Case Study

River Lune Millennium Bridge, Lancaster

• cable stayed footbridge
• linear and dynamic analysis
• local analysis of masts

Whitby Bird and Partners, are the designers of the Millennium Bridge which spans the River Lune in the UK. The new bridge provides Lancaster City Council with a much needed pedestrian and cycle route into the centre of the town and re-establishes a crossing at the site of an earlier Roman and subsequent medieval bridge. Lancaster's history as an important maritime port is reflected in the masts of the new structure which are intended to evoke an image of ships docked at the quayside.

A three 'legged' structure, the bridge connects to two levels on the south side of the river. The main steel trapezoidal box girder deck curves around the 40m high tubular steel masts to meet an old Victorian railway viaduct and the aluminium gangway structure connects to the historic quayside. The bridge has a main span is 64 metres and a total length of structure of 140 metres.

3D static and dynamic analysis using LUSAS Bridge analysis software helped develop and prove the design. Bar and thick beam elements modelled the various structural members. Forces and moments were extracted from the beam elements and converted to give stresses in the plates to allow design to BS5400 Part 3. Eigenvalue extraction was used to check the suitability of the bridge with respect to wind and pedestrian excitation.

As well as performing global analyses of the structure a local model of the top of the mast was created using thin shell elements. Stresses caused by the 6 cables being attached to this region were obtained and used to further validate the design.

Steven Boyd, the Project Engineer on the crossing said, 'We find LUSAS to be a very useful design tool. We have confidence in the accuracy of the results that are obtained and good technical product support is available should we ever need it.'