Composite materials: enter the dragon

The new lifting bridge over Rhyl Harbour in North Wales serves as an additional crossing for pedestrians and cyclists across the River Clwyd, from Rhyl’s West Parade to a newly created public area on the Kinmel Bay side of the river.

The opening, lightweight bridge, named Pont y Ddraig, or Dragon Bridge
The two composite decks were engineered by Gurit
(UK) Ltd and built by AM Structures, at Sandown,
Isle of Wight.

The opening, lightweight bridge, named Pont y Ddraig, or Dragon Bridge in English, has already become a landmark.

The two composite decks – the “dragon’s wings” – were engineered by Gurit (UK) Ltd and built by AM Structures, at Sandown, Isle of Wight.

The bridge consists of two mirroring, 8000kg, 30m long decks, hinged on a central caisson and lifted simultaneously by cables running up to an almost 50m central mast, which houses the pulley mechanism and lifting cables.

It opens several times each day, to give access to boats heading to and from moorings upstream.

The use of advanced moulded fibre-reinforced plastic (FRP) for the bridge decks saves weight and makes lifting cycles fast and energy-efficient. It also allowed a sculptured deck shape, which provides a striking, iconic sight when the bridge is opened.

Ramboll approached AM Structures in early 2009 to review the concept of the bridge deck construction and it approached Gurit UK, based in Newport, Isle of Wight, England, to review the structure.

FRP decks for Pont y Ddraig under construction at AM Structures, Sandown, Isle of Wight
FRP decks for Pont y Ddraig under construction at AM Structures, Sandown, Isle of Wight

Initial studies confirmed that the concept was feasible, with some minor changes in the geometry, and that the FRP decks would result in considerable savings compared with steel.

The lightweight and slender deck structure meant that detailed consideration of the dynamic behaviour of the bridge under pedestrian loading was required.

The bridge was designed with predominantly glass reinforcements with longitudinal stiffness enhanced by local planks of carbon fibre.

Gurit made extensive use of finite element analysis (FEA) to carry out transient dynamic analysis of the bridge, using load models from Eurocodes.

AM Structures built the decks using Corecell M-Foam, Ampreg 21 resin, and a mixture of glass and carbon reinforcements supplied by Gurit.