A HEAD FOR HEIGHTS
Sydney-based Structural Engineer, Angus McFarlane was the Structural Director on the Burj Khalifa in Dubai. Here he shares some of the challenges of working on the tallest human-made object ever built.
When Angus McFarlane was asked whether he wanted the job as Structural Engineer on what would become the world’s tallest building, he didn’t have to think twice about it.
“I immediately said ‘yes’,” Angus says. “I’d had a lot of experience working on tall buildings, including the Emirates Towers which rise to 355 metres over Dubai. The Burj Khalifa was to break completely new ground, however.”
McFarlane had been working in the UK during the early 1990s when a recession in the construction industry saw him looking for work. He moved to the Middle East where there was a building boom, and found himself in Dubai working on the iconic Burj Al Arab Hotel. He eventually became the Regional Technical Director with the global consulting engineering practice Hyder Consulting (now Arcadis).
It was with Hyder that Angus was given the opportunity to co-design the tallest man-made structure on Earth.
“It was an exciting project to be a part of, but most buildings are complicated things, and building the tallest is not really too different, you just approach it one piece at a time.”
The structural and the architectural teams arrived at a building that was to be Y-shaped in plan. This shape was chosen to minimise wind loading, but it also has the advantage of maximising views whilst maintaining the privacy of the occupants. The main structural system is a hexagonal-shaped reinforced concrete buttressed core which provides torsional stiffness.
Angus says one of the biggest challenges on the Burj Khalifa was the elevators. They are among the fastest on the planet, with a vertical speed of 10 metres per second. One of the elevators travels a record 140 floors.
“Elevators take up a lot of building’s volume; when you get towards the top, around 60 per cent of the building is lifts.”
Angus says initially the software running the elevators wasn’t tuned properly to the movement of the building and would bring them to an almost complete standstill. He says it took a while for the software engineers to solve this. Now, when the wind is blowing and the building is moving, the lifts are slowed to around 7 metres per second.
Speaking of movement, Angus says the Burj Khalifa is never still. When the sun rises and hits the eastern side, the Burj heats up and moves laterally around 300 mm. The spire (which itself would be taller than the Eiffel Tower if placed at ground level) sways up to two metres when the desert winds pick up.
Because the building is so heavy – about 500,000 tonnes – it presses the earth down 70mm, for 600 metres all around. “It’s like pushing on a mattress,” says Angus.
“We had to sink foundation piles 48 metres into the ground to transfer the enormous load. We drilled down through sand until we reached a layer of rock called calcarenite; it’s a type of limestone made from millions of marine creatures. Effectively the Burj rests on a layer of old seashells.”
One of the many records broken during construction was the height to which concrete was pumped; a staggering 606 metres. “We did that with just one pump and a hose from the ground,” says Angus. “Above that height, the structural material in the tower changes from concrete to steel.”
A particular challenge that Angus faced was the deflection of the floor slabs: “They were sagging where they met the glass curtain wall. My team and I redesigned them to a a more reliable, well-tested code of practice and used less reinforcement overall, but put more reinforcement where it was most needed…at the edges.”
More than 7,500 workers made the Burj Khalifa a possibility and when finally completed in January 2010, it became the most famous building in the world, even earning its own Lego Architecture set.
Everything about the Burj was and remains grandiose, from its towering height at 828 metres (almost twice as high as the Empire State Building) to its record breaking number of storeys (154 plus 9 maintenance floors). Almost a six years later, it’s a successful multi-use building with restaurants, a hotel, office space and residential apartments, surrounded by a dramatic man-made lake.
I asked Angus if he ever got the chance to go to the observation tower to take in the view? “I went to the top a couple of times before the building opened,” he says. “It’s a fantastic view when it’s clear, but if the wind starts blowing it kicks up the desert sand and you don’t see a damn thing.”