11 July 2019
One effect of advanced modelling designers of contemporary stadia is that architectural form has become increasingly complex. The availability of new materials, such as tensile membranes and PTFE, means that lightweight structures can be made even lighter and thus more susceptible to wind.
While it might not be intuitive to everyone, structures are inherently flexible – famously tall buildings and bridges but also stadia. Typically, the taller and more slender the building, the longer the bridge and the larger the stadium roof, the more flexible they can be. Upon completion Dubai Creek Tower will be over 800 metres tall. Mercedes Benz Stadium is close to 100 metres in height and could safely house the Statue Liberty. Lusail Iconic Stadium, the main stadium for the FIFA World Cup Qatar 2022, will when complete, span over 300 metres.
Pushing the limits of size can sometimes mean, with the right design ingredients – a lightweight roof supported by a cable net, for example that the structure can be very flexible when considered against the norms we have come to expect as engineers.
As little as 10 years ago the fundamental natural frequency of a stadium roof would have probably been somewhere in the region of 0.8 Hz
In recent years, we have seen this number plummet, first to around 0.5Hz and then to as low as 0.2Hz. This is as low as we might see for a 200 metre tall building and it has moved roof structures from a region where wind-induced excitation was relatively low, to one where it is potentially very significant and a driving factor in structural design.
So, what can this mean? Well, the more flexible a structure, and the lower the natural frequency, the more likely it is that the structure will respond to the excitation caused by the wind. And with that response comes considerably higher forces. Higher forces require more steel and more money to pay for the steel.
In one case recently, the response of the structure was big enough – over 100% increase in localised wind loads in regions of the roof that were the most dynamically sensitive – that the designers opted to include additional bracing supports to the roof structure. Fortunately, state-of-the-art wind-tunnel methods are available to derive accurate wind loads, and to challenge, optimise and verify the design performance for even the most complex of geometries.
*As published in PANSTADIA & ARENA MANAGEMENT 2ND QUARTER 2019
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