Photo: Thorkild Amdi Christensen

Meet the man behind Burj Khalifa

Wednesday 04 May 16
by Morten Andersen

H.C. Ørsted lectures

Twice a year, DTU invites prominent foreign researchers to lecture on their work, research findings, and the prospects within their field of research at the so-called Ørsted Lectures. The lectures are open to all. Watch or revisit some of the previous lectures and find out about upcoming lectures.

Towering records

Burj Kahlifa Tower is named after Abu Dhabi’s ruler, Khalifa bin Zayed Al Nahyan, who is President of the United Arab Emirates. At 829.8 metres, the tower is the tallest building in the world. The construction of the building began in 2004 and was completed in 2010.

Among the many other records held by the building is the world’s highest occupied floor at 584.5 metres and the largest number of floors—163. The tower boasts a mixture of exclusive apartments and hotel rooms.

Bill Baker is the man behind the world’s tallest man-made structure, Burj Khalifa in Dubai. Here, he talks about the engineering challenges of extreme high-rise buildings.

“It’s just a project.” It almost sounds as if he is downplaying their efforts, when William (Bill) F. Baker describes his team’s contribution to the Burj Khalifa Tower, which was inaugurated in 2010 and to this day is the world’s tallest man-made structure at 829 metres.

“It’s not the height record in itself that interests me. In fact, I hope that the record will be broken soon. What’s exciting is that you have to develop radically different systems, when you want to take a leap upwards. It’s simply not possible to upscale the structures from lower buildings.”

As chief engineer in Skidmore Owings & Merrill (SOM)—an architecture and engineering firm—Bill Baker developed the unique hexagonal core structure behind the world record (see the explanation below).

In addition to the actual height record, Burj Khalifa also holds a number of other records, e.g. the world’s highest occupied floor at 584 metres, and has received a wealth of prizes. Therefore—to put it mildly—the project has not lacked publicity in the world media. An overlooked detail is, however, that Bill Baker and his team have found some of their inspiration in Denmark:

“At SOM, we follow the work taking place within topology optimization at DTU with great interest.”

Engineer and architect equally important

Topology optimization is about distributing the material of a structure in an appropriate manner. By far the most structures in our daily lives are oversized in the sense that you can cut down on the material and still obtain the required load-bearing capacity. When you want to expand the boundaries of what is possible—for example by constructing the world’s tallest building—it is crucial to avoid unnecessary weight.

At DTU, in particular Professor Ole Sigmund and Martin P. Bendsøe, Dean of Graduate Studies and International Affairs, have been pioneers in the field of topology optimization (see box below). Their methods have, among other things, been used to keep down the weight of the Airbus aircraft.

“We don’t have a research collaboration as such, but we use tools that others have created on the basis of the research conducted by Sigmund, Bendsøe, and their colleagues,” says Bill Baker.

In other words, applied topology optimization is one of the main reasons why building Burj Khalifa was even possible. Another precondition has been the collaboration between Bill Baker’s team of engineers and the team of architects headed by Adrian Smith from SOM.

“The traditional division of roles, where the architect first makes the drawing and the engineer then has to come up with the technical solution makes no sense in connection with extreme high-rise buildings. The engineer has to be involved right from the start, which also happens to be the way we always do things at SOM. I believe this is one of the main reasons why we’ve been asked to do so many projects in the area,” says Bill Baker.

Bill Baker visited DTU in December 2015, where he gave an H.C. Ørsted lecture about his distinctive construction projects for students and staff.

Photo: Thorkild Amdi Christensen

Selected after two intensive weeks

SOM was awarded the contract by the client, EMAAR Properties, after an intensive idea competition lasting just two weeks. Here, Bill Baker offered a design that was inspired by another SOM project. To understand Baker’s idea, it is necessary to know the fundamentally modern way of designing very tall buildings. In 1963, SOM engineer Fazlur Rahman Khan (1929-1982) launched the ‘bundled tube’ concept. The best known example is Willis Tower in Chicago. At 442 metres, the building—which was completed in 1973, and is also known as Sears Tower—was the world’s tallest building for the next 25 years. The building is made up of nine tubes of different lengths.

"I actually also design single-family houses now and then. It’s almost a kind of therapy for me, I guess. "
Bill Baker, chief engineer and partner at Skidmore Owings & Merrill (SOM) in Chicago

Khan’s basic idea of distributing the static forces—also called ‘loads’ by professionals—which the structure must bear on several individual tubes tied together has been used in the construction of all subsequent very tall buildings. However, the principle has seen a number of significant developments along the way.

In Seoul, South Korea, the Chicago-based firm had designed a 580-metre building with a further development of the ‘bundled tube’ principle, a so-called extended core. “In fact, we never got round to naming the principle. But ‘extended core’ is probably what best describes it,” says Baker now.

The construction work started in 2007, but was delayed due to complaints from neighbours and was later completely abandoned.

“At some point, it seemed as if the project in Seoul would be approved if the height of the building was reduced. We changed the drawings and reduced the number of floors from 130 to 92. But in reality, I was convinced that the design could actually have accommodated a significantly taller construction,” Bill Baker recalls. “So I saw the new project in Dubai as an opportunity to bring the design to life.”

To Bill Baker, it is important to be able to describe the basic idea behind building projects in clear and simple terms. In words, not just based on mathematics.

  Photo: Thorkild Amdi Christensen

Surprise in the wind tunnel

But the principle from Seoul was not geared for the new leap in height.

“When we—using a wind tunnel—exposed the extended core design to the wind speeds that Burj Khalifa had to withstand, to my surprise it went completely wrong!” says Bill Baker. However, today he sees this apparent failure as a blessing in disguise:

“If we had been very close to the goal, we would probably have tried to adjust the design. But now we had to come up with something completely different. This led us to the buttressed core, which Burj Khalifa got."

The combination of the buttressed core and the topology optimization has significantly minimized the need for materials in Burj Khalifa. Had the building had a traditional design, like, e.g., the Empire State Building—the classic skyscraper—it would have required twice as much steel and far more concrete. The low material consumption was crucial for actually building Burj Khalifa.

The building’s tripartite structure—which is actually hexagonal—has proved to absorb the vertical loads caused by the structure’s own weight extremely well. At the same time, it also prevents twisting.

“We also found out that we would be able to reduce the loads significantly by choosing the ideal location for the building in relation to the dominant wind direction at the site. By rotating the building 60 degrees in relation to the original plan, the forces in the wind tunnel were reduced dramatically,” Bill Baker recalls.

Buttressed core carries Burj Khalifa

br> Burj Khalifa has a hexagonal core. The six sides has two different lengths, which means that each of the long sides is surrounded by two short neighbouring sides and vice versa. Roughly speaking, the three long sides only carry themselves, while the three short sides each supports their own tower with apartments and hotel rooms—the actual function of the building. Invisible from the outside, each of the three towers has a core extending from the short side of the centre core. This means that each side core supports the centre core in a way that resembles the support pillars in large medieval cathedrals. Bill Baker has named it the ‘buttressed core’ principle.

Illustration: Marianne Rom after Paul C. Martens 



Topology optimization

A classic example of topology optimization is the load-bearing structure for an aircraft wing. On the one hand, you want to use as little material as possible to keep the weight down, both to save fuel and to minimize the load of the wing’s own weight on the structure. On the other hand, the load-carrying capacity must be high, so that safety is not compromised. The conflicting considerations can be boiled down to an optimization problem that includes a set of differential equations.

In 1988, Martin P. Bendsøe, then professor of mathematics and now Dean of Graduate Studies and International Affairs, began to develop computer-based methods for topology optimization. He has later refined the methods in collaboration with Professor Ole Sigmund, DTU Mechanical Engineering.

Based on a prior definition of which outer form the design must stay within, the set of differential equations are solved. In practice, you will never get it right the first time, but after several runs, you will gradually reach a solution, which is considerably better than the one human intuition would be able to come up with.

A building is like an instrument

Fortunately, reality has confirmed the results from the wind tunnel:

“For the sake of safety, we had made room for adding equipment that could reduce the building’s oscillations, but it turned out not to be necessary.”

The top of the building is made up of a 244-metre spire. In spite of the enormous height and strong wind, the tallest point of the spire sways a maximum of only 1.5 metres.

According to Bill Baker, the basic robustness of the design is only a part of the explanation. To illustrate his point, the chief engineer gets up and makes his tie swing from side to side. He then hits a half-full coffee cup in front of him. And then a clothes stand in the meeting room.

“All things have a natural oscillation. This applies to a building as well. We have optimized the design to control the natural oscillation. This is actually very much like a musician tuning his instrument.”

Generally speaking, you cannot carry out a large project like Burj Khalifa without making adjustments along the way.

“But, in fact, things went very smoothly in this project. In connection with ordinary low-rise buildings, the contractor often comes across problems that actually require adjustments of the design, but which aren’t acute. And then one day, you discover that the small problems have become very large. This is not the case with extreme high-rise buildings. Here, you solve the problems right away, because you know they could be disastrous.”

Bill Baker has many significant buildings on his CV, both extreme high-rise buildings and buildings with long-span roof structures. 

  Photo: Thorkild Amdi Christensen

Mixed ice in the concrete

When you look at innovation within other technical areas, the obvious question is whether skyscraper could be even taller by means of new materials. Did you consider that in connection with Burj Khalifa? The question makes Bill Baker smile broadly:

“No! Because we wanted to actually see the project through! If a contractor is to embark on an extreme building project involving innovative materials, he will demand a price so high that the project is doomed in advance. We therefore knew we had to use well-known materials—concrete and steel—which the contractor had extensive experience with.”

Even with well-known materials, the challenges were huge. For example, we had to pump concrete up to a height of 606 metres—another world record held by Burj Khalifa.

“In this context, we were favoured by the fact that Dubai has some of the best concrete in the world, among other things because the local water is highly corrosive, making it necessary to have really good concrete,” explains Bill Baker.

The concrete was mixed according to a special formula. Normally, you don’t give much thought to the fact that setting of concrete involves a chemical reaction having heat as a by-product.

“With the quantities used for the construction, and with the high daytime temperatures in Dubai, we risked that the concrete would be so hot that the chemistry got of control. Therefore, the contractor mixed ice in the concrete instead of only water as you normally do!”

Leave mathematics aside

In addition to Burj Khalifa, Bill Baker can add a number of other high-rise buildings to his CV, e.g. buildings with long-span roofs. Furthermore, he has also worked together with artists on several occasions. Perhaps to get different ideas?

“No, not really. I like working with artists. One of the things we have in common is that we like to see our projects being realized. The artists I have worked with have all been very responsive. They are willing to make changes to a structure if there is a good technical argument for doing so, but at the same time there has never been any doubt that the creator of the work is the artist, and not me.” 

“I actually also design single-family houses now and then. It’s almost a kind of therapy for me, I guess. It’s a good thing to take a mental break from large-scale projects.”

Bill Baker also has another piece of advice for his engineering colleagues:

“Leave mathematics aside for a moment, and describe instead the basic idea of your design in words. If you find out that you need to use many complicated sentences to describe your idea, you’re probably not quite there yet! The best ideas are basically simple. It’s also a great help for you that you are able to provide a clear description of your idea. In this way, it will be much easier to explain to your colleagues and business partners what is going to happen. In a building project, there will constantly be conflicts between different views. It is therefore a huge advantage to make clear from the outset which principle has priority.”

Calculations become trivial

Not least at a time where computers play an increasingly larger role in an engineer’s working life, is it important to still be able to describe your solution in words, adds Bill Baker:

“It’s unfortunate when the engineer has to ask his computer how the building works. It should preferably be the other way round!”


Photo: Thorkild Christensen
William (Bill) F. Baker holds a degree in civil engineering from the University of Missouri. In 1981, he was employed at Skidmore Owings & Merrill (SOM), Chicago, USA, where he is currently chief engineer and partner.

Bill Baker is in particular known for his work with extremely tall buildings, among them Burj Khalifa, but his CV also includes a large number of other projects, e.g. several prominent buildings with long-span roofs. Furthermore, he has also worked together with artists on several occasions.

The interview for Dynamo took place in December 2015 in connection with Bill Baker’s H.C. Ørsted lecture at DTU. During the visit, Bill Baker and DTU Professor Ole Sigmund also agreed to work together in future on developing topology optimization methods for the design of high-rise buildings.