Watson Steel Structures Ltd
Sir Robert McAlpine
Arsenal Football Club
The roof has two parallel primary trusses spanning 204m along the length of the stadium. These sit on 11m high tripods at each corner and in turn support two 100m span secondary girders that span East West between them. This framework of main girders supports 32 tertiary trusses which span back to the perimeter of the stadium where they are connected to a continuous ring truss resting on perimeter posts.
The main girders, tertiary trusses and perimeter ring truss are all triangular in cross section and constructed mainly out of tubes. Several costing exercises were carried out during the design development stage and it was found that the tubular sections were the most cost effective because of weight savings, less surface area and the fabrication details are much simpler when using tubes in triangulated girders.
Watson introduced some new and innovative techniques into the fabrication process to ensure that the large complex individual elements were fabricated to the necessary accuracy. This involved using the X-Steel model of the individual components to produce a three-dimensional template-jig of the component, which was then orientated to provide the best build angle and level for the shop floor fabricators. Using this new technique Watson produced complicated fabrications to a high degree of accuracy that fitted perfectly on site.
The roof girders are 15m deep and 10m wide and were delivered to site as a ‘kit of parts’. The assembly was difficult and challenging because of the complicated 3D geometry and temporary works were provided to locate all the individual components until they were site welded. The girders were assembled in halves and then lifted into position using large cranes.
The designers had to account for the stresses induced by the dead weight deflection of each half and the temporary trestles were kept in place until the entire roof including the perimeter ring truss was complete.
This successful project is an excellent example of how a major project should be managed. The key factors that have contributed to its success are:
- a clear vision from the client
- well considered, detailed designs integrating the client’s requirements in a visually striking structure
- early involvement of the main contractor and key subcontractors allowed construction issues and design requirements to be fully coordinated and the best value solutions achieved
- application of leading edge technology into the design and fabrication process
- proactive and positive attitude from all parties.
Floating above this 60,000 seat stadium, the design of the roof structure posed major challenges to the team, being within a constrained site and with restrictions on its height.
The intelligent steelwork solution involves a “dished” roof profile, hung from the main structure, enabling the main truss and secondary girder depths to be well accommodated. For spectators, the uninterrupted sweep and clear lines of the roof draw the eye towards the pitch.
There have been many stadium roofs constructed in recent years, and this development represents an interesting step forward.