We take it for granted that the sporting action at the Olympics will be framed by a majestic stadium. But why do stadiums take the forms that they do? And why do these forms often differ quite markedly from each other? For example, contrast the awe-inspiring irregular lattice structure forming the 'Birds Nest' stadium at the 2008 Beijing Olympics with the understated elegant simplicity of the London 2012 Olympic Stadium (now the home of West Ham United FC). Or with the Olympic Stadium in Rio, where the sporting action is framed by pristine white roof trusses.
Modern stadiums are designed to give spectators an unimpeded view of the action, and to provide shelter from the elements. Each stadium may have a different required capacity and the "elements" will also differ (for example, the World Cup in 2022 will take place in the soaring summer temperatures of Qatar). The costs and availability of materials, machinery and manpower will also vary from place to place around the world. Thus, even before aesthetic considerations come into play, there appear to be many reasons why sports stadiums may take on a range of distinctly different forms.
However, there is another possible explanation: we simply haven't worked out what the 'best' structural form for a stadium is, and are still experimenting! (where 'best' could for example mean lowest cost, least use of material or smallest carbon footprint).
Establishing a Form
Whether designing a stadium roof or a tall building the traditional strategy is, as it would have been for a century or more, to start with a blank piece of paper, and to use the known design constraints to work up an outline concept. This can then be used to fix the positions and sizes of structural elements such as beams and columns. With the advent of powerful computers this last step can be performed rapidly, enabling the integrity of complex curved structural forms to be verified.
But how do we know that the form conceived initially is structurally efficient? The short answer is that we don't. We currently lack the structural engineering equivalent of the ideal Carnot heat engine, against which real-world engine designs can be benchmarked. This uncertainty means that the amount of steel or concrete used to form a structure may often be far greater than is necessary.
Looking to the Future
When designing a stadium roof the space above and behind the spectators constitutes the design space, where structural elements can potentially be placed. But where are the 'best' locations to place these elements?
Instead of using precedent or engineering intuition, so-called topology optimisation techniques can now potentially be applied. With topology optimisation the design space is modelled in a computer as a solid body, with low stressed regions progressively removed - a bit like to a sculptor chipping away at a solid block of stone to gradually reveal the final form. But this technique doesn't work well for skeletal structures like stadium roofs, where the final structure is likely to occupy only a tiny fraction (<1 per cent) of the available design space.
An alternative is to use so-called layout optimisation. In this case the design space is peppered with points, which act as the end-points of an array of interconnecting structural elements. Mathematical optimisation is then used to reveal the subset of elements forming, for example, the minimum volume structure. The solution can be used to benchmark other designs, potentially opening the door to structural efficiency ratings. The hope is that layout optimisation can also be extended to better take account of the multitude of real-world constraints the designers of large building structures face.
While efficient structures are invariably elegant, aesthetic considerations will often dominate in the design of flagship structures such as an Olympic stadium. But for the many other large building structures constructed each year, an approach which allows designers to significantly reduce material consumption would represent a major step forward; this is the subject of a current EPSRC-funded research project, involving the Universities of Sheffield, Bath and Edinburgh and a number of leading engineering design consultancies.