Engineering analysis and design software
Bridge design and engineering

Case Study

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Cathedral Green Footbridge

  • cable stayed swing footbridge

  • unique cranked back span

  • design assistance, thermal assessment, and development of bridge construction method

Cathedral Green Footbridge is an award-winning pedestrian and cycle cable-stayed swing bridge in the centre of Derby that spans the River Derwent and an adjacent old silk mill race. It has a 30m main span and an 18m cranked back-span that  counterbalances the main deck. The bridge rotates on a pintle bearing, with a central wheel to support its weight and swings to one side automatically when the river is in flood. Ramboll used LUSAS bridge analysis software to assist with the design of the structure on behalf of its client Derby City Council.

Overview

Rambollís competition-winning design for the bridge is unique in the UK and a novel variation on the cable-stayed swing bridge form. Its scissor-like shape was partly inspired by a tailorís shears echoing the heritage of the nearby silk mill. The end result is a single bridge that spans both the river and the old mill race (that has now been re-interpreted as a piece of dynamic modern art) in one economic structure.

Construction and modelling

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 cranked backspan. Three prestressed cables support the deck and carry the load back to a 20m high raked needle-like mast.

Ramboll used LUSAS finite element modelling to assist with:

  • Design of the deck, cables and mast.
  • Thermal performance assessment
  • Developing a bridge construction sequence.

Bridge operation

Because of the riverís flow rate, and the ability for its level to rise rapidly by up to 1m the bridge was required to open during flooding so as not to restrict the flow. In a flood condition, the 90 tonne bridge swings off the river and folds back into the newly constructed waterfront to Cathedral Green. When in either of its fixed positions the nose end of the bridge is supported on end bearings.

In use, the bridge swings utilising a pintle bearing that is always in uplift and a very large central wheel which supports the whole weight of the bridge. The tail of the bridge is held down using a wheel running in a concealed track rather than by using kentledge as with traditional swing bridges. Despite its weight but because of its design, the operation of the swing bridge mechanism can be effortlessly controlled by one person using a specially designed console, or even be winched to a closed position by hand in around 20 minutes.

The bridge was officially opened in April 2009.

A presentation entitled 'The Design of Derby Cathedral Bridge, a Unique Cable Stayed Swing Bridge' was given by Steven Wilson of Ramboll at IABSE-IASS 2011 Symposium "Taller, Longer, Lighter, London, UK. 20-23 September 2011. Reference to the use of LUSAS on this project.

Awards

  • British Construction Industry Award in the small civil engineering project category in 2009 

  • Commendation in the 2012 Structural Steel Design Awards.


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