Wind – A Lesson from Sandstone Point. By Simon Byrne.
Over the weekend the Red Hot Summer Tour was slammed by an unexpected and severe storm at Queensland’s Sandstone Point. According to weather reports, the wind gusts in the area reached 110 km/h. To the production team’s credit (Powa Productions and Regional Touring), the stage structure withstood the onslaught and through great crowd management, no-one was injured.
Obviously wind exerts force on a structure, but as the wind speed increases, the force is not linear. The force is actually proportional to the square of the speed. So the pressure exerted on a 1 square metre panel from a wind at 10 km/h is only about half a kilogram of force. In 20 km/h it rises to about 1.9 kilogram force, and for 67 km/h it is 21.5 kilograms force (which happened more than 10 days last year in Sydney).
And on that day in Sandstone Point where the wind gusted to 110 km/h? About 57 kilograms force on a single square metre of surface was exerted. That’s 114 times the force of 10 km/h, yet only 11 times the wind speed. The outdoor stage had very large sides so the lateral forces must have been huge.
Event structures have dramatically large surface areas upon which the wind can act, so the potential for extremely large forces to be exerted must be accounted for. Hundred’s of square metres in surface area could mean up to thousands kilograms force being imposed horizontally in unpredictable ways. Backdrops, speakers, lights, LED screens and even truss all present as surfaces upon which the wind acts.
Wind speed normally increases with height. So the wind that one feels on the ground is stronger above. Therefore, the forces from wind are greater at the top of the structure than at ground level.
And then there is lift. Lift is the component of force that is perpendicular to the oncoming wind flow direction. It is how planes stay in the air. That means all structures need to be anchored to the ground extremely well, otherwise they may literally take off (I understand that the FOH tent at Sandstone Point was held down by eight crew, for fourty minutes!). By way of background some planes only need wind of about 45 km/h to pass over their wings to take off. It is common for planes to be flipped over in storms if they have not been secured to the ground properly.
The event industry is particularly at risk for wind related accidents because we surround our temporary structures with thousands of people, that is the point. Also a promoter is pretty much guaranteed to make a loss if a show is cancelled (even with insurance which I understand can be difficult to claim on) so the financial incentive to keep it going is very strong. “The show must go on!” as they say.
Strong winds, together with temporary engineering, large crowds and financial pressure to keep the show running, combine to make a cocktail where accidents with injuries or death are more likely to happen.
There is absolutely no doubt that skilled and qualified people with a sound knowledge of wind loads need to be involved in making event temporary structures safe. But I believe we need to do much more. We need to help the wind experts appreciate the unique nature of live events where large audiences are packed in around temporary, and perhaps unique, untested structures, and it is quite possible that the promoter will insist that “the show must go on”.
I’m delighted that the production boys and girls at Sandstone Point obviously did things right!
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