There was a time when football and athletics stadia consisted of little more than two covered grandstands each side linked by open seating or terraces across the ends. Sun, rain and wind had every opportunity to nourish the grass in the middle, and the grass had to endure little more than one event every 14 days. Yes, the sight lines usually left a lot to be desired, and those stadia which attempted to combine facilities for football and athletics condemned football fans to a distant view of the action. But as a structural engineering challenge they hardly compared to long span bridges or tall buildings.
Not any more. A Premier League football club or the operators of a national stadium now expect their facilities to generate massive profits by offering increasingly sophisticated fans a whole range of services. These include weather protection, improved sight lines and proximity to the action. Coupled with the requirements of the 1990 Taylor report on crowd safety these demands turn stadium design into a complex challenge, as Sinclair Knight Merz design principal Steve Morley explains.
'Everybody wants high c-values - that's the height of your sightline above the crown of the head in front of you. This should be at least 60mm, compared to the 15mm in the old Wembley's lower tiers. But higher c-values mean steeper seating, and proximity to the pitch requires overlapping tiers, so what you get is a deep bowl with a pitch sized opening in the roof high above ground level.'
Maintaining high quality turf in these conditions is virtually impossible. Cardiff's Millennium Stadium is only one of the 'deep bowl' stadiums that have encountered serious problems with its pitch. And with no universally acceptable artificial alternative to grass yet available, designers are having to resort to extreme solutions to keep the turf green and healthy.
Buro Happold sports sector partner Paul Westbury says that translucent roof panels are an obvious possibility, as is a fully retractable roof. 'But in a 60,000 seater stadium it's the seating bowl itself that cuts off most of the sunlight and creates rain shadows.
'And grass needs ventilation as well, to agitate the blades and the roots. One option is windscoops, another is to open up the primary seating structure with ventilation louvres or by jacking up seats.'
A more radical alternative is to physically remove the entire grass surface between events and store it somewhere with unrestricted exposure to the weather. Several stadiums around the world have gone down this route, with mixed success. Most radical is the slide out pitch, where the entire pitch and its supporting concrete slab is moved out in one piece and parked immediately outside the stadium. Less costly but also less convenient is the palletised pitch. The former is used in a number of venues in Europe, including Schalke's new soccer stadium in Gelsenkirchen, Germany, (NCE 13 April 2000). Few UK city centre stadiums have sufficient surrounding space to make this possible, but Coventry City is considering such a solution.
The advantage of removing the pitch is that the stadium left behind is a concrete bowl suitable for a wide range of profitable non-sporting activities - especially if, as at the Millennium Stadium in Cardiff, it is coupled with a closing roof.
High technology like this is sometimes used in an attempt to create a hybrid soccer/athletics stadium. The basic problem is the significant difference in the size and proportions of the areas required. An international soccer pitch measures 105m by 68m, and there must be at least 6m between the touchline and the spectator area. A 400m running track, outside lane to outside lane, is 170m by 75m - with more width needed outside this for jumping events. The result, as the developers of the new Wembley found, is a fundamental incompatibility between sports.
'There's also the problem that except for Olympic finals, athletics rarely attracts crowds larger than 20,000 or so, ' Westbury points out. 'In practice, trying to combine both roles usually leads to complex technology like movable seat tiers or two level stadiums, massive costs and unacceptable compromises on sight lines. And no atmosphere for athletics.'
Although some previous Olympic stadiums, like Wembley, have lingered on largely unchanged as major soccer venues, their drawbacks are so obvious that most designers now go for the one-way conversion option. After the main athletics event is over, the stadium undergoes radical surgery to transform it into a permanent long term facility for one sport - usually but not always soccer. The Olympic stadium in Atlanta became a baseball stadium, Sydney's will host rugby and Manchester's Commonwealth Stadium (see main story) will eventually be home to the Manchester City football team.
The real challenge for the structural engineer is the dynamics of the structure, not strength or stiffness, says Sinclair Knight Merz's Morley. 'Modern stadiums inevitably feature very large cantilevers to give unobstructed sightlines and long spans behind the seating areas to maximise the revenue-earning potential of off-pitch activities, so they're inherently lively structures.'
Any structure with a fundamental frequency above 6Hz vertically is unlikely to give any problems. Elegant and economic designs, however, are almost certain to have fundamental frequencies well below this, and a full dynamic analysis will be needed.
At issue is the comfort of the spectators rather than their safety. Human response to structural movement is still an underresearched area - 'there's still a lot of debate about what level of movement spectators can tolerate before they feel unsafe and start to panic, ' Morley points out.