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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Longest Flexible-Arch Bridge Planned

By Kevin Wilcox

Construction will begin soon on the longest span bridge to employ a flexible arch that is shipped flat to a site for rapid deployment.

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This bridge over a railroad track at the Pleasington Golf Club in city of Blackburn, Lancashire, United Kingdom, was constructed using a precast flexible arch that is shipped flat to the job site. Emma Martin, Story Contracting

February 10, 2015—The arch bridge has been prized for thousands of years by engineers and builders drawn to the inherent strength and simplicity of the form. Many of the oldest examples in the world today were built by the ancient Greeks and Romans, and some of those are still in use. It wasn't until the early 1900s that the arch bridge began to fall out of favor as engineers focused on less expensive beam-and-slab systems.

In the early 1990s, Adrian Long, Ph.D., FREng, a professor of civil engineering at Queen's University Belfast, began what would become a 20-year quest to research why the arch bridge was falling out of favor and how the form might be adapted to modern construction methods, which favor speed of construction and the use of precast concrete elements.

This professional journey led Long, working with students, to patent a structural system in 2004 for an arch bridge using precast members. The system, developed in collaboration with the Macrete Ireland concrete company, is now called the FlexiArch.

"The biggest challenge was introducing a new form of arch, even though it performs better and behaves like a conventional arch. We overcame that [by] simply starting with short spans and gaining the confidence of contractors, who eventually became enthusiastic supporters of the system," Long says.

Work will begin soon outside Portsmouth, England, on what will be the largest span in the world to employ this technology. The bridge will span the Wallington River in Waterlooville, a town of 20,000 in the south of England. The crossing will be 53 ft long, comprising a series of 17 40 in. wide units, each with 35 concrete voussoirs —trapezoidal bars that are a modern version of the traditional, labor-intensive, wedge-shaped cut stones. Each of these units in the span will weigh 16 tons. Engineers anticipate that the bridge will be placed in a single day, using a single crane.

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Gravity pulls the voussoirs—trapezoidal bars that are a modern version of the traditional, labor-intensive, wedge-shaped cut stones—into an arch shape when the connected units are lifted by crane. Emma Martin, Story Contracting

"Every bridge is unique. I used to work in bridge design in Canada and no two bridges were identical," Long points out. "The biggest challenge here in this particular one was the fact that it was just a slightly longer span than the maximum done before. Actually, structurally it is not a problem at all. And lifting is not a problem at all on-site. The biggest problems are just handling it and bringing it to the site."

The FlexiArch technology combines the labor and cost savings of precast members shipped to the site with the strength of a conventional masonry arch, once installed. The flexible arch is constructed as a series of voussoirs that are placed with their wider sections at the top, touching on another, and the narrower section at the bottom. Polymeric reinforcement is placed on top and this is then topped by a cast-in-place concrete screed approximately 40 mm thick. This creates a system of connected voussoirs that can be shipped to a bridge site on a flatbed truck.

When lifted by a crane at the site, the polymeric reinforcement provides tensile strength and the screed provides shear resistance. A series of hinges form between each of the voussoirs as the force of gravity transforms the components into an arch, which is then placed on previously precast footings. The degree of taper in the voussoirs controls the ultimate geometry of the arch.

The arches are then be backfilled with either a compactable granular material or concrete; concrete increases the strength of the span by a factor or 10, according to research by the team. Concrete also has the practical advantage of cutting construction time, enabling the bridge to be placed into service faster.

Long says the FlexiArch system produces a bridge with initial costs similar to those of a comparable beam-and-slab span. But because the lifespan is likely to be much longer, the full lifecycle costs will be much less, he says. "If you try to compare the price with a traditional masonry arch, [the traditional method] would cost an awful lot more—by a factor of 2 to 3 times," Long adds.

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The connected voussoirs can be shipped to a bridge site on a flatbed truck. The angles of the voussoirs determine the radius of the arch. Marcus Brierley, Plain Communication

Long recalls that the first time such a bridge was erected, the building crews were skeptical. "You could almost see the guy was very much a practical engineer, saying 'Here's another nutty professor who has a system that has no chance of working.' But the first one they built, they lifted it on a crane [and] you could almost see the eyes popping out of their heads when they saw this thing actually forming into an arch instantaneously," he says. "It's amazing."

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