Tall order

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

Petronas Towers
Petronas Towers

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

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."

In the case of low-rise buildings-which are beginning to become more popular in programmes that call for preservation of indigenous architecture-architects can, and often do, make many decisions that have little impact on the overall viability of the structure. But supertalls are another category altogether.

"With structures that are at the limits of what can be built, engineering input right from the beginning is absolutely crucial," explains Michael Willford, Arup fellow and director of Arup's division of advanced technology and research.

Educating architects and structural engineers on the historical do's and don'ts is another way wind engineers can add value. If critical design mistakes exist, "It's much easier to think about variables at the conceptual stage rather than the change order stage," explains Edey.

They've made some sort of philosophical statement [with the Burj Dubai] that I don't quite understand. I've never come across a tall building before where, from the outset, the number one objective was to not have a damper in it. That seems bizarre to me. - Michael Willford, Arup.

"Quite often we see some pretty big differences between the predicted properties of the building and the final ones. That can have a significant impact on wind loads and it can provide some answers about how to deal with excessive motion," explains Roy Denoon, vice president, CPP.

Again, it comes back to having the right answers at your disposal. Perhaps that is why the more experienced structural engineers and consultants insist on wind engineers being involved from day one.

Denoon cites one study in which CPP tweaked the corners on a building just slightly-so much so that the alteration was unrecognisable from the street-but was able to reduce the wind load by 20%. For that case alone, CPP was able to remove around US$15 million worth of structural steel.

Another example comes from a project BMT is involved with where the building is designed to incorporate an unusual rooftop feature. The element is completely architectural in nature and function but extends 60-70 metres from the building.

By adjusting the aerodynamic shaping of that feature, BMT more than halved the wind load, and, at the time of writing, has almost completely alleviated any wind-induced sway problem.

"If we hadn't been involved, the client would have had to invest several tens of millions of dollars in damping systems," says Buttgereit. "The value of 'getting it right' is enormous," adds Denoon.

Blurred vision

Anytime someone takes the work of a creative person and begins altering this, adjusting that or changing the other, there comes a point at which the end product becomes unrecognisable from the original form.

Neither architects nor their financially flamboyant clients appreciate changing the architectural vision after they've invested in it.

"There are many factors that play into how influential the wind engineer can be on form finding," explains Buttgereit. "It often starts with how married the client is to a design he's bought."

"In other cases," continues Buttgereit, "it comes down to the quality of the input of the wind engineer. That usually depends on whether the consultant has prepared the client for subtle design changes."

While most experts agree that changing design elements depends on how well received the input from the wind engineer is, many are beginning to see a shift in the industry.

The shift is coming by way of architects and structural engineers becoming more familiar with the particular challenges of supertalls and a greater willingness to specify wind engineers very early in the concept stage.

"A lot of our RFPs these days seem to be getting us involved very early on initial evaluations and they're looking for our feedback," says Edey.

According to his experience, Buttgereit says that if wind engineers find that they can significantly improve the aerodynamic performance of a building through relatively minor adjustments, it tends to be reasonably well-received by both architects and clients.

Particularly in the Middle East, the client often buys a picture. He sees a piece of architecture, it appeals to him and that's what he wants. In some cases, that limits you in what you can do in terms of aerodynamic shaping. - Volker Buttgereit, BMT.

But, he is also quick to mention, there are cases when the client will not entertain the idea of shape change, which usually leaves them with structural penalties to pay or having to install costly damping systems to control windage and sway.

"The most cost efficient way of dealing with unacceptable dynamic behavior is by changing aerodynamic shape of the building," explains Buttgereit. "You can only do that if you're involved right at the start of the project."

Denoon finds that willingness or unwillingness to alter the form of a structure mostly comes down to word choice. "We never tell architects that they have to change anything," he says. "We just tell them that there could be advantages in changing something or another."

He is quick to point out, however, that, at the end of the day, the decision to alter the shape of a building should rest with the team of architects, wind engineers, structural engineers and the client.

Team involvement is crucial because any changes to the architecture impacts every other part of the building.

"Even if you just reshape the corners, you lose floorspace," explains Denoon. "If you want to maintain the same usable floorspace, you've got to go higher. If you go higher, your wind load increases, which increases your construction costs."

Knowledge on the ground

Especially within the Middle East, it's important for clients to understand that there are processes and principles that they simply do not understand. In these situations, clients trusting their team is every bit as important as the team having the right answers.

"It's not that clients are difficult, it's more that some of them are a little inexperienced," explains Denoon. "They don't know how much work goes into a supertall. It's a reflection of the maturity of the development process over here."

But, the local market is changing and beginning to understand more of the principles related to supertall buildings. They are also beginning to see some of the added value that comes from extending timelines and allowing more collaboration.

Still, there are those clients within the region that just aren't getting it.

"Sometimes you'll find a consultant that employs a wind engineer simply because Dubai Municipality has instructed them that they have to have a detailed wind study-it almost becomes like a parking fine they have to pay before they can drive again," says Buttgereit.

However, the problems with a lack of understanding don't end with inexperienced clients or developers.

Often times, the local government has been slow to grasp the seriousness with which street level wind will affect comfort and thus, footfall and ultimately, profitability of the project.

"On Sheikh Zayed Road, in particular, it's relatively calm when you're walking beside buildings but when you get to the gaps, you've got this howling gale," explains Freathy.

"Some of the most prestigious developments have large towers that comes down onto a podium where they've got public space planned...but the reality is that if these spaces are windy and no one likes to go to them, it detracts from the overall value of the project."

In Dubai particularly, the difference in the design wind speed is why you'd always want to consult a wind engineer. The thing that will sell the client the best is the likelihood that a wind engineer will probably save HIM money. - Paul Freathy, RWDI.

"In London," continues Freathy, "before you get planning permission to build something, you've got to conduct a test to find out the real street level wind environment. I've had a couple encouraging conversations with Dubai Municipality where they're starting to realise that maybe they should take this seriously, but it's not happening yet."

Fortunately, things are changing. Clients are beginning to understand the added value from more time for collaboration between the architectural and engineering consultants. "I think we'll start to see timelines stretching a bit more to allow for more collaboration in the coming few years," says Denoon.

The future of harnessing the wind

With the introduction of LEED and Estidama specifications and the movement toward green buildings, clients, architects and engineers are looking very seriously at ways to offset energy use in their buildings. It is impossible to talk about wind engineered buildings without bringing up the trend toward wind turbines on buildings and Atkins' Bahrain World Trade Center (BWTC).

"There is a lot of interest in [turbines]," says Buttgereit. "It's often the case that as soon as architects are faced with designing a low-energy building, the first thing they do is look to put wind turbines on it."

"They're a hot item right now," adds Edey. "I would have to say that we're looking at a number of buildings that people are considering. It seems that all of a sudden, turbines are the big issue."

"We're seeing a lot of push toward green buildings so we're definitely going to see a lot more people trying to integrate wind turbines into supertall buildings," adds Denoon. "We've been doing a lot of work recently on how best to do that."

However, opinions in the industry seem to be unanimous. When considering turbines on buildings, though they're popular, they're simply not that effective. On it's best day, BWTC recovers around 15% of operational energy. On average, the number is closer to 11%. "BWTC is a very unusual building. It's a freak project," says Denoon.

In a break from the norm, BWTC was designed around the concept of the turbines. Its shape and orientation were carefully specified to get the maximum efficiency out of them. On the whole, turbines tend to be a bit more of an afterthought.

"It's more typical that you end up with a contribution of about 2-3% of the total electricity of the whole building," says Denoon.

"My view is that putting small scale turbines on buildings is not very effective," says Willford. "If you want to invest in wind power, then go and find a windy site and install really big wind turbines that will actually do something."

According to Edey, you just have to look at companies that are proactively developing energy from wind and pay attention to where they're putting turbines.

"You'll see that they don't usually stick them on rooftops in urban areas,' he says. "They put them out in fields, out on clifftops and large open water fetches in order to maxmise their efficiency."

Another opinion on which industry experts seem to agree is that while they're not  overly energy-efficient, the political value of putting wind turbines on buildings is massive.

Pentominium is one of the projects that is very tall but has done some of the wind engineering things very right. The shape is really good so the wind loads are much smaller than you would imagine on a building of that height. - Roy Denoon, CPP.

Builders throughout the region are searching for evermore innovative ways to align themselves with the sustainable movement. One proven way to make a definitive statement is to put turbines on a building.

"Are they particularly good green value?" asks Denoon. "Maybe not. But they're a very strong statement about the green credentials of the building and the client."

While putting turbines on a building may be a strong symbolic gesture, their real contribution and economic payback leave much to be desired. "They're very expensive ventures," explains Buttgereit.

"That type of project needs a very rich client who wants them as a statement of sustainability. Returns on investment are not very high."

So, while turbines come with an inherent message that client A or developer B is doing their part for the environment, it is doubtful that energy companies will begin looking to urban rooftop wind farms as the future of energy production-no matter how many new buildings incorporate them.

A final word

The building trend in the Middle East is moving toward supertalls. Strip away all of the seemingly noble reasons, and fame and fortune are perhaps the two most important justifications for striving to build the world's tallest tower.

Supertalls bring higher-profile residences, which include more expensive penthouses, which attract wealthier inhabitants and increase overall property values. This is a relationship with which developers the world over are quite familiar.

There is however another relationship that those developers should be thinking about in this region. The taller the tower, the more it moves at the top. The more a tower moves, the less comfortable its inhabitants become.

Those living in penthouses, generally speaking, have a much lower threshold of acceptance for a tower that sways and creaks.

The relationship between that which is expected from an expensive property to that which is accepted by its residents is very strong. Put simply, no one wants to live in a tower that moans and groans in the wind, least of all, wealthy residents.

Wind engineers have been studying the effects of wind loads on the built environment since the 1960s. Wind load is the dominant factor in tall structures, and as such, wind engineers find places where they are uncomfortable, inefficient or unsafe.

If developer A is looking to invest a billion dollars on a new signature supertall, it just makes good business sense to consult a wind engineer as early as possible.

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