Olympic Stadium – The Roof

Athletics is a sport particularly sensitive to wind conditions. Should cross winds or head winds also be strong then it makes it harder for the athletes to reach their peak speeds. Furthermore, if the wind speed is to exceed a record high tailwind of 2m/s to be broken the IAAF cannot allow events to stand. Therefore, it is essential that the design of an athletics stadium reduce wind speeds to within this threshold as much as possible. In extreme adverse weather, it is common for athletics events to be postponed irrespective of the stadium design.

At an early design stage, computation fluid dynamic (CFD) modelling was undertaken on a number of roof coverage options to establish the optimum wind performance level, given obvious cost constraints. Physical wind tunnel testing was conducted to verify that the wind speed restrictions could be met. In comparison to previous years’ weather reports, the results suggest that the wind speeds at the recorded points will be below the threshold for the majority of the days of the athletic competition. From these results, the amount of roof cover was established resulting in approximately two thirds of the spectators’ seats being located under cover, which was deemed acceptable by the client.

To support this amount of roof, a number of design solutions were investigated including conventional cantilevered solutions. Typically cantilevered solutions are economically efficient up to a tipping point whereupon the amount of steel necessary becomes disproportionate. The amount of roof cover and the elliptical shape of the seating bowl allowed the design team to consider employing a ‘bicycle-wheel’ type roof, which can be extremely efficient both in terms of the materials employed and economically. The design developed with a truss around the perimeter of the stadium which is pulled into compression by a ring of cables at the inner edge of the roof, and radial cables which run between the compression truss and the inner ring, all of which are in tension. The overall ‘wheel’ of the roof is therefore in equilibrium and simply needs to be supported from below and any rotation resisted; which is why the columns that support the compression truss are angled both in section and elevation. This creates a self-stable structure, independent of the seating bowl to keep the dynamic forces of the two systems separate. The roof covering is a series of pre-shaped PVC-coated polyester fabric panels, approximately 1mm thick, which are clipped to the cables and tensioned into their final 3-dimensional form.