High society

The design of what will be the world’s tallest building presented its developers with a particular set of aerodynamic problems.

Once complete, Burj Dubai will claim the title of the world’s tallest building. At over 2,000ft it will be almost twice as high as the Empire State Building and over 500ft taller than the current record holder, the Petronas Towers in Kuala Lumpur, Malaysia. But to reach such dizzying heights, unique engineering design challenges have presented themselves.

It may come as a surprise to some design engineers, but during the development of the building – due to be finished in 2008 – the latest hi-tech predictive software tools (such as CFD) took something of a back seat.

Instead, historical precedent, good solid engineering experience and extensive wind tunnel testing were the keys to success. Indeed, architects Skidmore, Owings & Merrill (SOM) devoted much of its time to close analysis of its earlier designs. These include Chicago’s John Hancock Centre, the Sears Tower, and the Jim Mao Tower in Shanghai – China’s tallest office building.

SOM partner William Baker, Burj Dubai’s lead structural engineer explained: ‘We actually find that CFD is not sufficiently developed, It doesn’t model the microscale wind turbulence. That is a significant issue.’ Following wind tunnel testing, SOM uses its own in-house software to analyse the data. ‘Once analysed we ‘tune’ the building like a musical instrument. We call that a ‘period optimiser,’ said Baker. The major concern for SOM with a building of this size is ‘vortex shedding’.

‘When the wind blows there are two primary effects – drag, resulting from the wind impacting on the building, and lift, which comes from vertices peeling off the side of the building,’ said Baker.

Vortex shedding describes the eddys of wind that gather around the top sections of a building, generating areas of low pressure. With wind speeds sometimes reaching 120mph, these low-pressure zones fluctuate from one side of the building to the other setting up vibrations known as resonant frequencies, which can cause major structural damage.

Although the building’s original design was based on a competition held at SOM a year ago, its design has continuously evolved during computer and wind tunnel testing on a 1:500 scale model at a facility operated by consultant engineer RWDI in Ontario, Canada.

‘Immediately after the competition we went to the wind tunnel with our first ‘draft’. From the initial set of data we re-shaped the building several times,’ said Baker.

The results led the designers to develop a building that employs a number of set-backs every 10 storeys – a tapered arrangement that can be clearly seen in the picture, left. This disperses the perpendicular and long winds that create eddys, causing vortex shedding.

‘The reason you shape the building this way is so you don’t let the eddys get too organised. The rate they occur is related to the width of the building and the shape of its footprint,’ said Baker.

‘If you have a building the same shape from top to bottom, these forces can become organised. If they recur at the same rate as the natural frequency of the building there would be huge dynamic effects,’ said Baker. He added that in particularly extreme cases vortex shedding can completely destroy a poorly designed building. ‘Galloping gertie’, Washington State’s Tacoma Narrows suspension bridge, for instance, shook itself to bits when vortex shedding set up extreme resonant frequencies.

For this reason Burj Dubai features a wide base, and contains numerous set-backs and wings that will scatter the wind and reduce the possibility of vortex shedding having a detrimental effect on the structure.

It is the very nature of Burj Dubai – its height and the materials from which it is being built – that necessitates this close analysis. The Empire State Building, for example, is not faced with such problems as it is constructed of heavy steel.

However, a building twice as high requires materials with exceptional damping qualities. Burj Dubai will be constructed of a high-quality concrete called HPC containing blast furnace slag and microsilicates, resulting in a material that is almost as strong as cast iron, but is more resistant to vibration damage as natural cracking in concrete dissipates the energy. ‘Usually concrete is all about strength, but here we were more concerned with stiffness,’ said Baker.

Buildings are usually subjected to 100-year test – the estimate of the biggest storm the building would face in a weather cycle of this period. The Burj Dubai model was subjected to 500 and even 1,000-year tests to consider all safety aspects and ensure there was no instability.

Other design issues to be faced, says Baker, are the construction logistics for a building of this magnitude. It is all very well planning the world’s tallest building, but the actual construction is another matter. Work is progressing, however. The foundations are already being laid and the pilings have been started.