By Linda Ness PrEng., Linda Ness Associates
(Edited by Spencer Erling)
The stadium bowl façade is a uniquely designed system which embodies the visual simplicity and transparency of the iconic stadium structure, which itself is a beautifully engineered achievement. Peaking at 40m high off the elevated podium, 20 000m2 of perforated powder-coated aluminium standing seam sheeting sweeps round the bowl with a geometry of deceiving complexity. In some places leaning outward up to 30 degrees, the design and supply team were on a journey to resolve what essentially wants to be a static planar cladding system, into one that rakes warps and racks. (Ed: as derived from the old torture system, stretches, curves and wants to pull out of its supports under differing wind loading conditions.) (See picture 1)
The cladding design and supply subcontract was won on the back of a two part bid and negotiate process. Our team was awarded the subcontract in late 2008 and the cladding was completed early 2010.
Stainless steel played a major role in the suite of materials used for the ‘purpose designed component system’ that attaches the perforated sheeting to the bowl wall. Situated but a few hundred metres from the Indian ocean in the warm and humid Durban environment; corrosion issues played a major role. High strength duplex steel (see explanation below) offered an excellent interface with the higher yield strengths playing a role in structural challenges. (See picture 2)
A series of horizontal steel box beams collect a notionally vertically striated system of purpose designed protruding aluminium ‘fins’. Each ‘set’ of fins stand proud between the large steel box columns to form discrete bays of cladding. Horizontal sheeting rails are connected between the fins, and the proprietary powder-coated standing seam sheeting system with a 60% open punch-perforated tray clips over proprietary extruded halters, and the tray seams are finally zipped tight together. (See picture 3)
The solidity of the primary reinforced concrete framework ends some 20m off the podium level, and hands over to prefabricated steel box columns, on spherical pins, that are tied together at the head with the bowl compression ring, also a fabricated steel box. The columns were temporary stabilised during the erection of the compression ring. Release of the columns, cablenet
attachment and sequential tensioning of the tensile fabric roof structure made sure that the final static geometry of the bowl wall structures was not available for the cladding installation until at least the cable net was fully tensioned. Thus the cladding attachment system had to be a component based Lego-set. (See picture 4)
UNDERSTANDING THE STATIC GEOMETRY
The geometry of the bowl surface describes what can be loosely termed as a ‘ruled surface’, singly curved. As such the surface can be populated by a series of straight lines. From the North and South ends the vertical surface starts to rake outwards as the top edge rises. The outward lean achieves a maximum of 30 degrees in association with the maximum height of 40m at the East and West.
Closer inspection reveals a varying warp effect in each bay, which within the global geometry is handed and reflected, making some 50 differently warped bays repeat only three times each. It soon became clear that the connection complexity would lead the design effort. (See picture 5)
Another look reveals the architects’ desire to vary the set angle of the vertical fins in sympathy with the circulating column axes as the bowl sweeps around the 800m circumference. This means that the fins are skew to some degree in each bay, and any thoughts of traditional linear bracketry were finally left behind.
…AND IT MUST BE ABLE TO MOVE!
The wind loading forces on any lightweight large structure are significant, no less the effect on the Moses Mabhida Stadium. As the wind forces gather pattern around the bowl, the compression ring ‘breathes’ and flexes. Differential pressures on the fabric roof apply varied forces through the pullup tendons, causing deflection of the arch in sympathy. The columns supporting the compression ring are pinned in a way which allows them to spherically rotate at the base.
The end effect is a design requirement for the cladded bays below the compression ring to sway and rack in service. At the design case 50-year return wind loading, this relates to a 300mm vector shift at the top of the columns (some 22m high).
Warp, rake and rack… The only generic model that comes to mind is ball-and-socket: similar to car tow-hitches and roll-on deodorants. Sometimes the best seeds of engineering concept are drawn from everyday life. (See picture 6)
The primary connection component for the unique system was a machined and fabricated stainless steel ball with a single hole. Over 2 000 of these identically manufactured brackets slipped down the spines of the purpose designed aluminium fins, absorbing all of the raking and warping variations and bracketing the interface tolerances. The only two variations that evolved during design optimisation were variations in the fixing arrangements and plate thicknesses (rational design for varying wind loads up the height of the bowl.)
The Grade 316 machined ball is welded to a machine-threaded solid stalk, also grade 316, which allowed for half thread pitch adjustments out of the theoretical set plane during erection, and provided rotational release on elevation for each fin during racking motion – this eliminated lateral bending stresses in the fin extrusions.
The internally threaded ferrule is welded to a bent from plate 2101 Grade duplex steel channel. (Ed: This is an unknown material to the average steel structures person in South Africa. It differs from 316 stainless in that it has a third more carbon, more than double the manganese, 20% more chrome, 85% less Nickel and only 5% of the Molybdenum. It has a slightly higher Pitting resistance equivalent. What is impressive is that the yield strength is more than double that of 316 and the tensile strength is 30% higher and yet still has 30% elongation. So the material (developed by Outokompo) is pretty cost effective.) The channel slipped over the face of the support cold formed square hollow section beams. Allowance had to be made for both tolerance in the face dimension of the hollow sections, and tolerance to accommodate its final erected position. Unexpected rotation tolerances were excluded since the ball could be rotated in the fin extrusion!
Finally, the variation in the set angle of the fin to the support box beams was simply taken by the ability to rotate the channel about a vertical axis, on the box beam. (See picture 7)
The channel is fixed to the box beam with a designed arrangement of grade 316 stainless steel Tek screws. These were supplied with hardened steel drilling tips. Once aligned, the Tek screws used pre-laser cut holes in the channels as a guide to drill straight through the hollow section beams (in-situ). Full size mock-ups were load tested to investigate / confirm calculated Tek-screw groups for shear strengths and to determine any possible redundancy.
The setting out and fixing of the ball brackets was the absolute benchmark for the success of the system, there was no time for errors, rethinking or recycling. A common digital interface 3D model of the bowl structures was used by all consultants.
A centrally positioned Tek, the so called ‘swivel Tek’ on each bracket was located on the digital model, translated by a specialist land-surveyor to a physical point on the support box beam. Interface tolerances were checked, the ball bracket was slipped onto the box and the ‘swivel Tek’ installed. A digital inclinometer was used to set the pre-calculated and scheduled rotation angle for the bracket, and the balance of teks installed. Gaps were epoxied. (See picture 8)
The ball was screwed into position, and the fins threaded through, with site drilling through the extrusion to fit the single connection bolt through each ball completing the connection.
moses mabhida façade team
Façade engineer : Linda Ness Associates cc
Façade contractor : Façade Solutions cc
Specialist analysts : Endurasim (Pty) Ltd
Specialist drafting : Cladline cc
Steel fabricator : Rebcon Engineering (Pty) Ltd
Injection moulders : Extreme Manufacturers cc
Extrusion suppliers : Wispeco (Pty) Ltd Aluminium
Fujian Nanping Aluminium Co. Ltd
Sheeting Contractor : MJ Cheetah and Co. (Pty )Ltd
Sheeting Supplier : Hulamin (Pty) Ltd Roofing Solutions