in 2003 that replaced stiffening trusses with wind fairings to lighten the bridge, which is part of Interstate 678.

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While trimming 6,000 tons of weight is the main goal, the project also upgrades the aging 253,000 sq. ft. of roadway deck, said Tom Bach, vice president and chief engineer for the authority's bridges and tunnels division.

"We had an opportunity to take off the trusses," Bach added. "In the meantime, the bridge needed to be redecked. They were the bridge's original decks, and they were at the end of their useful life."

Redesigned to Weather the Wind
The bridge linking the Bronx and Queens over the East River first opened to traffic with four lanes in 1939, in time for that year's New York World's Fair. In the structure and shape of the design by noted bridge engineer Othmar Ammann, it was virtually a twin of another bridge built in that era, the Tacoma Narrows Bridge in Washington.

That close relationship has had repercussions to this day because the Tacoma bridge collapsed in an infamous accident in 1940. Beset by high winds, the bridge twisted wildly and fell into the Tacoma Narrows in a sequence memorably caught on film, earning it the nickname "Galloping Gertie."

Though there were no casualties - and though the Whitestone was not as long or as slender or subject to the same wind patterns - the Tacoma engineering failure led to changes ordered by Robert Moses, the longtime and influential head of what was then the Triborough Bridge Authority.

"There was a major concern around the country," Bach said. "Moses decided to be better safe than sorry."

The first step was to add diagonal stiffening cables in 1940. In 1947, the authority added two steel trusses on the bridge's sides to further stiffen the structure - a change coupled with the addition of two traffic lanes.

The trusses not only cluttered the aesthetics of the bridge, but they and the new lanes of traffic also added weight to the structure. The Whitestone took on another heavy wind-countering measure in 1987 with the addition of a tuned mass damper system in the middle of the bridge.

Since then, however, bridge design has evolved significantly.

"We've come a long way with aerodynamics," Bach said. "Now, normal flutters and vibrations from the wind can be addressed aerodynamically by shaping the bridge. Then you've eliminated the need for the truss. And if you eliminate the truss, you can get rid of a lot of dead load on the bridge."

That thinking - and the bridge's daily traffic count of 120,000 vehicles - led to the decision in the late 1990's to proceed with the weight-loss project.

Two Phases, But One Project Goal
The authority chose to launch projects separated by two years but closely intertwined in design.

"From an engineering perspective, they were the same project," Bach said. "We split them up mostly for timing and budget issues."

Indeed, the preliminary design took the entire scope into consideration, said Guang-Nan Fanjiang, project manager for the designer, Weidlinger Associates of New York.

"We had the whole program - with the wind fairings, new lateral bracing, and new decks - included in the original wind aerodynamic study in the laboratory," he said.

Splitting the work also allowed the project team to stage construction in a way that closed no more than one lane during peak-hour traffic, and that limited the amount of overall traffic disruption, Fanjiang said.

"If they had to deal with wind fairings when they were doing lane replacement, it would have taken longer," he added. "When you are doing the wind fairings and removing the trusses alone, you don't need permanent lane closures."

The first phase primarily involved removing the heavy trusses and replacing them with 390 wedge-shaped fiber plastic composite wind fairings - up to 25 ft. in width, 11 ft. deep, and 7 ft. long - that run across the entire span to deflect the wind and improve the bridge's aerodynamic stability. Nab Construction of College Point, N.Y., finished the project in 2003.

The current project to replace the decks is the bigger effort. It first requires demolition of the original 4.5-in.-thick, concrete-filled, steel-grid deck - as well as its crossbeams and stringers - on the six lanes that cross the span, which has a 2,300-ft.-long main deck and two 735-ft.-long side spans.

The next step is the installation of 408 new steel deck panels, for which the authority has a template from a similar project it completed last year on the Triborough Bridge, which connects Queens, the Bronx, and Manhattan.

Steel orthotropic decking, which consists of plates welded to diaphragms and topped by a 3/8-in.-thick, epoxy-and-grit surface, has been around for more than 20 years, but is more commonly found in Europe. While more expensive than the concrete it replaces, the steel deck will make the bridge lighter, stronger, and stiffer, said Vince Montanti, the MTA's facility engineer on the project.

"It also is quicker construction," Montanti added. "It's much faster to drop in a new panel fabricated off site than to prepare a deck for a concrete pour and then wait for it to cure-set."

Demolishing Old Deck, Installing New
The authority awarded the redecking contract to a joint venture of Perini Corp. of Framingham, Mass., and its financial partner, O&G Industries of Torrington, Conn., with Weidlinger remaining as designer. The project preparation took about 18 months starting in November 2003, with early efforts involving installation of a temporary work platform under the bridge, Montanti said.

"The contractor also prepared shop and fabrication drawings, installed gantry cranes, and installed temporary structural supports to strengthen the existing deck system," he added.

The early stage included the bid for steel fabrication won by Usiminas Mecânica of Brazil, said Gaby Antoun, Perini's project executive and operations manager.

"This is the new trend in the last four or five years on the East Coast," he said. "All of the bidders came from Korea, China, and Brazil, because they bid at half the price of the local market. Nobody from the local market bid this job."

Work began just after Memorial Day weekend this year, Antoun said. His crew of 100 workers is tackling the two 14-ft.-wide outside lanes and four 10-ft.-wide inside lanes one by one, starting with the right-hand southbound lane.

The crews saw out sections of a single lane, removing the slabs with four gantry cranes running on tracks, two on the main span and one each on the side spans. The removal of old decking and installation of the 60-ft.-long, lane-width steel plates takes place during off-peak hours, usually late morning and early afternoon, because crews operate from a second adjacent lane. Moveable median barriers allow for opening and closing the second lane to traffic as necessary.

On the first lane, crews demolished all of the old decking along the entire length before installing the steel plates, Montanti said. But crews working on the middle southbound lane in early fall were demolishing old deck and installing the steel plates during the same stage.

While the fabricator is applying the bonded aggregate surface to most of each deck panel prior to shipping, project crews are adding the aggregate onsite to the end sections, where the ironworkers weld the plates together.

The plan calls for next tackling a lane on the northbound side in order to balance the bridge's weight because having three lighter lanes on one side would strain the structure, Montanti added.

Working around a Strike and Traffic
The project already had a hiccup - a summertime strike that kept ironworkers off the site for 17 days. While electricians continued adding conduit and cable feeds across the bridge, Montanti said the project faced delays because ironworkers have major roles in steel cutting for demolition and in installation of the new panels.

The project team had not determined whether it might shorten a planned work hiatus scheduled for mid-December to late February, which is intended to avoid cold weather that could hamper welding as well as to prevent disruptions to end-of-year holiday traffic. During early fall, the team was developing strategies that might still allow it to finish three of the six lanes by December and the other three by late summer, as scheduled, Montanti said.

The project team is also adding new electrical, communications, lighting, and fire standpipe systems as well as a movable inspection platform under the bridge.

During the break, Antoun said Perini will have the last fabricated steel, shipped from Brazil to Brooklyn, transported to the site by truck.

Weidlinger's Fanjiang said that once the project is complete, the authority may revisit two other elements: the existing suspension cables, made of bundled, high-strength steel wires, which were not replaced in either phase; and the tuned mass damper, which for now is staying put. The authority has already hired Hardesty & Hanover, a New York-based engineer, to study and inspect the cables.

If stress patterns turn out as expected, the bridge may be in good shape for a long time, said the MTA's Bach.

"It hasn't been painless, but it has worked out pretty darned well," he added.