Built in the 13th century by the brother of Henry III, Tintagel Castle is a unique and cherished historical site. It’s known both for its striking location – perched on the rugged, windswept north coast of Cornwall – and for its long association with the legend of King Arthur.
Today, Tintagel Castle is administered by English Heritage and is among the most popular tourist destinations in England. However, the castle itself sits on a headland that can only be reached via an arduous approach involving many stairs. The route is daunting for most visitors, and difficult or impossible for people with mobility challenges.
To improve the site’s accessibility, English Heritage launched a competition for the design and construction of a footbridge between the mainland and the headland. Ney & Partners Civil Engineers and William Matthews Associates, an architecture firm, made the winning submission. They proposed an elegant cantilevered bridge with a striking feature: a small gap in the middle.
Most pedestrian crossings are structurally and geometrically continuous. The gap in the Tintagel Bridge design is a haunting visual element – and it creates multiple challenges for design and testing. The design team engaged RWDI to provide wind engineering consulting services to ensure the stability of this unique structure at its site, which is prone to extreme weather.
We needed to understand how the strong and changeable wind conditions at Tintagel Castle, combined with the steep and irregular topography of the site, would affect the bridge. To be sure that the structure as designed would remain safe and comfortable in all the conditions it would face, we conducted rigorous wind tunnel testing – which required a detailed model of both the bridge and its setting.
Drawing on satellite images and surveys conducted by English Heritage, we used detailed 3D contour maps to construct large-scale topography models with a CNC (Computer Numerical Control) router: a digitally controlled cutting tool. We tested a total of 21 possible wind angles, sometimes adding “upwind sections”: model elements that are not part of the structural system being tested, but which generate acute wind angles similar to those that might result from wind whipping along the coastline.
Our model-makers crafted the bridge model itself almost entirely by hand. It took a team of three experienced builders in our model shop a full three weeks to create a scale replica of the structure. With these detailed and accurate models in hand, we were able to begin studying the structure’s response to a wide range of wind conditions – all generated in our boundary layer wind tunnel in Milton Keynes, U.K.
Our studies covered numerous aspects of the bridge’s aerodynamic and aeroelastic performance – including its susceptibility to flutter and vortex shedding. We presented preliminary findings to the designers as we completed the tests, and sometimes the structural engineering team joined us at our testing facility to observe and discuss possible design tweaks to explore.
Had our tests shown that wind effects on the bridge – either the loads it would experience or the vibrations visitors might feel – would be excessive, we would have proposed design refinements to enhance the structure’s performance. Our tests revealed, however, that the existing design would meet the necessary requirements. We used our proprietary analysis tool, TopGun, to deliver the necessary design wind loads to the structural engineers.
Our rigorous testing was able to provide confidence that the bridge as designed would ensure the safety and comfort of visitors in all wind conditions – including in extreme storms to which the area is prone. No design refinements were necessary to optimize the bridge for local conditions. The design of the bridge is progressing and construction is expected to start in 2019.
Looking at the area today, a visitor might imagine that it would be impossible to both preserve the site’s dramatic appearance and also make the Tintagel Castle headland – with its cliffs and rocky terrain – accessible to visitors using wheelchairs, or to families with children in strollers. We were proud to work with the bridge’s designers to redefine possible at this remarkable heritage site.