A Minute With...

Mir Ali, an expert on skyscrapers

11/20/2013  8:00 am

The 109-story Willis Tower in Chicago recently lost its claim to the "nation's tallest building" when the Council on Tall Buildings and Urban Habitat (CTBUH) ruled that the new 104-story One World Trade Center in New York City, is taller. The decision hinged on the standards skyscraper experts use to measure buildings – a line of logic that Chicago Mayor Rahm Emanuel adamantly rebutted. University of Illinois professor emeritus of architecture Mir Ali – a CTBUH Fellow who served on the height committee in 2004 and has written or edited five books on skyscrapers – talked with News Bureau news editor Dusty Rhodes about how the council arrived at that ruling, and why he is not entirely comfortable with it.

The Willis Tower roof is 442.1 meters high while One orld Trade Center's roof will be only 417 meters high. So this d ebate boils down to wether the "mast" of One World Trade Center counts as part of the building's height. First, describe what a "mast" is.

image of professor mir aliIn 1971, about a year after the Council on Tall Buildings and Urban Habitat was established, the question arose: how to define the height of tall buildings. The council adopted the definition by the late Fazlur Khan (a co-founder of CTBUH with Lynn Beedle, who died in 2003) that the height of a tall building is the distance from the ground level at the main entrance to the top of the highest structural element of the building. By this definition, the top of the mast or spire (a vertical and slender tower rising steeply to a point), which is structurally integrated with the building’s main structure, should be considered as the highest point of a tall building. According to this height criterion that continues today, One World Trade Center is the tallest building in the United States.

Many skyscrapers around the world have masts. How do experts like you decide whether masts count in height measurement?

Any structural mast built integrally with the main structure is considered a part of the structure of a tall building. If it happens to be an antenna (as in Willis Tower) or a nonstructural upright element, it does not count in height measurement. Because of this definition of height, many owners/developers around the world have deliberately added built-in structural spires in their recently constructed tall buildings to achieve greater height for attracting attention and achieving status.

How long do you predict that One World Trade Center can hold on to the title of tallest building in North America?

I believe One World Trade Center was planned to replace the now-destroyed World Trade Center for economic and, more importantly, socio-political reasons. The city of New York and the Port Authority took an active interest in it. Construction of a super- or mega-tall building is justifiable, amongst other factors, by real estate marketability – that is, a need for commercial space and the profitability from such an enormous financial investment. Because of the currently prevailing economic climate in the United States, I predict we are unlikely to see a taller building built here within the next three to four decades. Let us remember that One World Trade Center is surpassing the height of Willis Tower after about 40 years.

You were previously a member of the CTBUH height committee. How would you have voted on this decision?

I was a member of CTBUH’s height committee in 2004 when Taipei 101 in Taiwan was built. It was a large committee consisting of many experts from around the world. We had intense discussions, and after considering all factors and opinions, we did stick with Fazlur Khan’s definition of height. Some additional height categories based on roof, highest floor, nonstructural spire, etc. were added, but the overall height definition was upheld as before.

How would I vote in the present decision with One World Trade Center if I were a member of the height committee now? I would vote yes. It is like following the constitution of a country whether it is up to your liking or not. CTBUH had a lot of discussions and deliberations on this and cemented the guiding criterion as a done deal. Khan’s definition was upheld by consensus. I believe nobody wants to change Fazlur Khan’s definition out of great respect. He is arguably and generally accepted as the greatest structural engineer in the second half of the 20th century, and he revolutionized tall building design with his innovations of structural systems for tall buildings. Also, if this definition is changed now, the heights of the Chrysler Building, Empire State Building, etc. down to Burj Khalifa have to be revised. That will be a “skyscraper” of a task.

I must admit, though, that I feel some degree of discomfort with this classic height definition. First of all, the public does not see a “spire” (structural or nonstructural), an indistinct, thin needle-like element at a great height, when viewed from the ground level, as part of a “building,” and they do not understand why it counts in height measurements since it is totally out of proportion with the main building’s bulk and girth. That explains why Mayor Rahm Emanuel vehemently discredited this height decision. Khan was a structural engineer, so was Lynn Beedle and so is Les Robertson (another CTBUH co-founder). It is no wonder that Khan went by a structure-based definition and others obliged.

To me, it would be logical to have the highest usable and walkable floor or the roof at the top of the main structure (that is, excluding the mast or spire), determine the height of the tall building. Even in terms of visual characterization, a spire is simply a spire, whether structural or nonstructural. In my opinion, the present height criterion is 42 years old and thus belongs to the past and should be considered for serious review as times have changed.

Because I knew Fazlur Khan personally and professionally and wrote a book about him (“Art of the Skyscraper: The Genius of Fazlur Khan”), I know he had a great deal of respect for truth and logic. This height controversy first came into the limelight in 1996, when the twin Petronas Towers in Kuala Lumpur surpassed the height of Willis Tower (formerly Sears Tower) because of their spires, although their highest occupiable floors were 46 meters below that of the Sears Tower. Khan was already deceased; we have no way of knowing what he would have said. Had he been alive then, I have reason to believe he would rethink his own definition and amend it.

Despite what I think, this definition is unlikely to change in the foreseeable future. Clearly, the public does not fully grasp the technicality of this matter and accepts willingly or unwillingly what the experts of CTBUH decide.



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