By Greg Whitaker www.constructionweekonline.com
We all know that the biggest, and most expensive of just about everything has been used on the Burj Khalifa, but did you realise that the project also broke new ground in machinery terms? We look at some of the tower’s PMV landmarks.
There’s more than 600 deep piles involved in the whole Burj Khalifa project, although the majority hold up the podium complex, with 194 used for the tower itself. Bauer Spezialtiefbau with Middle East foundations took on much of the piling work, which required bores to be sunk for cast in-situ piles, to a depth of 43 meters.
Known by some as the ‘Rolls-Royce’ of the drill rig world, the Bauer BG40 can deliver, as the name suggests, 40nm of torque. Of course, there isn’t a situation that we could imagine where you would need such heavy power for drilling pilling holes – half of this would be sufficient for most construction situations.
However, a reserve of torque means there is less stress put on the machine, so it can get on with what it is required to do. No less than 45,000 tonnes of concrete were poured for the foundations alone – that’s equivalent to 18 Olympic sized swimming pools.
Bores for the 194 deep piles were sunk back in 2004. Each of them was designed to be cast in situ, and as such needed to be very deep. Ground conditions at the Burj site were favorable – the soft, but not unstable soil proved easy to dig into. Other sites in the region are not so fortunate – naturally occurring limestone requires breaking with a breaker attachment first.
A machinery record was set in pumping concrete to a new height – though not exactly to the often-quoted measurement.
In late 2007 a string of high pressure concrete pumps needed to splutter the mixture up to 601 meters. The maker of the pumps held a photocall, and used a massive poster to announce the figure us a new world record.
However, shortly after it was discovered that the concrete needed to go a little further and so an extension was added to move the concrete to 606m.
The mix was able to reach such astounding heights by running through a high-pressure trailer mounted pump (a Putzmeister 14000 SHP D) at 200 bar.
With a 606m height difference, the concrete required approximately 40 minutes from the filling of the hopper to its discharge from the delivery line.
The concrete volume in the line amounted to approximately 11m3 with this installation height – meaning there was roughly 26 tonnes on the pump after every piston stroke – or five big elephants.
Over a period of about 32 months, the high pressure pump and two others delivered more than 165,000m3 of high-strength concrete which, going back to our preferred unit of measurement is about 66 Olympic sized
The external lifts used during construction were hardly slow, but the size of the building meant that it would still take about 35 minutes to get to the top. Added to this, thousands of people needed to be on site everyday, so once up there, workers would rarely come down.
Fortunately, some speedy new machinery has been installed so that residents and visitors to the building don’t have to wait around.
One record that has been broken by the Otis lift installations is in the speed at which many of the elevators travel – said to be more than 10 meters per second – this surpasses the record set by what is now the world’s second tallest building, Taipei 101.
No elevators are installed to travel all 160 floors of Burj Khalifa. Instead, they are strategically grouped to align with the floor layout, offering passengers a direct express service to their destination by bypassing other floors, though a pair of ultra-high speed double deck elevators are on hand to express visitors directly to the 124th floor viewing gallery.
As any architect will tell you, the amount of lifts needed in a skyscraper cuts into the amount of useable space, and therefore rendering most designs over about 70 stories uneconomic to produce. The Burj certainly has a lot of elevators – the official figure is 57 – but using the ‘double deck’ lift cars cuts down the amount needed.
Interestingly, if you want to go from the viewing deck to the pinnacle, it would be the equivalent of walking up the stairs of a thirty-storey tower as there is no lift to the top levels.
They’re graceful, mysterious and it seemed for a time everybody’s favourite topic. The high level cranes at the Burj were always enigmatic, enshrouding the operator of the very highest unit in mystery.
There were stories circulating about ‘the Indian on top of the world’ which speculated that he was paid a king’s ransom, and that he had been made an honorary UAE citizen.
All of this was really nothing more than idle gossip – the figure had become more of a mystery man through Emaar’s refusal to let the media have any access to him, and this was most likely due to the developer keeping the exact height of the structure a closely guarded secret – a figure which the high-level operators undoubtedly knew.
Despite this conundrum, there is quite a lot that we do know about the high-level cranes. For a start, there was not one, but three Favelle Favco cranes that served right up to level 156.
Given that the machines worked 24 hours for much of the project’s duration it would be safe to assume that there was a team of at least nine drivers and many other technicians to ensure safe operation.
(In fact Emaar recently confirmed that a 35-strong workforce were on hand to run the cranes, though this is a drop in the ocean compared to the total of 11,000 employees on the project.)
Figures suggest that the cranes shifted more than 63,000 tonnes over 45,000 hours. Usually the cargo consisted of steel reinforcement beams, but welding equipment, scaffolding, gensets and even tanks of fuel for the diesel powered cranes all needed to be lifted to the correct floor.
Installing the three high-level cranes was relatively straightforward as sections of the cranes could be moved up the tower with the completion of new levels.
Getting the towers down however, required a little more lateral thinking. The first high-level crane was moved in November 2007 down to level 99 in order to serve as a future recovery crane. The next high-level crane came down in October 2008, leaving one prominent machine apparently stuck at the top.
To get it down another small crane had to be lifted to floor 159. With a crane on this floor as well as the one on level 99, the dismantling process was ready to begin.
The process started with the crane climbing down from its working height of over 700 metres. The crane removed its own mast sections and lowered them to the ground until the boom and power pack were at the position of the Level 159 recovery crane.
From there, the Level 159 recovery crane dismantled the remainder of the main crane, lowering the pieces of boom, mast and power pack to the recovery crane at Level 99, which further lowered them to the ground.
The dismantling of the cranes at Burj Khalifa was a finely orchestrated set piece – except that the artists here were huge machines.
6 The Burj as a ‘Storm Machine’
There was much speculation on various architectural blogs that the temperature of the Burj Dubai can be as much as eight degrees different from top to bottom.
This led to some pretty wild speculations about the physics of the super-tall building.
An article in the German newspaper Das Spegiel provided one of the best examples with the most outlandish claim reading as follows: “The tower is so enormous that the air temperature at the top is up to eight degrees celsius lower than at the base. If anyone ever hit upon the idea of opening a door at the top and a door at the bottom, as well as the airlocks in between, a storm would rush through the air-conditioned building that would destroy most everything in its wake, except perhaps the heavy marble tiles in the luxury apartments.”
There is certainly truth in that that there would be something of a ‘chimney effect’, this is why skyscrapers and other tall buildings with high atriums feature revolving doors which are never fully open to the air outside.
However, the main problem would be an exposed shaft with 850m drop right into the basement, – presumably the facilities managers have got this worked out so that it could not be allowed to happen.
This hasn’t stopped chatter on the ‘net suggesting this downdraft might be so great that it could modify, or even cause, extreme weather over continents. The idea of a janitor being able to cause the next cyclone Gonu is an odd one.
Trevor Patt, a Harvard graduate in theoretical maths disputes this, though: “Given that the cubic volume of Burj Dubai is more than 8 orders of magnitude (100 million times) smaller than the cubic volume of even a very small or midget cyclone, I’m guessing the cyclone would be doing most of the negation – my understanding is that the Burj Dubai was tested for winds up to 55m/s or 125 mph, which should make it a decent bet to survive a category one cyclone at least.”
So, no need to bolt a hurricane cage over your villa just yet then.
What were those cranes?
The three cranes on the tower were all diesel Favelle Favco units, of various specifications. This type of diesel-hydraulic crane is popular on ‘supertall’ skyscrapers, due to a useful turn of speed and power. However, one of the main challenges was actually getting the fuel to the required height – there are no petrol stations on the 159th floor.