On a miniscule site that gives new meaning to the phrase ‘close to the flightpath’, the team building Heathrow’s new air traffic control tower found an ingenious way to hoist the control room 87 m into the air.
Anybody who thinks they've got problems with access to a cramped site should pause for a minute. Working in the middle of functioning hospitals or city centres is nothing compared with this logistical challenge: building the tallest and most safety-critical building in the heart of the world’s third busiest airport.
The building under construction is the new air traffic control tower designed by Richard Rogers Partnership, which will eventually control all aircraft movements in and around Heathrow. This £50m nerve centre will take over from a 45 m tower, which is too short to monitor aircraft movement around Terminal 5. In order to solve this problem, the new structure will be 87 m high when it is completed. But how do you build such a high tower on a tiny site that’s squeezed between parked and taxiing aircraft? A team led by engineer Arup came up with a novel answer …
“I still can’t get used to that,” shouts Richard Matthews, engineering leader of the project for multidisciplinary consultant Arup, barely audible above the shriek of jet engines. “You look up and there is the wing of a jumbo just a few feet away.” The site of the control tower is slap bang in the middle of a set of aeroplane stands sandwiched between two parking bays. The bays are occupied by one jet after another, while other planes are constantly taxiing past on their way to or from more exotic locations. The site itself is only the size of a single bay because BAA didn’t want to close the adjacent bays down. “This fact has given us some huge constraints. It played a major part in the way we tackled its construction,” says Matthews.
The team initially considered building the control tower on site, and examined two options. The first involved building the control area, or cab, on top of a concrete mast. “We were going to try and build the cab 80 m up in the air with aircraft floating around. We did look at this but it didn’t work,” says Matthews.
The other option was to build a slim steel mast, build the cab around its base, and then jack it up to its final height. The problem with this plan was getting the cab past the cables that would be needed to stabilise the mast.
The team then came up with a “third way”, which was a neat variation on this theme. “The big idea came when we abandoned the idea of jacking the cab up the mast and instead decided to jack up the cab and feed the mast in underneath,” explains Matthews. “When we came up with this idea we thought, ‘hang on a minute, if we are jacking it up as a complete unit why can’t we prefabricate it off site?’”
Building the cab off site at an area near Terminal 4 solved several problems. “It gave us an awful lot more room. Although it’s airside [beyond passport control], the security issues were not as high,” explains Peter Czwartos, the project’s production leader for construction manager Mace.
The cab was built by initially constructing the mast section that extends inside the cab, then fitting the lift docking station, at its base. Structural mullions were installed, and then the entire roof was constructed on the ground and craned into position. Finally, the glass was fitted and the interior fitted out.
prefabrication of the steel mast sections required a lot of forethought. “It was on the very limit of what could be transported by road. This made the interior planning of the mast very difficult,” says Arup’s Matthews.
The solution was to make the mast very slim, at just 4.6 m across. This feat made possible by cable staying the finished mast at three points in order to secure it.
Because of this, the mast’s two lifts were originally going to be exterior wall-climbers to save space. Unfortunately however, it wasn’t possible to guarantee that they would work in all weather conditions. The answer was to have an external lift that would operate most of the time, and another one inside next to the internal stairs.
The tower was also tested for movement in a wind tunnel. This revealed that the narrow structure could create turbulence in strong winds, causing the cab to move. This problem has been taken care of by adding a series of winglets to the mast, which stop the buffeting.
The prefabrication programme ran in tandem with the considerable enabling and substructure works that were needed on the site. The control tower needs two sets of electricity cables in case one fails. Both have to be carefully knitted in with existing services, including fuel lines under the airport’s hardstanding. Piles were sunk and the pile cap created ready to receive the prefabricated mast and cab.
The move took place on the night of 29 October 2004 after the last aircraft had departed. The cab was moved as a complete unit, with the temporary works needed to jack it up already in place. Three computer-controlled, hydraulically-powered flat-bed units with 48 wheels each were needed to moved the whole assembly – which weighed more than 800 tonnes – from Terminal 4 to its final position near Terminal 3, 1.8 km away. “We had planned it in minute detail and did a lot of risk analysis,” says Czwartos. “We had contingencies in case a tyre burst or a hydraulic line failed. If a power pack failed, we could run in using the other two units – there was huge redundancy built in.” According to Czwartos the move went very smoothly in just two hours. The next day, work started on inserting the mast sections.
Mast sections 12 m long come to site with the stairs, lift shaft and service risers already installed. Temporary works to lift the cab up into the air to allow the mast section to be inserted underneath were also in place. The temporary works consisted of three towers, each with a strand jack at the top. The jacks pull in or release a series of steel cables that are in turn attached to a large yoke that fits around a flange on the mast. A computer controls the three jacks so they work in harmony lifting the yoke, complete with the mast, upwards or downwards.
To start with, the cab is jacked up to a height of 15 m. The new 12 m section of mast is placed on top of a reusable 3 m section and the whole assembly is rolled along a short section of track underneath the suspended cab. The reusable section has a doorway cut into it so workers can get inside the mast and up to the cab. The cab and the mast sections fitted earlier are dropped down onto the new mast section. These are bolted together, then the whole process is repeated. Temporary cables are used to stabilise the whole assembly, with a further series of computer-controlled jacks controlling the length and angle of the cables as the whole assembly moves upwards. The mast will be completed in seven lifts. When the final section is inserted, the cab and existing mast sections will weigh 1150 tonnes.
What’s left to do
The mast sections are being completed at the rate of one every seven or eight days, so the most technically challenging part of the operation will soon be over. In a few weeks’ time, the temporary cables will be swapped for permanent ones. These are massively oversized to minimise any movement of the tower. In addition, Europe’s first active mass damper is being brought in from Japan. The damper system consists of electric motors and large weights, the former driving the latter in the opposite direction of the tower’s movement to “dampen” it. A building containing plant and IT equipment will then be constructed around the base of the tower. In March 2006, National Air Traffic Services will move in to install and safety-check the air traffic control equipment, and should complete their work by November 2006. The timing should allow the system to be fully embedded before Terminal 5 opens in early 2008. “NATS wanted the tower a year before Terminal 5 opens as it’s a double hit if you are trying to commission a control tower and a new terminal at the same time,” says Arup’s Matthews.
architect Richard Rogers Partnership
structural engineer Arup
construction management Mace
steel manufacture Watson Steel
lifting and temporary works design Rolton Group
transportation and lifting contractor Fagioli PSC
lift supervision Dorman Long Technology