Composites have a long history of use throughout the infrastructure industry. They are often utilized as reinforcing materials in repair and retrofitting of existing infrastructure applications. Composites also are the material of choice for corrosion-resistant pipes and tanks. And more bridge decks are now made from FRP than ever before.
A lifting bridge is made from lightweight composites to save energy
When the Denbighshire County Council in northern Wales bid out a pedestrian and cycle lifting bridge to cross the entrance of Foryd Harbor where the River Clwyd enters the sea, the desire to establish an iconic aesthetic was a top consideration. Of equal concern was the need to minimize the energy required to operate the lifting bridge in an efficient manner. The new pedestrian bridge – called the Pont y Draig, or Dragon Bridge – connects the town of Rhyl to Kinmel Bay and is the final link in 15 miles of traffic-free cycling across Conwy and Denbighshire Counties.
Consulting engineering firm Ramboll approached composites manufacturer AM Structures Ltd. and materials supplier Gurit to review its concept of an FRP bridge deck construction. Once Ramboll landed the project, it turned to Gurit to carry out the detailed engineering of the decks, which AM Structures fabricated.
The design combined an iconic central mast-like structure and caisson with two wing-like FRP spans that lift simultaneously to enable boat traffic to pass. The two bridge decks are each 30 meters long and just 6 meters at their widest point, creating a slender and elegant profile, particularly when the wings are raised.
To meet both longitudinal and torsional dynamics, the decks were fabricated using a mix of materials: They are predominantly glass-reinforced fiber and epoxy resin, plus four planks composed of carbon-reinforced fiber placed at key corners to enhance longitudinal stiffness. The structures were fabricated with Corecell™ M-Foam, Ampreg 21 (an epoxy wet laminating system) and glass and carbon reinforcements supplied by Gurit.
“In addition to the client’s desire for an interesting sculptural shape, the use of advanced molded FRP for the bridge decks reduced the weight of the spans, which led to a reduction in energy required to operate the lift cycle several times each day,” says Dr. Mark Hobbs, senior engineer with Gurit’s engineering services. “Optimizing the use of carbon fiber just where needed provided the necessary robustness without driving up the cost of the bridge materials unnecessarily. It’s quite a sophisticated structure.” With cables similar to a sailboat’s rigging, the 48–meter mast and wing-like decking is visible for miles when lifted.
The Denbighshire County Council held a competition among local primary schools to name the new eye-catching bridge. The 11-year-old winner aptly named the FRP structure the Pont y Ddraig Bridge, meaning Dragon’s Bridge in Welsh. When the two decks open inward, they resemble dragon wings. The name is also a nod to the Welsh flag, which features a red dragon in the center.