Tall order

Oliver Ephgrave examines the challenges facing engineers in Jeddah

ANALYSIS, Projects

RELATED ARTICLES: French spiderman up for challenge of Kingdom TowerEngineer: Kingdom Tower is practical challengeExperts: tapered form of Kingdom Tower is ideal

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 the solution of 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.”

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 the same footprint as 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.”

Leclercq elaborates on the need for rigidity. “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 really nauseous.”

However, Leclercq is quick to point out that this should not be an issue on Kingdom Tower. “As long as a good structural engineer is involved, they will take care of that movement. That shouldn’t 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 continues: “That shape has got the wow factor. I never fail to marvel at the design of Burj Khalifa. It is truly a magnificent building. If I was in Saudi Arabia and I saw the same structure, I’d still be amazed. I wouldn’t get tired of looking at it.”

Leclercq adds that the architect’s treatment of facade may provide the tower will a distinct identity. “Although it uses the same footprint as 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,” adds Leclercq.

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. “With Kingdom Tower we now have a design that reaches around 1km in height. Later on, someone will push for 1 mile, and then 2km,” adds Kim.

Kim believes that, technically speaking, a 2km 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.”

Yet Kim states that it is highly impractical to build a 2km-high tower. He adds: “In terms of practicalities, we don’t need to built at 2km, but someone with a lot of money might still want to do it.”

He points out that building at such height will incur many structural challenges.

“There might be constraints for the structural engineering – we don’t know many things. When you go up to one or 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.

For WSP’s Leclercq, the technical limit at the current time is 1 mile. “I truly believe that 1 mile – 1.6 kilometres – 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,” says Leclercq.

DSA’s Kelshaw is similarly cynical on the feasibility of a 2km tower. “I don’t know why people would want to build something 2km tall. From a developer’s perspective that can’t be feasible. Just to think about that is mind blowing and I can’t see it happening in my lifetime.”

Kelshaw also states that extremely tall towers may struggle to attract tenants. “Is the market there? Are there people that want to work and live at that height? That is an unknown market. Getting people to work and live in such a tower will be a challenge in itself,” he argues.

Yet Leclercq disagrees and asserts that there will be always be an appeal to build and occupy the tallest building in the world, no matter how high.

“Is there such as thing as too high? I think mankind is always going to be challenged by finding the next frontier. I think there’s also a market - people will always want to be in the world’s tallest tower,” he concludes.

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