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Tall Order

Tall Order

Jeddah, Kingdom Tower, Smith+Gill

Oliver Ephgrave discovers the numerous design and structural challenges behind the delivery of the 1km-high Kingdom Tower

Kingdom Tower may have been described as “highly constructible” by AS+GG partner Gordon Gill, but the design and build of a 1km-tall tower is far from simple.

One of the main issues is wind load, which increases with height. Commenting on Kingdom Tower, Gill said: “The three-petal footprint is ideal for residential units, and the tapering wings produce an aerodynamic shape that helps reduce structural loading due to wind vortex shedding.”

A top engineer behind the project, Bob Sinn, principal at Thornton Tomasetti, states that the wind load solution was one of the main structural challenges. “All tall buildings are dominated by wind loads, and the reduction of this force was one of the big challenges early on. All of the structural elements were sized for strength, without the need for additional materials.”

Bart Leclercq, head of structures for WSP Middle East, believes that the design of Kingdom Tower provides a sound aerodynamic solution. “The shape of the building is quite stiff in itself – it’s a similar footprint to Burj Khalifa. The taper reduces the wind load at the top. Because it changes shape every few floors, the wind loads go round the building and won’t be as extreme as on a really solid block. There will be local disturbances, so it’s a really good design from an aerodynamic perspective.”

Sinn adds: “I don’t know if it’s an ideal, but I’d say that the tapered form is an excellent solution in dealing with the wind load. It turned out to be very beneficial to us.”

As well as solidity, a supertall tower also requires rigidity. Leclercq elaborates: “You have to make sure a tower is not too flexible and people aren’t getting nauseous in high winds. You have to put enough stiffening elements in your building. For example, sheer walls in combination with concrete cores in the case of a concrete building. It’s the same thing for a steel building – you have to provide really solid structural walls that take care of the wind load. The building may be strong enough, but if it is not stiff enough then people will get nauseas.”

However, Leclercq is quick to point out that this should not be an issue on Kingdom Tower. “A good structural engineer will take care of that movement. I really don’t think that this will be a problem.”

Steve Kelshaw, managing director of Dubai-based DSA Architects International, believes that the tapering form is the best model for a tower of this height, despite the aesthetic limitations. “I don’t think you could do it any other way – if you built a square design up to that height, I don’t know how it would work,” he says.

Yet there are other options, other than the taper, according to Sinn.

“Anything that changes shape is best. For instance, it could be sloping, stepping or have holes in it. I’d say that the worst shape, aerodynamically speaking, is the prism. The old World Trade Centre in New York is a textbook example of the worst shape.”

Leclercq adds that the architect’s treatment of facade may provide the tower will a distinct identity. “Although it uses a similar footprint to Burj Khalifa, the designers can be really playful with the facade. The facade of Burj Khalifa is quite astonishing and the Kingdom Tower might look completely different from Burj Khalifa.”

A big challenge for supertall buildings is vertical transportation, which includes elevators and fire escapes. Leclercq explains: “When you work on a building of that height you find that a large area of the floor plate has to be occupied by vertical transportation. This means that you have large areas that are unlettable.”

He refers to the unbuilt 1.4km-high Nakheel Tower in Dubai, for which WSP provided structural design. “The Nakheel Tower design had 47 lifts, just to get people up and down, so you can imagine the enormous amount of space that this required. The lettable area is reduced the higher you go, and that’s a problem.”

According to AS+GG, Kingdom Tower will contain one of the most sophisticated elevator systems in the world, with an estimated 59 elevators in total. This will include 54 single-deck and five double-deck elevators, in addition to 12 escalators. Elevators serving the observatory will travel at a rate of 10 metres per second.

A 1km-tall tower may seem staggering, but is this the buildable limit? Most probably not, according to the chairman of the Council on Tall Buildings and Urban Habitat, Dr Sang Dae Kim. “We now have a design that’s 1km tall. Later, someone will push for 1 mile, and then 2km.”

Kim believes that, technically speaking, a 2km-tall tower might be possible at the current time. He continues: “At this point in time we can build a tower that is 1km, maybe 2km. Any higher than that and we will have to do a lot of homework.”

Thornton Tomasetti’s Sinn also believes that 2km is an achievable height. “Kingdom Tower is certainly feasible. It’s not a structural challenge. Technically I think a 2km-tall tower could be done, but I don’t think it will be done anytime soon. Such as building will have more architectural and practical constraints than structural issues. There have been very strong advances in reinforced concrete in the last 20 years,” he adds.

Yet Kim believes that there will be structural challenges for a 2km-tall tower. “There might be constraints for the structural engineering – we don’t know many things. When you go up to two kilometres, we don’t have much information surrounding the conditions.” Kim also notes that there may be issues with floor lean due to the shortening of columns over time.

He also agrees that it is highly impractical to build a 2km-high tower, and adds: “We don’t need to built at 2km, but someone with a lot of money might still want to do it.”

For Leclercq, the technical limit at the current time is 1 mile. “I truly believe that this is within range. Over that, it may be possible if there are improvements in concrete quality. But 2km is too big a figure – it’s just a step too far at the moment.”

Kelshaw also states that extremely tall towers may struggle to attract tenants. “Is the market there? Personally I would feel uneasy. Getting people to work and live in such a tower will be a challenge,” he argues.

Yet Leclercq asserts that there will be always be an appeal to build and occupy the tallest building in the world. “Is there such as thing as too high? I think mankind is always going to be challenged by finding the next frontier. There’s also a market; people will always want to be in the world’s tallest tower,” he concludes.

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