By: Arthur Schurr
In Northern Ireland’s County Fermanagh, an engineered alternative for Thompson’s Bridge in Derrylin proved that sometimes two spans are not better than one.
Derrylin is steeped in history. Featuring the 17th century Callowhill graveyard and the remains of one of the oldest castles belonging to the Macguire Clan (the ancient lords of Fermanagh), Derrylin also is a modern, prosperous village on the road between Dublin and Enniskillen (the largest town in County Fermanagh and home at times to both Samuel Beckett and Oscar Wilde). But history is by no means Derrylin’s only attraction. Running through Derrylin, the Swanlinbar River is a wildlife marvel. It is one of the few rivers in Northern Ireland that retains a significant and viable population of the rare freshwater pearl mussel, as well as oysters, the Atlantic stream crayfish, otters and kingfishers. But another feature has captured attention recently—Derrylin’s Thompson’s Bridge.
Constructed in 1936, the original Thompson’s Bridge was a continuous two-span reinforced concrete overbridge. The concrete deck slab spanned between five longitudinal concrete beams supported on a concrete substructure. With each end of the superstructure supported simply on rubber bearings, the central support was a reinforced concrete pier (not oriented in the direction of the river’s flow), which was continuous with the deck beams.
Though ostensibly commonplace, the bridge served an important local and regional function, carrying the A509—a primary single-carriageway route through Northern Ireland. With the bridge reaching the end of its lifecycle and facing increasing loads from heavier vehicles, the Northern Ireland Roads Service sought to replace it. In 2009, the Northern Ireland Roads Service awarded the approximately $2.5 million bridge-replacement project to a century-and-a-half-old building and construction firm, McLaughlin & Harvey. They, in turn, called on AECOM, a familiar collaborator.
“In 2003 we worked together very successfully on a bridge project, one where our design proved to be a key element in winning the project. For Thompson’s Bridge, they wanted a more commercial way to address the design challenges for the bridge’s replacement,” explained Robert Rocke, associate director of bridges in Scotland for AECOM. Providing structural engineering on the project, AECOM is a global provider of professional, technical and management support services. “So, we found several areas where we felt we could advance a better engineered design, a more efficient bridge that would greatly reduce maintenance costs over its life cycle.”
Two to one is odds-on favorite
The new design for Thompson’s Bridge boasts a 32-meter, single-skew-span, concrete-composite integral bridge with the abutments supported on a single row of precast concrete piles. Though seemingly straightforward, the design provided numerous advantages and featured considerable innovation.
By choosing a single-span scheme, the designers eliminated the need for a supporting pier in the river. That change is not insignificant. Taking the pier out of the river removed an obstruction in and disturbance to the river, improving hydraulics and flow. And by skewing the end supports, the team was able to reduce the bridge’s length, enhancing economy. Removing the river pier also eliminated the cost and risk of midriver construction, as well as the long-term maintenance liability of the pier and the pier bearings. The one-span solution addressed many needs on several fronts. But that was only the beginning.
“Each amendment the project team made was intended to provide a better, more efficient bridge for the Roads Service; that was everyone’s foremost goal. Nothing was done solely for the sake of innovation alone,” added Rocke. “But there was quite a bit of innovation on this bridge in terms of design, materials and technique. In particular, two aspects deserve greater attention.
“First there is the pioneering work of two Queens University associate professors, Dr. Su Taylor and Dr. Des Robinson, particularly their work with fiber reinforcement. Their work shows great promise for extremely high-strength reinforcement that has few, if any, corrosive limitations. And the other aspect of the project was the design of the deck slab using compressive membrane action (CMA). In the UK we have standards that allow us to use CMA both to assess the strength of an existing bridge slab and to design a new bridge slab. And monitoring CMA’s use on this bridge could significantly increase our knowledge of the mechanism.”
Road’s scholarship Classified as a demonstration project, the fiber reinforcement Rocke speaks of is rebar made from basalt fiber-reinforced polymer (BFRP). Used in the bridge deck slab, BFRP boasts nearly twice the tensile strength (900 megapascals) of steel, yet weighs 75% less. In terms of cost, it is approximately 30% to 40% less expensive than stainless steel, but about three times more expensive than common steel rebar. But Taylor cautions that the numbers deserve even closer examination, especially in terms of cost. For her, in fact, Thompson’s Bridge was a welcome opportunity to illustrate exactly what BFRP is capable of.
“It was somewhat of a rocky road, so to speak, getting to actually install BFRP. At one point I had to get the funding secured, as there was a bit of red tape. So we had a bridge, but no funding. Then that bridge was built before we could use BFRP. Then we had the funding, but no bridge to build. Fortunately, we got it sorted out with Thompson’s Bridge. And it is very good that we did. Used properly, BFRP enhances durability and sustainability, and both are key issues throughout Europe for the design, construction and life-long performance of civil infrastructure.
“BFRP is a mineral material that is mined, melted and extruded into fibers, in this case 12-mm-diam. bars. Unlike steel rebar that is subject to corrosive forces, salt and weather don’t affect BFRP. But there has been hesitation in using it, as it has a lower elastic modulus. The industry perception is that it engenders higher deflection and service problems. But the arching action in a bridge-deck slab actually improves service performance. When you combine arching theory with corrosion-resistant bars you get a more sustainable bridge-deck form.”
Though not yet fully accredited for use on bridge-deck slabs, BFRP has Det Norske Veritas approval (an extremely rigorous European standard similar to Lloyds Register Certification) for use in marine structures. Taylor reasons that marine structures face even greater corrosive forces, so its nonmarine use should be a relative given. However, because of unfamiliarity with it, BFRP also presented some challenges on this demonstration project.
For Thompson’s Bridge, Taylor and Robinson worked closely with the project team to install BFRP. Used in the middle two-thirds of the bridge span (with the remaining third of the bridge span using traditional steel rebar), BFRP was combined with CMA and self-consolidating concrete (to eliminate the need for vibrators). Cast in place over four longitudinal, precast-concrete, flat-bottomed U-shaped beams, the 22-meter-long, 10.9-meter-wide, 16-centimeter-thick deck spans 1.6 meters between the beams. Discrete optical sensors were embedded in the concrete to monitor BFRP’s performance. Before it could be monitored, though, it had to be installed properly.
“Again, BFRP bends much more than rebar. So the ‘steel’ fixers had to make sure to use the correct concrete spacers on-site. There’s a bit of a learning curve when using any new material, and this is no exception,” added Taylor. “But it’s worth the effort, especially when you consider cost versus value. Yes, in the short term BFRP appears slightly more expensive than common steel rebar. But many cost comparisons are done by weight, not volume. In examining the cost-per-use and the whole-life performance, BFRP is actually much less expensive.”
Putting on the pressure
As Rocke explained, BFRP’s use in Thompson’s Bridge hinged on its combination with CMA. Also being tested on the bridge, CMA enhances load capacity above that predicted using standard flexural theory in most design codes, making the use of BFRP possible in this context. By combating the brittle behavior and low modulus of elasticity—perceived drawbacks of BFRP—CMA eliminates these concerns. “Over the past 20 years, many concrete bridges in Europe have exhibited problems associated with reinforcement corrosion,” Robinson explained. “Their repair or replacement causes significant disruption and congestion, costing travelers and agencies quite a lot. In addition, bridge-deck slabs must carry heavier loads than ever before. At present, CMA is not taken into account in flexural design approaches; it is not in the current Eurocode design standards.
“But when a laterally restrained slab is loaded, an arching thrust develops—which enhances the flexural capacity. This enhancement has been recognized by a number of bridge authorities and highway agencies and is now being used by some consultants for design. It is our hope that Thompson’s Bridge will further that awareness.”
Pedal to the mettle
With sensors on the bridge, test results were easy to quantify. And the news was good. Taylor explained:
“The BFRP-reinforced concrete-bridge-deck slab in Thompson’s Bridge exhibited lower deflections than the similar steel-reinforced bridge-deck slab. The deck slab was capable of supporting a wheel load of nearly 40 tons with no detrimental effect and with strain values well within the service-load range. There was no visible cracking in the top of the slab and hairline cracking in the soffit in the 1.4-meter slabs. All of the test regions showed excellent recovery in deflection and strain after unloading. I think it demonstrates clearly that BFRP rebar provides an alternative corrosion-resistant system with improved whole-life performance as compared to corrosive steel rebar.”
Rocke agreed, and he added some further observations.
“The use of BFRP and CMA was not just an exercise; it was invaluable research into finding an economic and sustainable alternative for bridges. BFRP and CMA reduce maintenance costs for the client.
“And as a result, deck slabs may ultimately need far less material, allowing them to be produced less expensively. In fact, I suspect that if you have CMA for a given thickness of slab, you may be able to separate deck beams further apart to reduce the weight and expense of a bridge.
“The use of BFRP and CMA is really a stepping-off point that demonstrates the value of the materials and technique.
“Recognition is due to Su Taylor and Des Robinson for driving forward research in this field. They are changing industry practices.
“Also, the Northern Ireland Roads Service has shown a forward-thinking receptiveness to new techniques in facilitating this test.”
To trinity and beyond
Derrylin is steeped in history. But sometimes history is the key that can unlock the future. In the case of Derrylin’s Thompson’s Bridge, the replacement of an old bridge—using a new material innovatively combined with an established technique—holds great promise for the future of bridge construction around the world.
One thing is certain, the engineered alternative and use of innovative materials and techniques on Thompson’s Bridge proved that sometimes two spans are not better than one.
About The Author: Schurr is a New York–based freelance writer who covers transportation infrastructure.
