NotesKhalifa Stadium Upgrade: The firms and roof engineering
When the delapidated Khalifa Stadium, Doha, Qatar, required refurbishing for the 2006 Asian Games, engineering consultants Arup (Sydney) converted a 20,000 seat stadium into a 50,000 seat stadium with a dramatic yet delicate roof and signature lighting arch.
The Stadium formed part of the Khalifa Sports City redevelopment in Doha, Qatar. Arup's primary role was to detail the design and produce the documentation of the Stadium's main roof and lighting arch. This was technically a very demanding task. Arup worked with a number of design and contruction teams in a number of different countries, including Birdair Inc., USA (manufacturing and installation of fabric panels), Schlaich, Bergermann & Partner, Germany (engineer safe methods of lifting and stressing cables), SEACAD, Malaysia (3D steel model of the roof and lighting arch and shop drawings), Wire Rope Industries, Canada (supply, fabrication and shop drawing of cables) [shop drawings = these are drawings, diagrams, schedules, and other data specially prepared for the work by the contractor or sub-contractor, manufacturer, supplier, or distributor to illustrate some portion of the work], International House General Trading, United Arab Emirates (day-to-day running of the project, coordination of consultants and sub contractors), VSL, Switzerland (safe stressing of the roof and lighting arch), Eversendari, Malaysia and United Arab Emirates (developed the temporary steel work design and directed the activities of Eversendari on site), Arup, Sydney, Australia (conceived the structure of the roof and lighting arch, developed the structural concept, delivery of engineering drawings and design on time and to budget, participated in the development of safe erection procedures for the roof and lighting arch, detailed design of the steel and cable elements, drafting of the entire structure in 3D, using Bentley Structural software), Midmac Sixconstruct, Belgium (responsible for the delivery of the entire project), Pfiefer, Germany (cable erection). GHD, Sydney (project management), Cox/PTW, Sydney, in association with GHD, Qatar (architects). The project commenced in 2003 and was completed in February 2005.
Using tension to give a cloud-like appearance, the stadium comprises only stressed cables and 15,000 square metres of lightweight polytetrafluoroethylene (PTE) [teflon] coated glassfibre membrane. The roof is supported by a delicate cable net structure. Comprising a series of radial cable stays and trusses, the cable net is secured on one side by compression arches and on the other side by the main catenary. The arches and catenary meet, so that the opposing tension and compression forces are balanced, with no eccentricity. They are tied down at the northern and southern end of the stadium by concrete buttress foundations.
The geometry of the cables and main catenary were determined such that the stressing of the main catenary, pulls and secures the radial trusses into the roof shape. This arrangement confers stability, allowing the roof to resist the strong and varied wind loads that are common in the surrounding flat terrain. Arup tested the arrangement in a wind tunnel laboratory to determine the worst possible wind loading on the roof, testing 12 combinations of severe upward and downward loadings.
The Lighting Arch
Mirroring the roof arch, a slender lighting arch extends across the opposite side of the stadium, providing a striking visual counterpoint. The arch was designed to support sports lights, loudspeakers and fireworks. Leaning at a steep angle without visible means of support, the steel arch spans 265m across the stadium. Soaring 75m into the sky at and angle of 24 degrees from the vertical, the slender cable-stiffened arch comprises two 1.1m diameter circular hollow sections. The arch is made from a slender Vierendeel truss that is anchored to the same buttress foundation as the roof (Vierendeel truss = Named after the Belgian engineer, Arthur Vierendeel [1852-1940], who developed the design in 1896. This truss has rigid upper and lower beams, connected by vertical beams. The joints are also rigid. All members of the Vierendeel truss are subject to bending moments. This truss has the one distinct advantage, namely, the elimination of diagonal members).
Careful consideration was given to the loading of this lightweight structure, because of the stadium's extreme desert environment. The stadium was designed to withstand a temperature range of 5 to 85 degrees centigrade, and models were tested in a wind tunnel to ensure that it would withstand the strong wind loads. Arup engineers developed and analysed cable systems, which had no obviously visible means of support, by using their own in-house GSS Relax software. A form-finding process was carried out on both structures to find the most efficient geometry and prestress field, ensuring that they were stiff and able to resist the applied loads.
The stadium's structural connections are attractive and striking in appearance. Arup developed a series of elegant cigar-shaped columns and struts to support the main roof arches. Arup investigated and developed design methodologies that minimised their tonnage by 15%. The key connective devices for the cables were clamps. On both the main roof and the lighting arch, clamps form the geometry and ensure that the pre-stress forces are transferred through the structural system. As well as carrying out the design in accordance with international codes and guidelines, Arup formulated specifications and coordinated the prototype testing to verify the clamping procedure and design.
Arup created 3D computer models of the main roof and lighting arch. These models allowed the transfer of detailed information during the design, fabrication, and construction stages, ultimately reducing the time spent on design and drafting. 3D modelling also facilitated the explanation of key details and 'structural logic' to the client and project team. This was vitally important for the complicated geometry-dependent Khalifa Stadium, and allowed Arup to communicate information ranging from the overall architectural impact of a connection to its finer build-up details, fabrication sequence and buildability. The modelling also facilitated Arup's checking of the shop detailer's model. This allowed a heavier reliance to be placed on the shop drawings and ensured that fabrication matched the design intent, resolving any issues prior to fabrication and erection.
Des Barrett, Curator Science and Industry.
(The curator acknowledges the assistance of Peter MacDonald, Senior Associate, Arup, Sydney, in compiling these notes).