It's a common misconception that the biggest challenge in designing supertall buildings is how to withstand the effects of an earthquake. But as Gensler principal Ben Tranel points out, the toughest effect to deal with is actually the wind.
And Tranel should know, having recently completed work as technical director on the 632m Shanghai Tower, China's tallest and (currently) the world's second tallest building.
While the Shanghai Tower uses a five-storey high, 1200 metric ton Tuned Mass Damper (TMD) to mitigate the amount of sway, its design also has a second, unique way of countering the effect of wind the building's distinctive curved facade.
"The tapering and twisting shape increases the effect of vortex shedding," says Tranel. "Air hits the building, clings to it as it goes round, and is then released. The sooner it's released, the lower the pressure on the building."
Although initially being interested in the twisting effect for aesthetic reasons, Tranel says the team at Gensler did have some intuition of how it would behave structurally.
"We developed different iterations and tested them with wind tunnel engineering. From those we determined which had the most impact within the bounds of construction outcomes. The greater the twist, the greater the complexity of construction."
The tests determined that a twisting, asymmetrical shape with a 120° rotation was optimal, cutting wind loads on the building by 24%, and resulting in US$58 million savings in building materials.