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World’s largest composite bridge

inlandpipeStay-in-place girder forms net MaineDOT global record for vehicular structure

The 32-ft.-wide, eight-span, 540-ft.-long Knickerbocker, the largest composite bridge in the world, opened earlier this year in Boothbay, Maine. Built in 1930 with a major superstructure rehabilitation in 1983, the existing Knickerbocker Bridge was comprised of a 38-span timber bridge, approximately 535-ft. long. This two-lane highway bridge, which carries Barters Island Road over the Back River in Boothbay, is close to the tidal waters with only 4 feet of clearance at high tide.

Being in a harsh marine environment, it was key that the replacement bridge be constructed to better withstand the elements. The solution came in the form of hybrid composite beams (HCB) manufactured by Harbor Technologies in Brunswick, Maine. Developed by HC Bridge Company, LLC, a firm specializing in the development of hybrid-composite structural alternatives, the lightweight beams are made using fiber reinforced polymer. Used in conjunction with concrete and steel, the beams make a stronger, longer-lasting bridge with an installed cost that is comparable to the traditional materials used to construct bridges.

According to John Hillman, HC Bridge president, the HCB combines the strength and stiffness of conventional concrete and steel with the lightweight and corrosion advantages of advanced composite materials. The result is a cost-effective alternative for major infrastructure projects with sustainable structures that are lighter weight, safer and longer lasting than conventional bridges. The hybrid-composite beams also have been used on bridges in New Jersey and Illinois, as well as a Class 1 Railroad Bridge for BNSF Railway Company.


Hillman said that the Maine Department of Transportation (MaineDOT) decided to move forward with use of the hybrid composite beams for this project after spending years watching the technology develop. He noted that, in an effort to improve the quality of new bridges in the state and to help foster economic development through growing Maine’s composite manufacturing base, a Composites Initiative was made part of a public law. With a start date of Sept. 1, 2008, this law served as a catalyst to advance the development of composite bridges and created the funding mechanism for the Knickerbocker Bridge.

Roughly $11 million dollars for construction of bridge projects, which include composite components, was funded as part of the Initiative. For this project, the preliminary design report recommended replacing the existing bridge with adjacent precast box beams. This recommendation was later changed to HCB after MaineDOT discovered, researched and tested the new HCB technology at the University of Maine. Since the existing bridge had a low load rating, it could not be used to transport or off-load the heavier precast box beams, thus it would have necessitated a trestle or large barge and crane.

The construction documents were prepared by Calderwood Engineering of Richmond, Maine, with assistance on the HCB design from Chicago-based Teng & Associates, Inc. Eric T. Calderwood, P.E., chief engineer at Calderwood Engineering, noted that corrosion and long-term durability were key factors of design since this structure is over the salt water, and there are times that the water is very close to the bottom of the beams. These factors made HCB attractive as a design solution.

“We have a good history of the use of composites in a marine environment here in Maine as there are boat yards all along the coast today that use composites, so it is a material that we have confidence in for this kind of exposure condition,” he said. “Further, HCB was attractive because the weight of the beams during erection would give them a considerable advantage over precast.”

In order to comply with the hydraulic criteria for the new bridge, the HCBs were designed to match the recommended 33-in. depth box beams in order to maintain the required vertical under-clearance. Further, similar to the proposed precast box-beam bridge, the HCB framing system was limited to two 60-ft. end spans and six 70-ft. interior spans resulting in an eight-span bridge with a total length of 540 feet. The beams were also made continuous for live load with negative moment reinforcing steel cast over the piers in the 7-in. concrete topping slab.

According to Calderwood, the design was impacted at the substructure units because the dead load was significantly less than the original precast design. However, the engineering team had to design for significant wave action. “One of my considerations when using something new is how it will function,” he said. “In this case, we had a full-size beam made up at the University of Maine’s Advanced Structures and Composites Center (AEWC) with arch filled and deck cast on it as it would be in the field. This beam was then fully instrumented and tested for fatigue considerations, and then once fatigue was fully satisfied, it was tested to destruction. The beam had approximately four times the capacity required.”

Most important, Calderwood noted that from a structural engineering standpoint, HCB offers an enhanced safety benefit because flexural components need to fail in a ductile manner to ensure that distortions are visible and allow for repair and closure/evacuation prior to ultimate failure. “Protection against a sudden, non-ductile failure is inherent in the use of the HCB technology because the design of the beams is actually governed by deflection,” said Calderwood. “Any decomposition or structural loss of capacity should be readily visible by any maintenance crews and or the general public.”


Construction began in the spring of 2010 with the installation of seven pile bents comprised of concrete-filled pipe piles with rock anchors tensioned into bedrock. Each bent was constructed with only one row of piles to allow for deflection under temperature loads. Only one bent was constructed with two rows of piles to provide longitudinal fixity. The only expansion joints are located at the abutments.

In the fall of 2010, prior to completing the substructure construction, the general contractor, Wyman & Simpson, began erection of the HCB units. Using HCB, the contractor was able to drive the trucks with the beams on the existing bridge and off-load them using the same barge and crane needed for the substructure. With the HCB, the contractor was able to ship the beams across the existing timber bridge even with the posted load restrictions.

On average, the HCBs were erected at a rate of approximately eight beams per day. After setting the first four spans of the bridge, the contractor placed the concrete for the arches in the HCB units. By placing a hopper with a steel tube into the tops of the beams, it was possible to fill each beam in about 20 minutes. Again, the contractor was able to place all of the concrete compression reinforcement in one span within one day. Further, the reduced shipping and installation costs associated with the advanced composite structure made the solution cost-competitive with the conventional precast box-beams on a first cost basis.

“One of the biggest advantages of HCB is the extreme light weight for shipping and erection,” said Kim Suhr, vice president, Wyman & Simpson. “Each of the 70-ft.-long beams for the Knickerbocker Bridge weighed only 5,000 lbs. As a result, four HCBs could be shipped on a single truck instead of one truck per beam as would have been required for precast concrete beams. Further, we were able to erect the beams using a small crane instead of mobilizing a 200-ton crane. This resulted in a significant cost savings. Further, no deck forms were required.”

Once the beams were filled, the contractor began placing reinforcing for the deck pour. With top flange widths of 4 feet, the beams were placed tip to tip so that no deck forming was required. Scupper details, screed rails and reinforcing details were no different than those for a comparable precast concrete bridge.

The first half of the deck was cast in October 2010. After working through the winter to complete the remaining piers, the contractor completed installation of the second half of the HCB superstructure in April 2011. The bridge was officially opened to traffic on June 11, 2011. “I will definitely design with HCB again,” said Calderwood. “I think it is a wonderful product and when used in the right location, it will benefit bridge owners for a long time to come.”


As a result of the success of this project, four additional HCB bridges are in various stages of design and construction, including multi-span bridges and longer spans on the order of 120 feet. With economies of scale and further advances in fabrication automation, it is now possible with the HCB to make sustainable structures using advanced composites a mainstream component for reconstruction of the world’s deteriorating infrastructure. “What makes the Knickerbocker Bridge unique is not only the HCB framing system, but the fact that this will be the longest composite vehicular bridge in the world—the first to be made continuous for live load—and that this has been accomplished with a structure that was no more expensive than a conventional concrete box beam bridge,” said Hillman.

Suhr also noted the long-term benefit of HCB—its anti-corrosive properties. “I am confident that this bridge will have a longer life span in this corrosive salt water environment than if it was constructed of simply steel or concrete,” he said. “We definitely will use HCB again.”

Nate Benoit, P.E., project manager, MaineDOT Bridge Program, concurred, noting that the department is already quoting other projects using HCB. “We believe HCB is a viable technology and another tool in the toolbox for bridge designers,” said Benoit. “HCB has proven to be a successful technology as demonstrated by the Knickerbocker Bridge Project. HCB offers corrosion resistance, is light-weight and it can be used with an accelerated construction schedule. We expect to see more of the technology used for bridge infrastructure projects.”

The project has garnered much attention from transportation groups nationwide, including the Technology Implementation Group (TIG) of the American Association of State Highway and Transportation Officials (AASHTO). The purpose of TIG is to identify and champion the implementation or deployment of a select few proven technologies, products or processes that are likely to yield significant economic or qualitative benefits to the users.

In June, TIG met in Bangor, Maine, to develop a promotional campaign highlighting various transportation departments that have already deployed the technology in order to educate their peers as to the benefits of the technology. According to Ken Sweeney, MaineDOT's chief engineer, this promotional campaign was started to bring engineers and people from around the country together to see the technology. — HC Bridge Company LLC,, 847/722-4072