Designing a new foot and cycle bridge over the River Derwent in Derby has given our bridge team the opportunity to progress bridge technology at a significant site of industrial history.
The competition-winning design is the face of modern engineering, and is unique in the UK as a variation on the cable-stayed swing bridge form. A 38m main span and an 18m kinked backspan are supported by three prestressed cables carrying load back to a 20m high raked mast. The design of the backspan was dictated by the site: it crosses a mill race, reinterpreted as a dynamic piece of public art, enabling access to the riverside walkway whether the bridge is open or closed.
One big challenge was the river's flow rate, which is not only fast but can rise rapidly by up to 1m. Under flood conditions, the bridge swings off the river, folding back into the newly constructed waterfront to Cathedral Green.
When it either of its fixed positions, the nose end of the bridge is supprted on end bearings. In transition, the bridge rotates on a central pivot bearing under the mast. The tailend bearing is continuously supported on a concealed track. The deck is a stiffened plated box girder, tapering in two directions. Its closed section provides the torsional properties needed to carry the out-of-balance forces induced by the geometry of the kinked backspan.
LUSAS finite element modelling contributed to the design of the deck, cables and mast, plus the assessment of thermal performance and the development of the the bridge's construction sequence.
The 90 tonne structure was delivered to site in four pieces following the construction of piled foundations and the installation of a temporary construction deck.