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Reaching for the skies

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MetalWorks Transport

Farnborough airfield's new owners plan to expand facilities - including the construction of new-design aircraft hangars You remember Farnborough from years of TV coverage of all those annual air fairs.

With TAG Aviation now in the driving seat as leaseholder, the air fairs will continue - but the enormous airfield is now set to become the UK's leading airport for business aviation.

The tulip-shaped control tower whose stem is embraced by the terminal structure is nearing completion - and so too is the first phase of the hangar building programme, a 290m-long three-bay wave of Kalzip curves which can accommodate six big, Boeing 737 business jets.

Right at the beginning of the £50 million project, TAG appointed Buro Happold as engineering consultant and then ran a beauty contest among 15 architects for the masterplan and design of the airport facilities: hangars, control tower and terminal building. Of the three finalists, Grimshaw, Gensler and Geoffrey Reid, the latter was appointed with the general idea that the best solution for the hangar was a big arched structure.

The thing about hangars is the awkward shape of aircraft. There is no simple way to accommodate them without seeming to waste a lot of space. Hangars have to be very wide to accommodate wings and high to allow for the tall tail at the back. There is not much you can do about the wings. For the high tails, all sorts of solutions have been tried out including having the front part of the hangar raised up to accommodate only the tail section. The familiar solution is a crinkly metal arch that accommodates aircraft tail feathers because it is higher at the middle. But for big planes, existing solutions are either ungainly or, as in the case of the arch, require additional structure such as tie rods which limit their effective height.

The arch solution first mooted was too big - and a tad too obtrusive for what is an often secretive activity. Although the brief asked for the design to create a sense of place and arrival, around half all business travellers want to be able to slide very unobtrusively in and out of the country. So privacy is a ruling factor and one of the design requirements was that travellers could arrive and, without getting out of their cars, be processed by Immigration and Customs and drive right up to the aircraft on the apron.

Following talks with the client, the Geoffrey Reid team, headed up by associate director Colin Calderhead, and the Happold team under senior engineer Thomas Gabele, decided that the solution was to have three interconnected smaller arched structures.

The original 45m depth would be retained and each arch would span 93m. Geoffrey Reid's Calderhead says: 'This is where Buro Happold really interpreted what we wanted, which was to get the apex as low as possible and bring the eaves down low - which makes a massive difference to the scale - and makes a response to the sweeping hills in the sweeping nature of the hangar roof.' The height is dependent on the minimum clearance needed for the aircraft's empennage.

There are plenty of uncomplicated singlearch hangars around for small aircraft, but in arched structures of this size, there is a need for some kind of tie to stop the roof spreading. It is usually installed high up, but that means the arch has to be even higher to allow clearance for rudders under the tie.

At Farnborough, Gabele's better-mousetrap solution was to install the essential tie underground. The tie is a 600mm x 600mm post-tensioned concrete beam buried 500mm or so below the hangar/apron slab, tying together the ends of the arch. This meant that the arch could be designed as a simple compression structure without complicating bending moments. It's not quite as simple as that because each end of the arch actually sits on an A frame with 3m-long legs fixed to foundation pads through which the below-ground tensioning cables are threaded. So there are some bending moments around the A frames, but they are not serious. After this the hangar design turned out to be quite simple: three rows of five 90m span, 3m deep arched trusses supporting the purlins for the three giant undulations of the standing-seam roof.

Post-tensioning was applied, half straight after the trusses were erected and, as the loads were applied, the remaining tension was tuned to bring the structure into a state of equilibrium. Gabele says that there was not a lot of room for tightening tensioning nuts in the two mid sections where tie beams ran contiguous with the bolts on the ends of the tension cable for the next bay. The back walls are skinned in straightforward cladding although the front, where the aircraft go in, is a bit more complicated.

Arched hangars usually have sliding doors running on tracks fixed to the overhead tie beam. Here the upper part of each arch has a weatherproofing infill across the top - with vertical slots through which pilots manoeuvre the tail. The main doors, by Jewers, slide from either side, and the slots overhead have roller shutters. But without the overhead tie beam (or the problem of getting power across the slot) the doors were installed to roll - and be powered at ground level.

Calderhead selected Kalzip because its standing seams help to emphasise the curve and have a shadowing effect. He says: 'The other key thing is that it's a natural mill-finished aluminium, not anodised. It will lose its shine, but there is something attractive about the quality of the use of a natural material.'

Different buildings call for different shapes - and materials. Most of the control tower, whose form springs out like a tulip, is clad in square aluminium shingles 400mm to 500mm square and laid diagonally. They vary in size because the tower, elliptical in plan, swells out towards the top - where the roof is clad in Kalzip. The tower is actually a concrete slip-form structure to which a steel sub-structure for the cladding is attached.

Equally conventional is the 4,000m 2 three level terminal building wrapped around its base. It also accommodates TAG's corporate offices and it is clad in the same mill-finish aluminium shingles. Calderhead says: 'The form of the terminal is reminiscent of a flying wing clad in aluminium into which holes have been punched for windows. This wraps around the tower and hovers over a band of ground-level glass.' In contrast, he points out, 'the hangar is about a roof that lets you do what you want inside'. In fact the deep overhanging eaves and the space inside the A frames are to be used for offices and workshops - a planning decision which obviates the too familiar addition of excrescences around any of the available flat walls on the average airfield. This hangar will be replicated nearby when business traffic expands.

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