Meeting tolerance limits of one thousandth of an inch would pose a challenge on any construction project. But when the job is in an active shipping channel, where crews often work from barges, the task is particularly daunting.
Spanning 1,000 ft and positioned 60 ft above the surface of the Taunton River between Fall River and Somerset, Mass., the Brightman Street Bascule Bridge replacement, opened last November, is one of the world's longest of its kind. All components of the four-leaf bascule design had to line up perfectly. In addition, the bridge sits on a skew to accommodate optimal roadway and navigation channel alignment.
"Each of the geometries is different for each of the leaves of the bascule," says John Smith, associate vice president with HNTB Corp., the lead engineering firm on the project. "There were no standard dimensions for the bridge. We were doing a bascule on a skew, so the precision required demanded a lot of dimensional control."
The five-year, $214-million project, led by a joint venture of Cianbro, Bloomfield, Conn., and The Middlesex Companies, Littleton, Mass., replaced the original bascule bridge, which opened in 1908. Since it featured less than 30 ft of vertical underclearance, the old bascule was opened frequently to make room for ship traffic, causing significant impacts. Additionally, the original structure's draw span was too narrow and off-center within the channel.
The Massachusetts Dept. of Transportation awarded the joint venture the contract in 2007. HNTB estimates that the new bridge design should reduce the number of openings by 90%.
The concrete bascule piers had to be constructed to very tight tolerances, allowing the machinery equipment erected on top to meet tolerances within one thousandth of an inch.
"It was like building a 7-story piano and tuning it on top," says Kaven Philbrook, project manager with Cianbro. "At seven floors up, most building contractors would be happy if they are within an inch [of tolerance]. Here, we needed the concrete within a quarter of an inch. Then you make up that quarter-inch within the grout pad to the machinery."
Concrete testing on site was a main focus of the QA/QC team. The bascule concrete was pumped to 600 ft, requiring Cianbro/Middlesex to perform a test-pump to simulate several dry runs. The results provided information on slump and air loss at varying temperatures—findings that proved useful when the actual concrete was placed.
Precise concrete mixes were also critical for the lightweight concrete partially-filled grid deck and the heavyweight concrete counterweights. "There were very careful controls on the mix design," Smith says. "They did testing before each load was placed to see how the two mixes lost water weight overtime. They kept tracking it."
To ensure the highest quality and craftsmanship on the bascule pier concrete, the team used a jump form formwork system supplied by Doka Forms. The system was engineered to guarantee dimensions, which were critical to achieving the tolerance required to set the bascule mechanical equipment.
The bascule erection was performed utilizing two, 300-ton ring cranes situated on barges. To minimize the number of times required to close the channel to marine traffic, crews preassembled the tip sections of the bascules and raised them into place.
Once the pieces went into place, fine-tuning continued to get the bascule alignment just right. "We were down there with an emery cloth, buffing a tooth to make sure we're getting the proper percentage of bearing," says David Schill, senior project manager with Cianbro. "We're talking about thousands of tons of steel and concrete and we were there with a piece of emery cloth. It's scary how finely tuned this thing is."
High Steel Structures fabricated 125 steel tub girders for the approach spans. The new alignment also required new connections to adjacent roads, including a new trumpet interchange between Route 6 and Route 79 in Fall River, a partial diamond interchange and several new signalized intersections. A number of local roads required reconstruction, ultimately resulting in a higher volume of traffic across the bridge.
Facilities for pedestrian and bicycle circulation between Fall River and Somerset were also incorporated into the design of the bridge, approaches and local street network. Bike paths, bike lanes and wide outside lanes were constructed to provide a continuous route for public use.
In keeping with the team's high expectations for quality, the entire project was completed on time, within budget and with zero lost-time injuries.
Brightman Street Bascule Bridge, Fall River and Somerset, Mass.
Owner: Massachusetts Highway Department
Design Firms: HNTB Corp
General Contractor: Cianbro/Middlesex A Joint Venture
Concrete Flatwork: Algar Construction
Bridge Machinery: Steward Machinery
Bascule Steel: PDM