Super tall: it's what's inside that counts
Building tall structures is not just about creating the right image, writes Christopher Sell.
As the UAE continues to forge its identity on the world stage, the scale of high-rise buildings within the region continues to increase. Within the next few years, a number of tall buildings are earmarked for completion, which the developers hope will have a similar impact as the Burj Dubai, Burj Al Arab or Emirates Towers. But awareness is growing that adopting the techniques and methodology employed for smaller towers in high-rise structures is simply not enough. It is becoming clear that tall towers require an implementation of practices that are as unique as the building itself.
So important has the issue become that a two-day conference held at the end of January saw the great and the good of the construction industry - from Arup, Atkins and RWDI to RMJM and Hyder International - discussing the more acute issues that prevail within the subject of high-rise construction. Optimal tower design, creating viable tall building communities and innovative solutions to wind engineering, were just some of the more in-depth issues on the agenda.
The overriding feeling was that developers and contractors must be more aware of the idiosyncrasies of the region, be it meteorological or topographical, and appreciate that the rules for high-rise construction do not necessarily correspond to conventional towers.
Dr Andy Davids, director of structures, Hyder Consulting says the key issues include the physical forces at work in the region, specifically the ÃƒÂ¢Ã¢â€šÂ¬Ã‹Å“Shamal' wind, a wave of high-pressure that funnels through the Gulf between Saudi Arabia and Iran, which can last three to 40 days and possesses unique characteristics that could affect super-tall structures.
A key feature of the Shamal, according to Davids, is that wind velocity distribution differs once it reaches heights of over 400m. This would mean that, in general, a tall building may not be affected, but when you factor the Burj Dubai is mooted to be over 700m, clearly wind measurements will not be uniform. "This means the peak velocity can be hitting the middle of the building, so perhaps it's something to take notice of," he says.
Davids adds that he would like to see less reliance on wind studies which are traditionally taken a few metres above ground level, whose readings would bare little correlation for skyscrapers.
Paul Freathy, director, RWDI echoes Davids' view that there is too much dependency on wind extrapolation figures: "Wind profiles don't always behave as you expect, which is what you must bear in mind for super-tall towers." Such variations led to the team at RWDI to approach the testing for the Burj Dubai in a novel way. Ordinarily, a tall building would be wind tunnel tested using a 1:600 scale model, or 1:400 in exceptional circumstances. However, Freathy reveals that for the Burj Dubai, a 1:50 scale model was constructed to ensure absolute confidence in the wind test figures and ensure that vortex shedding (potentially strong cross-wind motions) were within prescribed limits.
Furthermore, Freathy says that designers of super-tall structure should consult wind-testing consultants in the early stages so that they are able to offer input to the design to ensure physical forces are within the required limits. He adds that RWDI was involved for at least two years during the design of the Burj, and the company employed this method with the design of Taipei 101, currently the world's tallest building (although the Burj is scheduled to surpass this by May 2007). The original design had square corners and resulted in loads on the building being too high. Consequently, small steps were put into the corner of the building design, which resulted in a 25% reduction in vortex shedding.
"Wind tunnel testing is still by far the most suitable testing platform," says Freathy. "But you must make sure you pick a good consultant who has a test house with the relevant experience and you absolutely must not use Computational Fluid Dynamics (CFD) to assess design loads or pressures - it is not there yet."
But pushing the envelope of building design has led to further areas that must be addressed, says Wulf Binder, product group manager, roofing and cladding, Novelis Deutschland. Binder claims that new fire regulations, introduced in the EU in 2002, which take into account flammability, smoke development and combustible dripping, have specific resonance for the Gulf, with its emphasis on composite cladding. "After two years of exploration, we found that all parties involved are not following the 2002 EN 13501 regulations," he says, before demanding that those responsible adhere to it in the future, citing the massive fire risk.
"A 5,000m2 faÃƒÆ’Ã‚Â§ade cladded with composite cladding is equivalent to 19,000 litres of fuel," says Binder. And 1kg of polyethylene, which is what most composite cores consist of, causes 2,300m3 of highly poisonous smoke, consisting of carbon monoxide, cyanide, carbon dioxide and soot."
After two years of research in the Gulf market, Binder found that all towers in the Gulf have composite faÃƒÆ’Ã‚Â§ades, except the Emirates Towers, and stressed that, in the future, only solid aluminium sheeting should be used. "Consultants must take action to ensure safety in tall buildings and ensure new regulations are followed," he says.
A modification in the UAE's approach to building would also be constructive, adds Davids. He suggests that there needs to be a change in the mentality of contractors operating in the region to see a project as a whole rather than as individual pieces. "One thing all contractors have to learn is to piece all the parts together; currently they tend to treat them as packages rather than a holistic object, and that has to change," he says.
Finally, engineering a tall building is something that can be achieved relatively simply by following fundamental methods. However, engineering a community within the building is another matter entirely and one - with the raft of high-rises emerging on new projects such as Business Bay, Jumeirah Beach Residence or the 381m Sky Tower in Abu Dhabi's Reem Island development - which is increasingly important.
Dr Falah Mustafa, CEO Bayt Al Khidma Properties acknowledges the importance of ensuring a community settles in relative harmony and is aware what must be done to ensure this attitude stays. "Tall buildings are about power and ego and certainly wealth and status," he says. "But they are responsible for noise, pollution, traffic jams and high service charges, so successful modern urbanism is about getting more positive externalities than negative."
The key to a successful high-rise community, says Mustafa, is a creative approach, strategic planning and a good infrastructure backed up with utilities. The perceived tardiness of Abu Dhabi's growth, could, says Mustafa, count in its favour. "Some people may accuse Abu Dhabi of being slow, but I think while we may be slow, we want to get it right." A US $5 billion (AED18.3 billion) expenditure on the infrastructure of Al Reem Island - the single biggest real estate development in the Gulf - is testament to this endeavour.
With an increasing awareness of the disparate elements of high-rise tower construction being demonstrated, Dubai's thirst for iconic developments could be satisfied in a more secure fashion than perhaps was seen in previous years.