By Pamela Hetherly, PE, and Lisa Dickson, PG
Last year, the American Society of Civil Engineers (ASCE) updated its Report Card for America's Infrastructure and gave the nation's bridges a grade of C. Nearly 5% of the nation's 150,000 structurally deficient or functionally obsolete bridges are located in New England, where long winters wreak havoc on the region's aging bridges.
Anticipating the expense that would be involved to replace these structures, in 2002 the University of Maine's Advanced Engineered Wood Composites Center and the Maine DOT began exploring alternatives. The resulting “bridge-in-a-backpack,” a term coined by Center Director Habib Dagher, refers to his creation's portability: forms to build a bridge as long as 60 feet that each fits into a large duffle bag.
Steel reinforcement not necessary
Fiber-reinforced polymer has been used for decking for more than a decade. But because the Federal Highway Administration hasn't developed standards for use of composites, engineers haven't specified the material for the substructure. Dagher and his colleagues believe their solution will be widely accepted.
Instead of I-beams and rebar, tensile strength is provided by 12-inch-diameter tubes of carbon fiber-reinforced polymer — which are stronger than fiberglass-reinforced polymer — that fold and store like a textile.
At the work site, the tubes are placed on steel forms built to length and curvature specifications. A vacuum pump infuses an epoxy resin into the tubes and left to cure for six hours to create hollow tubes. The 100-pound arches are then lowered into place, two or three feet apart, with a boom to fit the bridge profile, and are filled with concrete. The arches cure within 10 days.
(The arches are the latest hybrid bridge design in use. Wilmette, Ill.-based HC Bridge Co. has developed straight beams that combine with steel and concrete. Read more about that design in the April 2009 issue of PUBLIC WORKS; “Cost-competitve composites,” page 47.)
Corrugated composite decking is installed above the arches and covered with granular fill and subbase before being paved with asphalt. The head-walls are constructed using either composite or traditional precast concrete.
Because the composite shell keeps road salt and pavement de-icers from penetrating the arches and deck, the structures are expected to last 100 years — twice the design life of a typical bridge. So although composite can be twice as expensive as steel, developers argue that savings in transportation, labor, equipment, and maintenance offset the additional upfront investment.
Dagher expects the solution's cost to eventually drop about 20% due to typical market factors such as increased competition and availability of materials.
Three weeks of fatigue testing in which the substructure was subjected to 50 years' worth of truck traffic demonstrated that a 35-foot span handles a maximum load capacity of 75,000 pounds.
|THIRTY-EIGHT-FOOT-SPAN BRIDGE COMPLETED IN TWO WEEKS
Award-winning bridge-in-a-backpack uses half the concrete of precast.
The initial challenge for the 38-foot-span Auburn (Maine) bridge over the Royal River in southern Maine that was completed in October 2010, was in designing a substructure capable of accommodating the loads induced by the superstructure arches.
The self weight (dead load) and traffic loading (live load) for traditional spanning structures typically creates vertical reactions on the substructure. The abutments had to be designed to resist these vertical loads in addition to the high horizontal thrust forces that the bridge-in-a-backpack system's flat arch shape places outward on the abutments. Because the arches are embedded in the abutment concrete, the substructure elements had to be designed to handle the potential bending moments created by this continuity. In addition, geotechnical site conditions warranted use of deep foundations on the project.
The project's design team was able to incorporate all of those parameters and develop a cost-effective abutment design that consisted of cast-in-place concrete pile caps on a single row of steel H-piles.
That style of abutment used approximately 50% fewer piles and allowed for more rapid construction and lower material costs compared to traditional pile-supported footings, which require two rows of piles. The design team eliminated the added expense of battering piles, which are typically necessary to resist horizontal loads.
The headwall also presented design challenges.
Although mechanically stabilized earth wall systems usually work well with the bridge-in-a-backpack method from a constructability standpoint, using them for soils below the 50-year storm elevation is not recommended since potential loss of backfill material can create significant problems — including washouts. The design team evaluated a variety of systems but needed an alternative that:
In designing a headwall system to interact with the arches, the design team worked with Advanced Infrastructure Technologies, owners of the patented composite tube technology, whose team includes members of the University of Maine's Advanced Engineered Wood Composites Center — developers of the bridge-in-a-backpack technology.
The final solution included concrete steps cast on top of the reinforced concrete overlay to support a precast concrete modular gravity (PCMG) wall, or T-wall, system by The Neel Co. By founding the wall system on cast-in-place concrete steps, they used a proven technology in a new way.
This system satisfied each of the requirements:
Using the wall system on the headwalls provided consistency in aesthetics and construction methods because the wingwalls, adjacent to the headwalls, were also constructed with walls. Once the steps were cast, all remaining wall construction was rapidly completed with the precast modular units.
10% of state bridges to be composite
In late 2008, the state and university replaced the 70-year-old Neal Bridge in Pittsfield for $581,000. The 44-foot structure comprises 23 arches spaced at approximately 2 feet on center along an asphalt deck. The head walls were constructed with a composite sheet pile system. Although two years isn't enough to determine the long-term return on investment of these bridges, the Neal Bridge deck has performed as well as expected so far, with no need for resurfacing.
In 2009, the Town of Anson replaced the 28-foot McGee Bridge for $89,350. It was the lowest bid and $5,000 lower than the next lowest. Later that year Gov. John Baldacci launched the Composite Bridge Initiative, which requires the state DOT to use composite materials in at least 10% of replacements and new construction and made $6 million in grants available.
Now investment group Advanced Infrastructure Technologies is marketing the concept nationwide. All designs are engineered to exceed American Association of State Highway and Transporation Officials load standards for single-span bridges from 25 feet to 70 feet and multispan designs exceeding 800 feet. The standards-issuing organization hasn't yet endorsed the technology, but Maine is moving ahead regardless. As of December 2010, the state had contracted the company to build seven bridges with spans ranging from 24 feet to 60 feet.
Dagher expects more bridges to be built with the bridge-in-a-backback forms nationwide in 2011. (See sidebar, page 33.)
—Hetherly, PE, (firstname.lastname@example.org) is a technical advisor to Kleinfelder/S E A Consultants. Dickson, PG, (email@example.com) heads up Kleinfelder/S E A Consultant's office in Augusta, Maine.