Petronas Towers, Kuala Lumpur, Malaysia.



Tall order

by Jeff Roberts Wednesday, 20 August 2008

Wind engineers can add value, add safety and, ultimately, save money if they're involved in your supertall.

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After speaking with engineers from the world's most renowned wind engineering firms about design challenges for supertall buildings, to a person, their advice was identical and it was simple: "Get us in early."
What used to be testing for basic wind loads, orientation and shape, has changed completely because of four universally accepted realities: first, because clients are looking for more signature buildings, the architecture of high-rise structures has become much more complex. Simple engineering methods don't apply anymore.

The great thing about architects is that they can take engineering requirements and turn them into something very interesting and beautiful. - Michael Willford, Arup.
Second, structural engineers are now able to apply element analysis much earlier in the design, which leads to saving materials and, ultimately, money. Third, people are taking a more sustainable view of how they use resources.

Clients are beginning to realise more clearly the cost/benefit of sustainable design. Fourth, the higher a building goes, the larger the issue of serviceability-or, comfort of inhabitants-becomes. Comfort is based on perceptions, which is significantly more difficult to quantify.

"All tall buildings move so it is a matter of educating the public to that reality as well. You want them to move; you want flexibility," explains Tom Edey, director of operations for the Alan G. Davenport Wind Engineering Group at the University of Western Ontario.

"The further up you go in a building, the more the motion increases. What is an acceptable level of motion?"

It is also important to remember that nearly all net wind data comes from a typical weather station, which is measured at approximately 10m above the ground.

Once you start trying to extrapolate data from 10m up to 800m-which is what most wind codes are based on-it becomes very clear that the real challenge is actually defining the correct properties of the wind approaching the tower.

"When you do something out of the ordinary, like a supertall building, you shouldn't be using a code," explains Paul Freathy, managing director, RWDI International.

"Codes tend to be quite conservative so you pretty much always get lower loads if you use a wind tunnel than if you base your numbers on a code."

In fact, Dubai Municipality allows structural engineers to use a lower wind speed if they design using a wind tunnel compared to if you use a code, explains Freathy. "The difference is about 30% reduction in wind load. That's huge."

Experts agree that the key reason for wind testing tall towers is that you need to know what the right answers are before moving forward on a project. Finding the right answers in the ever-heightening skylines of the Middle East means getting wind engineers involved from day one.

Get 'em in early

Particularly with supertalls, wind can be the governing factor in design for the foundation and lateral stability system, so wind engineers need to be involved from the early concept phase. If this happens, it allows them to track the design as it evolves and offer safety and money-saving recommendations.

"Small changes in external shape can make quite a difference in the way the building behaves aerodynamically, explains Volker Buttgereit, managing director, BMT Fluid Mechanics.

"For example, if you had just 10% rounding on the edges, it might make 30-40% difference in the loads that design is designed for."



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