Briton Fabricators Ltd
Dean & Dyball
Derby City Council
Cathedral Bridge is a new pedestrian link and a cycleway which has also been designed to form a meeting place where people can sit and take in views of the surrounding area.
The new swing bridge had to respond to site constraints associated with a lowbanked, fast-flowing river that can rise quickly by up to 1m. The bridge provided a flexible structure low enough to integrate well with the flood-plain environment, swift enough in its mechanics to respond to water fluctuation as needed; its sharp steel profile evokes associations with the tools – scissor, needle – used throughout the local history of textile manufacture.
The basic form is derived from tailors’ shears, in particular the action of the hinged blades as they open and close. The bridge deck evokes one of the blades with the pivot along its length and the force to open and close the blade applied at the opposite end.
It was the client’s wish to re-instate the 18th Century mill race once used by the famous Silk Mill. This was achieved by designing a single bridge that could span both the river and the mill race in one economic structure. The bridge’s kinked back-span links across the mill race while also contributing structural efficiencies to the design, since it counterbalances the main bridge deck which spans across the Derwent. The 20m high needle-like mast is connected to the hollow box steel section bridge deck by three pre-stressed steel cables: these provide support along the deck’s thicker outer edge, keeping the overall structural depth slender. Operation of the swing bridge mechanism is effortlessly controlled by one person standing at the tail end of the bridge on the western bank, using a specially designed consol.
Feasibility analysis took prominence in early design phases as this complex bridge had to be tuned to function under variable loading conditions both in its open and closed positions, as well as in its transitional mode. During transition the bridge rotates about a vertical axis on a central massive cast steel pivot bearing located under the mast. The tail end bearing is supported along a concealed track concentric to the pivot point, and a roller mechanism concealed beneath the elbow of the bridge deck provides continual vertical support. The nose end of the bridge is entirely unsupported during transition. Projected deflection values were calculated to within a hair’s breadth, pre-cambering the structure for efficiency of construction. Robustness was built into the design to account for unexpected behaviour during the building programme, but the first swing of the bridge came in absolutely level.
Keeping the structure as lean as possible reduced the use of materials, all of which were sourced and fabricated within 15 miles of site. Ecological sensitivity was taken into account. The design is futureproofed to accommodate potential rises in water levels. Analysing the structure to achieve the slimmest possible steel deck depth meant it was possible to calculate a generous tolerance for floodwater rise.
The skewed swing bridge provides a novel and successful solution to linking parts of the city which previously only had road bridges, hostile to cyclists and pedestrians.
The box girder deck is cable-stayed and slender, and the inclined mast is of Y-shape cross section. The fabrication, particularly of the mast, is of very high quality.
Steelwork shows its capabilities yet again.