Bill Baker of Skidmore, Owings & Merrill LLP: Tall Buildings and the Burj Khalifa

Mark Meshulam is an expert witness and consultant for architectural windows, glass, and building facade.

Bill Baker is the partner in charge of Structural and Civil Engineering for the Chicago office of Skidmore, Owings & Merrill LLP. One of his most recent accomplishments, completion of the world’s tallest building, the 160 story, 2,717 feet, Burj Khalifa (formerly the Burj Dubai) in Dubai, has literally shaken the world of architecture.

I spoke with Bill Baker in Skidmore’s office in the Railway Exchange Building, the historic 17 story terra-cotta beauty sporting the iconic Santa Fe sign on Michigan Avenue. The edifice oozes architecture, having been designed in 1902 by D.H.Burnham & Company, and having housed Graham, Anderson, Probst & White, The Chicago Architecture Foundation, VOA Associates, Harding Partners and Goettsch Partners. In 1909 Daniel Burnham drew up the Plan of Chicago in this very building.

Q: Can you tell me about some of your more prominent projects?

Mr. Baker:
I have led the structural design team for many prominent tall buildings including the Burj Khalifa, formerly known as Burj Dubai in Dubai, which is the world’s tallest building. Here in the US, I’ve worked on many, many buildings, the latest which is the Trump Tower here in Chicago, which is the 2nd tallest building in the United States. We are currently very active not only around town, but also in China, as well as the Middle East and Europe.

We don’t just do tall buildings; we do every type of building. I am currently working on a single family house in California. We also do engineering for artists for large sculptures. Over the years our firm has done many diverse projects in Chicago, such as the engineering for the Picasso sculpture in Daley Plaza to the Sears Bldg Tower, and now the Trump International Hotel and Tower. We also designed the John Hancock Center.

Q: Were you involved in those projects?

Mr. Baker:
No, I came in later. I was involved in the renovation of the Sears Tower and John Hancock. I wasn’t involved in the original towers. They pre-dated me a bit. (Note: Mr. Baker joined SOM in 1981. Sears Tower, 110 stories, 1,450 feet, was completed in 1973. John Hancock Center, 100 stories, 1,127-feet, was completed in 1970.)

Q: In the design of a tall building, generally it seems there is a set of core design concepts involved. How do you see the transition of those core concepts from your early days in the business until now?

Mr. Baker:
Well, it is really important to have an idea, to have a concept. Lots of times you see buildings out there and you can tell they really didn’t know where they were going and the results show. We try to have a clear hierarchical idea of what’s the most important thing about the building, what’s the 2nd most and 3rd most important idea, so when you have conflicts, you know how to resolve them. What’s the main idea?

A lot of times we try to do the preliminary design of very tall buildings by hand. If the idea is so clear that you can calculate it by hand, maybe you are on the right path. And if it’s that easy to calculate, maybe it’s that easy to build. You must have clarity of vision. That is what we try to do.

Sure, the structural systems have changed. We are getting a better understanding of issues of scale. You can’t just take a system that works on a smaller scale, blow it up and make it bigger. The Sears Tower really can’t be any taller without major changes, because it would just be too big, too much floor area.

As an example, if I was twice as tall, I would be twice as wide, I would be twice as thick and I would weigh eight times as much. We are finding that you get this disproportionate geometric relationship, which is the reason you cannot scale things.

As buildings get taller and taller, you have to evaluate new systems that address the issues of scale, access to natural light, things of that nature. Some of the older schemes just won’t work to go any taller than they currently are. They were very good solutions for the problems they were solving, but when you go to the next problem, maybe you have to come up with something new.

Q: In addition to the architectural concepts, the structural concepts for handling wind-induced and gravity-induced loads seem to have evolved quite a bit.

Mr. Baker:
Yes, quite a bit. You have to remember, these systems are developed by a team of engineers, architects, interior designers, electrical engineers and mechanical engineers. Even urban planning get involved in how these systems come about.

It starts when a client comes up to us with a problem. They want a building that is a certain height in a certain location with certain functions in it. Is it one use, two uses, three uses? Is it just an office building, is it just residential, is it just a hotel, or is it all three? Is it retail? Is there an observation deck?

All these things affect how the thing looks. SOM is an integrated practice where we have all the disciplines sit together under one roof. Everyone has an opportunity to comment on everyone else’s discipline.

When starting the design, we each have an idea of what we are trying to achieve, how we will achieve it, and the ideas we bring to it. Through discussions these ideas will merge with other people’s ideas and you’ll come up with something different than anyone brought to the table to start with. And it’s not unusual for an engineer to have a major impact on the architecture, and the architect to have a very major impact on the engineering.

As far as the evolution of structural systems, if you go back to the 60’s and 70’s generally the buildings were tubular, where you have closely spaced columns on the outside of the building. Sears(Willis) Tower, Amoco(BP) and the World Trade Centers towers in New York were like this. The John Hancock had fewer columns but you had the major diagonals on the outside.

Nowadays we are seeing that tubular design still exists but we are also seeing buildings where there is more structure on the inside, using the core of the building where the elevators are as a major part of the structural system.

The reason for that is partially the economy. When I look at a building, I see giant beams coming out of the ground. How you make that beam efficient? In that core around the elevators you have a lot of blank, solid walls. Using them, you can have a relatively efficient structure. In a tall building, sometimes even the cores can be too slender for their great height and you must supplement them with either a perimeter system or an outrigger where you reach out to the perimeter system. This is what the Trump Tower has.

One of the things in more recent buildings that you didn’t see earlier is a lot more glass and a lot more open space between columns at the perimeter. The World Trade Center, had columns every 40 feet, and the columns were about 20 feet wide, so you had very narrow strips of glass in between. You were in a very tall building but it was hard to see out because you were inside this cage.

Now let’s go forward to the Amoco building here in town. Now it’s called the Aon Center. There you have columns that are 10 feet on center. The columns are 5 feet wide and there is 5 ft of glass in between. But it’s still 50% solid.

Now let’s go to Willis (Sears) Tower. On both the two preceding buildings all the structure is on the outside. On Sears Tower, some of the structure goes to the middle. You might say that the building is comprised of 9 tubes. Where the tubes come together, that where the interior structure is. The perimeter columns are 5 ft wide, but they are spaced at 15 feet on center with 10 feet of glass in between. Now the building is 2/3 open.

Now let’s look at the Hancock. The columns are 40 feet on center, but you have the diagonals, which I personally like, but some people object to. You have to deal with those, but you have a lot more open glass because of the system.

Looking at the modern systems such as the Trump Tower, the columns are 30 feet on center. The building has a lot more glass, a lot more openness, a lot more views to the outside, which one would say is the fundamental part of being in a tall building. That is one of the trends you see. These buildings tend to be more open on the outside and less cage-like as opposed to what they were in the 60’s and 70’s.

Q: Are there systems where the curtainwall cladding contributes structural strength back to the building structure?

Mr. Baker:
There have been proposals over time to use the cladding as part of the structural system. But generally, that’s not done.

There used to be an idea called the “stress skin.” When you look at an airplane, the skin of the fuselage and the wings is part of the structure. But the exterior wall on a building has a lot more functions than a piece of aluminum sheathing can accomplish.

The building cladding must be mainly “see-through.” It is made from different materials with different thermal properties. Glass moves differently than aluminum, which moves differently than steel, which moves differently than concrete. The exterior wall sees thermal swings which the inside does not see.

I think yesterday, here in Chicago, we had a 40 degree temperature change in one day. That is not unusual. That is a lot of thermal growth that the wall has to absorb in a way that no one notices (laughs). And so, generally the exterior wall is hung on the structure in a way that it is able to breathe and grow and shrink as it needs to through the weather fluctuations, while still keeping the weather out.

Q: When you have the structure focused more toward the core, which I assume is the center of the building, would that allow the building to lean more under wind load?

Mr. Baker:
Yes, the cores are usually in the middle of the building, but no, the criteria don’t change. The buildings we are building today are as stiff as or stiffer than in the past. The criteria may have gotten stricter over the years. We try to, using design and economy, minimize the amount the buildings move.

Q: In the worst wind loading condition on the Burj Khalifa, what type of movement do you expect?

Mr. Baker:
It doesn’t move a lot, actually. As far as the cladding goes, it sees very small movements because the building acts as a giant beam. A lot of shorter buildings have what is called “moment” frames, where you have columns and girders, and rigid connections between them. Under load they do what is called “shear racking.” One story will translate relative to the floor below because the columns and the beams may flex even though the joints might be rigidly connected to each other maintaining a 90 degree angle.

Shear racking affects the exterior wall and the interior partitions. You have to do special detailing so the building doesn’t squeak and make noise during movement.

The Burj Khalifa is a bunch of walls. There is not a moment frame, so there is very, very little racking. The building does move, but it moves by one side compressing, the other side expanding, due to the forces, but the movement at the head of each floor is very minor. This building is probably much quieter than most buildings.

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