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Heathrow NATS air traffic control tower
Heathrow Airport, Harmondsworth, London, UK
Heathrow NATS air traffic control tower
associated engineer
Arup
Mott MacDonald
date  c.2002 - 2006, operational April 2007
era  Modern  |  category  Building  |  reference  TQ065759
photo  © Thomas Nugent and licensed for reuse under this Creative Commons Licence
London’s Heathrow is the world’s busiest two-runway airport. Ongoing expansion, including the recent building of Terminal 5, highlighted the need for a new, centrally-located air traffic control tower. Constructing a tower in the middle of an operational airfield posed challenges that a prefabricated solution helped overcome. The control room is located on a cable-stayed steel mast, and the new tower is more than twice the height of its predecessor.
Heathrow Airport became London’s new civil airport on 1st January 1946, though aircraft had been using the the site from 1930, when it was known as the Great West Aerodrome and later RAF Heston. The first passenger terminal, the Europa Building (renamed Terminal 2, demolished in 2010), opened in 1955. Heathrow's original 39m high air traffic control tower (TQ075758) was constructed nearby in the same year.
The new tower was partly made necessary by the construction of Terminal 5 (T5, begun 2002) on the western boundary of the airfield, in a position that obstructed sight lines from the original tower to the terminal apron. A higher tower was required in a more central location, and a site some 500m west of the original tower was selected — west of Terminal 3 and closer to T5.
Britain’s National Air Traffic Services (NATS) controls the movement of some 1,350 aircraft at Heathrow every day. Constant visual contact is necessary, so the height of the new tower, 87.5m, and its precise positioning were established by assessing the sight lines to all parts of the airport.
The NATS tower consists of a base building, a mast and an air traffic control room complex at the top. It was constructed under the T5 Agreement, which was a joint contract between the British Airports Authority plc (BAA) and all the main suppliers. The superstructure was designed by Arup, and the substructure by Mott MacDonald, and architect is Rogers Stirk Harbour + Partners.
Part of the project was to develop an appropriate construction strategy. An in situ concrete pylon would have required an onsite concrete batching plant and regular airport closures for craneage. A cable-stayed steel tower design was chosen instead, not least because the diameter of the mast would be approximately half that of an equivalent concrete tower, making less visual impact and satisfying planning authority concerns.
For the control room, the requirements included 360 degree views, a compact layout and the housing of the latest equipment. The presence of taxiways and stands at the base of the tower dictated a low viewing angle. The design process led to a four-level structure resembling a giant top hat, with 10m high inclined glass facades, offering probably the world’s largest cone of vision from a control tower. The visual control room on the uppermost level is the highest in the UK.
Construction began with the removal of an aircraft stand and its underground refuelling mains, followed by the laying of more 2km of cable to bring two separate power supplies to site. The foundations consist of 1m and 750mm diameter piles with pile caps up to 4.1m deep.
The mast was prefabricated in sections at Bolton and Sheffield, prefitted with stairs, lift cores and services risers. In plan, it is an aerodynamically rounded triangle, with a diameter of 4.6m — just less than the 4.8m size limit for transportation. All but the top two sections (for the control room complex) were fabricated from heavily braced 3m long steel tubes. The tubes were heat treated for stress relief before the bracing was removed and the pieces welded into 12m lengths. Each section has a 30mm thick outer steel skin, stiffened with vertical plates spanning 3m between horizontal hoops. The hoops also formed end flanges for bolting the sections together internally on site.
The 31m high control room structure was assembled and equipped at a site on the southern perimeter of the airport, near Terminal 4. A temporary piled foundation was constructed and the top two sections of mast erected as its core. The site was far enough from taxiways for cranes to be used without restrictions.
The control room complex tapers in elevation and is circular in plan, with a maximum diameter of 17m at the roof. Its lowest level houses air-handling plant and a lift docking platform. The second level acommodates staff facilities and includes a cantilevered walkway for access to the window-cleaning cradle. The third level gallery is used for servicing the equipment room located immediately below the fourth level visual control centre. The equipment includes the tower's communcations and radar gear.
The column-free visual control room contains desks for 13 air traffic controllers. Its floor area is set back from the glazing to protect the controllers from air currents circulating near the glass. The room’s temperature is regulated by air-conditioning vents in the floor and air jets around the perimeter.
The control room structure consists of 24 radial portal frames. Each is composed of a steel roof truss with moment connections to vertical members that double as glazing mullions. The floors are concrete laid on steel decking, supported by the portal frame legs, in turn supported by a steel ring beam wrapping the first level of the structure, just above the connection points for the three sets of cable stays.
The 50 tonne roof and its acoustic lining, access walkway, decking and waterproofing was constructed and fitted out on the ground. It was then craned into position, and the glazing installed. The largest double-glazed units, 5.5m by 3.5m, weigh 1.2 tonnes each and were placed by a crane using a purpose-built lifting apparatus.
Once the the control room complex was complete it was transported 1.4km northwards across the south runway and main taxiways to the tower site. The whole assembly weighed about 860 tonnes, or almost 900 tonnes including temporary steelwork. That is far more than a loaded Boeing 747 (around 400 tonnes), so the pavement's loadbearing capacity had to be assessed first.
During a night-time airport closure on 29th October 2004, six transporter units were coupled in pairs beneath a three-point lifting frame attached to the base of the structure. Each pair of hydraulic flatbed units had 48 wheels, all controlled by a single computer. The move took less than two hours, and the assembly was set down less than 10mm from the centre of the tower’s foundations.
The control room structure was then slowly jacked into its final position and mast sections slotted in below it. The jacking operation, one of the most complex ever carried out in the UK, used a technique borrowed from the petrochemical industry. Three 16m high temporary towers were erected, supporting jacks connected to vertical steel members from which hung a yoke that gripped the base of lowest mast section at the core of the control room using hydraulic jaws. The permanent and temporary works were now effectively one structure.
The jacks then raised control room off the ground high enough for a section of mast to be slid into position beneath it and bolted into place. The whole assembly was lowered back onto the foundation, the hydraulic jaws released and the yoke lowered to the new base of the mast, where they were reconnected ready to repeat the cycle. A total of five lifts were carried out, all keeping the top of the tower within 25mm of the vertical.
Three tensioned guy cables, each of 180 tonne capacity, controlled the tower’s stability during the mast lifts. A further three were available for additional rigidity in adverse weather (when lifting couldn't take place).
The mast was designed to resist the erection stresses, which were greater than those it would experience in service, and the thermal stresses generated by the axial stiffness of the cable stays. The sections had been prepainted in pale grey glass-flake epoxy to minimise the thermal stress caused by solar absorption. The mast houses a staircase, a lift and mechanical, electrical and information technology services. An external lift is located on one side.
Once the mast erection was complete, work began on the construction of a three-storey building encircling the base of the tower, its plan echoing rounded triangular shape of the mast above. It houses the NATS offices, administration and training rooms, technical equipment areas and most of the mechanical and electrical plant.
The temporary guys were removed and permanent stays installed when the tower and base building had been completed and all services between them connected. The six stays, two on each corner of the mast, are 150mm diameter steel locked-coil cables, connected to a collar below the control room ring beam, and anchored at ground level near the corners of the base building. The anchorages take the form of giant hooks for the 500kg anchorage pins. The stays are stressed to 10 percent of their normal working capacity to provide axial stiffness. The reserve strength allows the tower to operate safely in the event of cable replacement.
Extensive modelling of the dynamic performance of the tower under wind loads was used to ensure the comfort of the air traffic controllers as they work round the clock in all weathers. Wind tunnel testing was used to develop the aerodynamics of the external lift in an effort to reduce its potential drag on the tower. And small projecting steel plates (strakes) have been added to the tower to control vortex shedding and reduce the effects of crosswinds.
The damping of lateral vibrations was achieved using small viscous dampers on the cable stays and two 5 tonne hybrid mass dampers below the control room floor. The mass dampers are tuned to the as-built frequency of the tower. Accelerometers in the control room complex detect movement and prompt activation of the dampers, moving suspended weights in the appropriate direction.
Finally, a 100m long glazed pedestrian link was constructed between the middle storey of the base building and the end of Terminal 3's Pier 7. The link bridge is supported on three circular columns and was prefabricated in 30m lengths, then craned into position.
The new air traffic control tower topped out at the end of 2005, an occasion marked by the tightening of special gold bolts at the base of the tower. In March 2006, it was handed over to NATS for installation of the control systems and staff training. It cost more than £50m to construct and weighs around 1,150 tonnes.
In February 2007, the tower was in service and a full transfer of operations from the old tower was completed officially at 2am on Saturday 21st April 2007. The original tower was closed, after 52 years of service, and demolished in 2013.
During July to December 2009, engineer consultancy Ramboll investigated a range of retrofit options the upgrade of the façade design to improve glare reduction, using a technique called virtual prototyping.
Architect: Rogers Stirk Harbour + Partners
Temporary works design: Dorman Long Technology
Infrastructure engineering: TPS
MEP engineering: DSSR
Lighting consultant: Speirs and Major
Fire consultant: Warrington Fire Research
Contractor (substructure): Laing O’Rourke
Cost management: Turner & Townsend and E.C. Harris
Infrastructure contractor: AMEC
MEP contractor: AMEC
Transportation and jacking: Fagioli PSC
Logistics: Amalga
Steelwork: Watson Steel Structures Ltd
Facade contractor: Schmidlin
Lift installation: Schindler
Fit-out contractor: Warings
Research: ECPK
bibliography
"Creating Heathrow’s new eye in the sky" by Richard Matthews, Civil Engineering, Vol.161, Issue 2, pp.66-76, London, May 2008
www.airport-technology.com
www.building.co.uk
www.dormanlongtechnology.com
www.heathrow.com
www.nats.aero
www.newsteelconstruction.com
www.ramboll.co.uk
www.rsh-p.com
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Heathrow NATS air traffic control tower