Analysis and design of footbridges for the Dubai Metro light rail project

• 3D modelling of steel truss footbridges

• Eigenvalue analyses to derive mode shapes and mass participation factors

• Assisted the design of large number of footbridges in a timely and efficient manner

The Red and Green Lines of the Dubai Metro light rail scheme are being constructed as a Design-Build contract by a consortium of international contractors. Atkins, one of the world’s leading engineering and design consultancies, is the lead designer to the major civil contractor of the Dubai Rapid Link (DURL) Consortium and is carrying out the full multi-disciplinary design and project management of the civil works for the project. Atkins used LUSAS Bridge analysis software to assist with its analysis and design of a large number of steel truss footbridges used to provide pedestrian access to the elevated stations on the project and to permit the crossing of roads along the route.

For each footbridge, 3D thick beam elements modelled the primary structural members of the streel truss, crossheads, and concrete pierhead and pier. Piled supports were modelled using the equivalent cantilever method. Joint elements of appropriate stiffness and freedoms represented the articulation of the bearings. Lumped masses, applied throughout the model using joint elements, and with an appropriate eccentricity, modelled the mass distribution of the cladding and finishes to the roof, walls and floor.

From a close inspection of the mass-participation factors, eigenvalues and frequencies obtained from each LUSAS analysis the primary longitudinal, vertical, lateral and torsional modes could be identified. Graphs of total mass participation plotted against frequency showed quite clearly when particular amounts of structural mass became excited for certain freqencies. In addition, animations of selected eigenmodes permitted easy visualisation of the true structural response.

Design and checking

Steel design was to BS 5400-3:2000 and BS EN 1993-1-8:2005. Post-LUSAS analysis calculations carried out on the main footbridge types included vibration serviceability checking to BD 37/01to ensure that the structure was not overly excited by pedestrian use, and aerodynamic checking to BD 49/01.

Manuela Chiarello, engineer on the project said: "When we did the analysis of the longest non-travelator footbridge type with LUSAS and checked for susceptibility to aerodynamic excitation as defined by BD 49/01 we found that, whilst the structure passed the vortex excitation check, it didn’t comply with the divergent amplitude response check (for galloping) essentially because it was an enclosed structure that was just taking too much wind load, so a wind tunnel test was additionally carried out. This proved the structure wasn’t actually susceptible to galloping."

David A Smith, Regional Head of Bridge Engineering at Atkins said: "The use of a modular design approach in conjunction with detailed LUSAS analyses of the main footbridge types enabled us to design the large number of footbridges required for the project in a very timely and efficient manner."

The Red Line was officially opened in September 2009. The Green Line opened in 2010.

Other analyses undertaken by Atkins on the Dubai Metro light rail project using LUSAS include:

• Seismic analysis of viaduct substructures to derive bending moments for use in designing and reinforcing the piers and piles.
• Rail track / structure interaction analysis to derive and check stresses in the rails and resultant forces in the structure.
• Re-design check of Abu Baker station concourse for a revised roof loading configuration.

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