Case Study

Gateshead Millennium Bridge

• cable stayed opening footbridge

• staged construction analysis

• nonlinear dynamic analysis

Gateshead Millennium Bridge is a striking arched opening bridge designed by Gifford and Partners with Wilkinson Eyre. Made of steel, and designed with the aid of LUSAS Bridge analysis software, the bridge stands 45m high and spans 105m across the River Tyne to provide a link between the newly revived Newcastle quayside and the Gateshead quayside opposite.

The 130m long deck is parabolic in elevation and of steel box section that tapers in plan towards the centre of the deck. It carries a pedestrian footway that varies from 3m to 5m in width as well as a 2.5m cantilevered cycleway. The main arch is also parabolic in shape and tapers both in plan and elevation. Whilst small river craft can sail beneath the bridge, for larger craft the cable-stayed double-arched structure pivots at the abutments through an angle of 40 degrees to give the 25m navigational clearance as specified by the client, Gateshead Borough Council. At the fully open position the suspension cables lay horizontal holding the pair of arches together. Huge 14 tonne castings on either side support bearings which withstand the outward and radial thrusts imposed. 

According to Shapour Mehrkar-Asl, one of the engineers on the project, "Using LUSAS was essential on the design of this bridge because a full 3D staged construction analysis to model the construction sequence, and a step-by-step integration with large angles, to model the opening and closing sequence was required". In all, 374 elements modelled the structure with 2-noded 3D bar elements, 3D engineering thick beams, and corresponding 3D joint elements being used. A whole range of detailed analyses were carried out on the structure including: linear static, frequency (eigenvalue) extraction, buckling (eigenvalue) analysis, and staged construction using a nonlinear analysis with a linear elastic material.

The staged construction facility in LUSAS allows a complete model to be built but lets the user activate or deactivate selected elements to suit the stage of construction being considered. It is particularly suited to cable stayed structures of this type where prestressing of the cables using initial stress or strain, and replacement or severance of cable stays is to be investigated. Dead, live, wind loading, and temperature effects were also assessed, with rigid supports being used for all models with the exception of the opening and closing model which required spring supports with contact gap elements.

As Shapour Mehrkar-Asl explains, "The forces and displacements produced from the LUSAS analyses were used to decide upon the design and staged construction method". He continues, "The calculation of the initial forces in the tendons to give the required profile for the bridge deck, allowing for any camber, were used in the staged construction analysis, and were an essential part in the design of the bridge."

A dynamic analysis of the opening and closing sequence using step-by-step integration is still to be carried out at the time of writing. For this analysis, elements will be amended to allow for a 40 degree rotation of the whole model with contact joint elements being used to allow for support separation. In addition, the 3D engineering thick beam element will be replaced with a nonlinear 3D engineering thick beam to accommodate the rotation in the model.

Additional LUSAS staged construction analyses were also carried out to investigate the lifting and transporting of the bridge from the construction yard, where it was assembled, to its permanent position. Additional elements representing the lifting assembly can be seen on the LUSAS model shown. 

The bridge was officially opened in May 2002.
 
"The calculation of the initial forces in the tendons to give the required profile for the bridge deck, allowing for any camber, were used in the staged construction analysis, and were an essential part in the design of the bridge."

Shapour Mehrkar-Asl, Project Engineer, Gifford and Partners

More pictures, additional information and live web cams are available at:-

• Gateshead Borough Council's dedicated bridge web site

• ArchitectureWeek