A war of engineering words is brewing over the Tintagel Castle footbridge.
The row has emerged after former Institution of Structural Engineers (IStructE) president John Roberts accused the footbridge’s designers of presenting a “false” description of the Cornwall bridge’s structure.
Roberts claims that the final designs prove that the bridge is actually a single span with pinned movement joint, not two separate bridges as claimed.
However engineers behind the bridge as well as Historic England have disputed the claims, sticking by their plans.
The plans outline “two 30m cantilevers reaching out and almost touching in the middle” and state that “the new crossing is not a single bridge but two independent bridges springing from either bank and almost touching in the middle”.
The planning application also reveals that “a pair of 40mm diameter pins between each half ensure that the bridge halves are aligned vertically and laterally”, which Roberts believes contradicts the claim that the structure is “two independent bridges.”
“This feature was clearly one of the key reasons why the design was selected as the winning entry,” Roberts told New Civil Engineer. “The competition entry said that there would be a clear joint between the mainland and island halves and that the narrow gap between them represents the transition between the mainland and the island, here and there, the present and the past, the known and the unknown, reality and legend.”
He added: “In spite of all the hype and all of the publicity the bridge is actually a single span with a pinned movement joint, and is not two independent bridges at all.
“It is very disappointing that the engineers have allowed such a misleading description to be published in the press, and a salutary reminder that architects are not engineers and should not be allowed to misunderstand, and more importantly misrepresent, the fundamentals of bridge design.”
The pins are required to lock the two structures together and stop a large step opening up between them when differential movement occurs. In planning documents submitted to Cornwall Council, it said a horizontal gap from 5mm in extreme summer to 85mm in extreme winter could open up.
However, despite Roberts criticism, Ney and Partners project manager Matthieu Mallie maintains that the bridge is still two separate structures and will still have a gap in the walkway.
“It will be built as two cantilevers, the bending moments are the same as if it were two cantilevers and each can take the dead and live load independently without having to act as a single span.”
Each slender cantilever will span 33m across the 66m gap. The trussed form of each structure is made up of two welded plate box girders around 250mm deep by 140mm wide for the top chords and a third girder at the ends forms a bottom chord which divides into two arches as it springs from the support. It will be 4.4m deep at the root, tapering to a 170mm deep beam approximately 20m from the support.
English Heritage has also stuck by its decision to award the contract to Ney and Partners and William Matthews. “The Tintagel Castle footbridge will be formed of two independent cantilevers, which reach out and almost touch in the middle,” a Historic England spokesperson said.
“The cantilever design is an important part of the construction philosophy, as it enables the bridge to be constructed without the need for any support scaffolding, which would impact the monument and SSSI [Sites of special scientific interest].
“These locator pins have been part of Ney and Partners and William Matthews associates design from the beginning and details of the pins have been included in all documentation, including planning documents, which the public and stakeholders were invited to view online and in person at public consultations in Tintagel.
“Though the pins will transfer the shear force between the two cantilevers, this does not change the fact that they are designed to be independent (i.e the bending moments are zero at the gap and maximum at the abutments).”
The bridge was picked from a shortlist of six designs which varied in structural form, however it was the only submission to propose a two cantilever bridge solution.
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