Making it real
Maine takes two new composite bridge technologies out of the laboratory and into the real world
By Kathryn Buxton
On a cool day in early October, a bridge construction crew prepares for the delivery of beams to the shores of the Back River near Barter Island. Although they don’t look like anything out of the ordinary as they wend their way through Boothbay on a tractor trailer rig, the beams aren’t typical concrete or steel. They are hybrid composite beams (HCB) – each a glass fiber shell encasing a core of foam and high-strength steel fibers. The concrete that is at heart of the beams’ compression design will be placed on the construction site
The HCB bridge is the first of its kind in Maine, and one of only four in the world. It is being built by Wyman & Simpson of Richmond for MaineDOT from a design by John Hillman of HC Bridge in Chicago. The beams were manufactured by Harbor Technologies in Brunswick.
Calculating the ‘unknowns’
Old and new bridge building technologies can be seen side-by-side at the construction site where the new HCB bridge is going up next to the 80-year-old timber-pile Knickerbocker Bridge. The new HCB bridge is expected to have a considerably longer lifespan than the old timber structure – at least 100 years.
And while it is not the first hybrid composite bridge to use Hillman’s revolutionary design, at 540 feet, it is the longest and only multispan HCB bridge built to date, and that has presented some interesting construction challenges.
“There are some unknowns with this project,” said Eric Calderwood of Calderwood Engineering. Calderwood is the overall bridge design engineer on the project. “Composite beams aren’t as stiff as concrete and steel, and we’re looking at how much creep we’re going to get.”
“It’s a function of the materials and understanding their ability to sustain loads over time,” said Calderwood.
Calderwood explained that with a conventional bridge beam, the beams are more flexible and as a result, the effect of the weight of the bridge deck on them is known almost immediately. In the case of the HC beams, that shifting or “creep,” while likely to be only a quarter of an inch, will take place over a much longer period of time. Because the technology is so new and has never been used on a bridge of this length before, there has been a certain amount of educated guessing.
Calderwood also talked about other details on the HCB bridge that are not part of a typical bridge project. Because the composite material may react to ultraviolet light reflecting upward from the water, the underside of the beams has to be painted with a special gel coat to protect them. Also placement of the utility lines required some special design consideration. “We had to come up with tubes to carry the conduit across the bridge,” said Calderwood. They used tube steel set on the beams and hung the conduit between the tubes.
Nathan Benoit, who is the MaineDOT project manager for the bridge, said that “it isn’t as complicated as it seems.” Benoit said that the HC beams are merely an innovative application of materials that have been used for years to build bridges – concrete and steel. Said Benoit, “It simply is a matter of taking advantage of the best properties of these materials. That’s the beauty of the design.”
Kim Suhr, Wyman and Simpson’s project manager at the Boothbay HCB bridge site, also sees the benefits of the new designs. Like MaineDOT’s Benoit, he praises what he calls the “simplicity” of the design and said that construction has gone smoothly. While the bridge was originally scheduled to be completed by summer of 2012, the current schedule calls for completion in September 2011.
“This is a pretty neat design,” said Suhr and, considering this is the state’s first experience with an HCB bridge design, the project has gone very well. In fact, most of the project challenges have been very typical for a bridge of this size, such as driving the pile and setting the rock anchors for the piers.
One unique challenge was a result of the relative light weight of the HC beams. Two of the beams had the concrete placed at the Harbor Technologies facility in Brunswick and, concrete is being placed on the job site for the other 62 beams. The advantage was the lightweight beams are easier to set in place; the challenge was anchoring the beams to prevent them from shifting should a Nor’easter or tail end of a hurricane hit the region.
“We didn’t want these blowing down the river,” said Suhr.
The Boothbay bridge is just one of several composite bridge projects that either have been recently constructed, currently are under construction or soon to begin. The state has made a major commitment to composites, in part because of the much publicized bridge-in-a-backpack technology developed at the University of Maine’s AEWC Advanced Structures and Composites Center in Orono. MaineDOT created a program to take the state’s homegrown composite bridge design from the AEWC laboratory to the real world. The state’s investment in the HCB bridge in Boothbay also supports business development in the state. Last year, HC Bridge announced a partnership with Harbor Technologies of Brunswick to construct the beams.
With the HCB bridge underway in Boothbay, one bridge-in-a-backpack complete and another nearly done, Wyman & Simpson is currently the contractor with the most composite bridge experience in Maine and, possibly, in the world.
Brian MacFawn is the contractor’s project manager for the two bridge-in-a backpack sites: one in Auburn and one in Bradley. The Auburn bridge wrapped up construction in October. The Bradley bridge was paved in early November and will require some additional “clean up,” including surface paving, next spring.
MacFawn said that despite the ease implied by the bridge-in-a-backpack label, construction of the composite arch bridges at both sites had some unique contracting challenges. The design uses carbon fiber tubes that may be inflated on the site and infused with resin, but for one of its two bridge-in-a-backpack projects, the tubes were infused with resin at the manufacturing facility and then transported to the site. (The arch tubes, when deflated, fit into a container about the size of a large backpack – hence the nickname.) The composite arch tubes are secured to a headwall at either end of the bridge and then filled with concrete.
“The Auburn job was harder in the sense that it was a very deep hole,” said MacFawn. He explained how in a typical bridge, crews build up, constructing the abutment and closing up the excavation earlier in the construction. For the Auburn construction, the hole had to stay open longer and later into the construction process. “It was a deeper cut and an enormous hole to maintain a safe excavation,” said MacFawn.
Greener, faster, cheaper?
Working two of the bridge-in-a-backpack projects actually proved to be a benefit, MacFawn added, because lessons learned at one site inevitably paid off at the other. He said that Wyman & Simpson will be among a group of engineers and contractors getting together to share experiences and provide information that can help MaineDOT and the AEWC Center refine the design and construction process.
MaineDOT’s Benoit said the aim of the department’s composite program is precisely that: to create a better bridge through the use of composite technologies developed right here in Maine.
Benoit said that in the first composite arch bridges, there have been many lessons learned. The bridges have incorporated different headwall designs, and the effectiveness of those designs is being evaluated. MaineDOT also is looking at ways to reduce the cost by using fewer arches and reducing the time it takes to construct the composite arch bridges. “Greener, faster, cheaper,” he said is the goal – to make the technology more marketable.
“We’re working on streamlining the process to make them friendlier to construct and more cost-efficient,” said Benoit.