Unusual Footbridge Will Link To Remote English Island
Crossing will provide access to King Arthur-related artifacts.
To link the mainland with an inaccessible island site of the 13th-century Tintagel castle in Cornwall, England, engineers are cantilevering both halves of a 68.5-meter-long pedestrian bridge in small pieces and without heavy equipment.
With only footpaths for access, “design was driven by buildability,” says Matthieu Mallié, a partner of Belgium-based structural engineer Ney & Partners. Working with William Matthews Associates Architectural Practice, the team in 2016 won an international design competition for the crossing run by site owner English Heritage.
Aiming for an unobtrusive, low-profile bridge, the designers ruled out cable-suspended structures, says Mallié. They also turned down slender arch structures “because of very high thermal stresses” that would arise, he adds.
The solution was to cantilever the bridge in six prefabricated sections from rock anchors on either side of the chasm. A 4-cm midspan thermal expansion gap, nearly 60 m above sea level, will also give visitors “a sense of transition between the mainland and the island, the present and the past,” according to the owner.
With bridge depths reducing from 4.4 m at the abutments, the deck comprises top and bottom chords of painted steel box girders, as shallow as 14 cm, says Mallié. The chords are cross-braced with 3- to 6.5-cm-diameter high-stress stainless steel tubes to withstand the corrosive marine environment.
With limited access, main contractor American Bridge U.K. Ltd. rigged cables between a scaffolding-type tower on the mainland and rock anchors at a high point on the island. The cable crane’s 5-tonne capacity dictated bridge module sizes, says Mallié. Abseilers are bolting adjacent chord box sections together through heavy stainless-steel finger joints.
Accounting for most of a $6.3-million site improvement project, the new bridge is due for completion this July, replacing a natural stone arch that collapsed centuries ago.
Without the new crossing, around 0.25 million visitors a year must negotiate 148 steps via the chasm bottom to reach scattered remains of the site of the legendary King Arthur and his Knights of the Round Table.