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1991 May: How It Was Built

Stansted, particularly the terminal building, is highly sophisticated constructionally. Here Colin Davies examines the chief elements one by one

The whole point of the design of the Stansted terminal is that the concourse is a single volume. Travellers always know where they are and which way they are facing because they can see the four glass walls of the huge room in which they stand.

The problem for the designers was that this room was also a single 600,000m³ fire compartment. Divide it into four, and provide automatic sprinklers and smoke vents in the roof, was the unsurprising advice of the fire authorities. Apart from destroying the unity of the space, this would also have compromised the second principle of the design: that the roof should be completely free of mechanical and electrical services.

There were a number of reasons for this decision, both practical and aesthetic. Introducing high-level services would have meant providing access for maintenance either an unsightly network of high-level walkways, as at Heathrow Terminal 4, or temporary scaffold towers erected on the concourse floor. Neither of these was acceptable to the designers who for functional reasons wanted all the services to be accessible from the undercroft. They wanted to preserve the conceptual purity of their design. The roof was to be a simple, lightweight canopy, with no risers, ductwork, or light fittings, and no plant to disfigure its clean lines. The architects were also anxious to minimise the number of vulnerable interruptions to the single-layer waterproof membrane.

Their response was as unsurprising as that of the fire officers: call in Margaret Law of Ove Arup & Partners. Law is the architectural profession’s favourite fire expert because she takes an analytical and scientific rather than a conventional approach to fire safety. She was able to demonstrate convincingly that the single large volume might be a positive advantage in the control of smoke, which is the main hazard in all building fires.

The normal ceiling height in airport terminals is about six metres. To preserve the visual unity of the space, the Stansted concourse would have to be about twice as high - something that was potentially difficulty to justify to the client. But the designers could now argue that the extra height was necessary, not just for aesthetic reasons, but because it would act as a smoke reservoir, allowing plenty of time for the building to be evacuated before the fumes reached floor level. The smoke could be extracted not upwards but downwards, through the tops of the ducts in the centre of each structural tree. The advantage of a single volume was that, though the extract fans would be thrown into overdrive, if one failed its neighbours could cope with the additional load.

But what about the requirement for sprinklers? Here the strategy was to isolate the main likely causes of a fire. All the high risk areas - offices, kitchens and shops, some turning over gallons of potentially explosive duty-free liquor - are enclosed in so-called ‘cabins’. These have one hour fire-resistant structures, smoke curtains and fire shutters. This leaves only the travellers themselves and their baggage outside the protected areas - a sufficiently diminished risk to obviate the need for overall sprinklers.

So the architects got what they wanted. It was a typical piece of Foster problem solving: a combination of rigorous analysis and determination to preserve the purity of the architectural concept.

Structural trees

The basic structure supporting the floor of the main concourse is reinforced concrete and mostly invisible to the traveller. Round columns, spaced either six or 12 metres apart, support a concrete waffle slab either 850mm or 450mm thick, forming the floor of the concourse and the ceiling of the baggage handling and services undercroft. Architecturally, however, the most important part of the structure is the grid of steel trees at 36 metre centres, which support the lightweight, services-free roof. Surprisingly, the roof was erected first, providing a weatherproof enclosure for the construction of the concrete beneath. The trees are therefore independent of the concrete, piercing the waffle slab and resting on pad foundations in the ground.

Each tree has a trunk, formed by four massive steel columns, and four slender branches. These branches are braced by tension members attached by a single bolt to a pyramid on top of the trunk. The trees support a square grid of steel members which act as the main roof beams. Each square is infilled by a lattice steel dome. All steel members are circular in section.

This bald description hardly does justice to a structure which is visually incredibly delicate, floating above the concourse as if supported by air. The trees appear to be holding the roof down rather than up. And yet the engineers assure us that there is plenty of redundancy in this structure and safety margins are massive. It makes one wonder why other supposedly lightweight steel structures - even those supporting thin fabric membranes - are so heavy and cumbersome.

Services pods

The trunk of a tree does not just hold up the branches, it also delivers nutrients from the ground to the leaves; and so it is, by analogy, with the structural trees at Stansted. Since there are no mechanical or electrical services in the ceiling or walls, air, light and power have to be delivered to the concourse from the undercroft via ducts in the trunks of the trees. Four rectangular air supply ducts, arranged in a square, form the walls of a central return-air duct feeding down to a walk-in fireproof blockwork plenum enclosure in the undercroft.

The return-air duct also acts as an automatic smoke vent. Within it sits a metal spiral staircase, rising from the floor of the undercroft and giving access to the powerful uplighters fitted to the top of the supply ducts, Around the outside of the trunk are fitted secondary services such as power sockets, hose reels, emergency lighting, illuminated signs and passenger information services. The whole thing is attached not to the concrete floor, but to the steel superstructure so that it is not affected by differential movement.

The various components of this assembly, or ‘pod’, were thoroughly researched and tested with the help of numerous experts in addition to the main services consultants. The uplighters, which combine with the reflective dome above to form what is in effect a giant light fitting, were designed by Claude Engle. Models to test the behaviour of the air from the supply ducts were built and tested until the right duct profiles and velocities were established to ventilate the space adequately while minimising draughts and noise.

The pods are not particularly beautiful objects in themselves. Their beauty lies in the fact that they completely eliminate all the ugliness an unco-ordinated services installation can inflict on a big building. No ducts, no walkways, no roof top plant rooms, no temporary access scaffolding, no suspended ceilings or raised floors, no strip lights, no transfer grilles and so on and on and on. The elegance and simplicity of the whole building depends upon these amazingly compact and fully integrated pieces of hardware.

Concourse roof

The roof domes are not true domes but are made up from four intersecting part cylinders. This is important because of the nature of the waterproof membrane - a single layer of hot welded ‘Sarnafil’ pvc. The absence of double curves greatly simplifies the job of tailoring the membrane to fit the form. It is mechanically fixed to ordinary Z purlins on a profiled metal deck, with two layers of ‘Rockwool’ insulation between. At the valleys between the domes, the deck simply bridges the gap, forming a triangular duct to take the horizontal rainwater pipes.

Rainwater drainage

Getting the rainwater down from 39,204m² of continuous roof was never going to be easy and a design strategy which outlawed visible service connections made it doubly difficult. A conventional system, with outlets every 18 metres, would have needed 300mm diameter pipes falling at least 2.5 metres to the perimeter of the building. Concealing these pipes would have been impossible. A radical solution was therefore required.

It was found in a system previously unknown in this country, but used quite extensively in Finland and Russia. Alvar Aalto used it, for example, in the Finlandia Hall in Helsinki. It is called a UV system, which stands for the Finnish for ‘full flow’. In a conventional system, water spirals around the wall of the down-pipe, leaving a core of free air, but in the UV system the downpipe runs full bore. This creates a siphonic action which pulls the water along the horizontal pipe. The rainwater outlets are special sumps which remain closed until a sufficient head of water has been built up to promote the siphonic action.

The system therefore offers two big advantages: the pipes can be much smaller, because they are running at full bore, and there is no need for a fall in horizontal pipes. At Stansted, downpipes are on average only 89mm internal diameter and there is plenty of room for the horizontal pipes in the triangular duct between the domes. The only visible parts of the system are the slender exposed downpipes at each bay outside the flank walls of the building, tapering as they approach ground level.
Roof aerodynamics

The roof is a practical, economical structure and its form has emerged from the usual rigorous analysis and option-testing design method that mark Foster’s modus operandi. But perhaps it is also subtly symbolic. The impression that it bears some relationship to aeronautical design is pervasive, though hard to pin down.

One detail, however, is a genuine piece of aeronautical design. Because the waterproof membrane is fixed mechanically rather than stuck down continuously, there is a risk that in certain conditions it will be lifted off the deck by negative wind pressure. To prevent this, ‘spoilers’ are fitted to the eaves to reduce turbulence and promote a smooth, laminar air flow over the perimeter of the roof. These spoilers take the form of curved, rolled aluminium panels fixed by an ingenious expanding bolt device which leaves the face completely flush. The comparison with the leading edge of an aircraft wing is inescapable, both functionally and visually.

Rooflights

Internally the roof is transformed visually by the soft, glowing quality of the daylight. This is achieved by a combination of rooflights, suspended daylight reflectors and white ceiling panels. Each dome is provided with four triangular rooflights, amounting to 11m2 or three per cent of the total roof area. They flood the deep space of the concourse with energysaving daylight and allow orientating glimpses of sky and sun. Sealed units of double low emissivity glass are fixed to upstands by a combination of glazing bars on the upper edges and bolts through the extended top pane of glass on the lower edges. So the rainwater is allowed to drain off on to the roof without obstruction.

As always with rooflights, the main problem is glare. To overcome this, it was necessary to reduce contrast between bright sky and dark ceiling. The solution is simple and elegant. Triangular metal panels suspended on wires beneath the rooflights bounce the daylight back on to the adjacent ceiling creating a halo of reflected light. At night the effect is reversed. The panels reflect the light downwards from the powerful uplighters in the trunks of the trees and partially mask the dark triangles of the rooflights.

Ceiling

Several alternative solutions were explored for the ceiling panels. The basic problem was to reconcile conflicting demands of acoustic absorption and light reflectivity. A soft fibrous material might have coped well with the sound, but would soon have become faded and dulled by airborne dust. A metal panel would have been cleaner and more durable, but noisy. One proposal was to use tensioned fabric panels which could be ‘tuned’ to absorb the different frequencies of background noise in the concourse, but this was soon rejected on the grounds of cost. Perforated aluminium was the next option, but also proved to be too expensive. The final compromise was microperforated triangular steel trays, filled with a white, light-reflective, sound absorbing quilt.

Curtain walls

The external walls of the concourse are all glass: 3.6m wide x 2m high sealed double glazed units, with a low emissivity coating and an inert gas filling. This gives a very respectable level of thermal insulation at a U value of 1.6 for the whole wall. The inert gas will not definitely stay sealed in for more than four or five years, but this is reckoned to be more than long enough for reduced heating and cooling bills to cover its minimal cost. All glass is toughened for safety. In the flank walls, where there is no projecting canopy to provide shade, the glass is interleafed with two layers of translucent pvb which is calculated to reduce solar heat gain by more than 60 per cent.

These are not curtain walls in the strict sense, since they do not hang from the roof, but stand on the floor. They have to span the 12 metre height of the concourse in one go, and withstand a high windloading of 1 .4 kiloNewtons. The problem for Foster therefore was to design a strong support framework which would match the incredible visual lightness of the roof structure. As usual, the solution is logical and elegant.

All mullions and transoms are 120 x 80 mm rectangular hollow steel sections, forming a completely flush-framework. This is strong enough to span 3.6 metres horizontally, but not strong enough to span the whole 12 metres vertically. An extra, bracing component is therefore added to the mullions in the shape of a 114mm diameter tube, welded via cleats to the inside face of the rhs. In effect the two act as the booms of a vertical truss, tapered at the ends where the bending moment is less.

Structural silicone is a favoured Foster method of fixing glass to a grid. It was first tried out on the Hongkong Bank and was briefly considered for the big glass panels at Stansted. In the end, however, it was rejected as too risky. Instead the fixing is of a conventional mechanical kind, with thermally broken aluminium strips screwed into prepared holes in the rhs frame.

So far, this description has covered only the static structural parameters of the wall. The real problem, however, was a dynamic one: how to cope with a possible differential movement between wall and roof of anything up to 120mm. This movement is of two kinds, thermal expansion, and deflection under wind load. It is magnified by the fact that unlike the concrete floor, there are no expansion joints in the roof structure. Thermal movement is therefore cumulative across the whole width of the building. The walls have to be stabilised at the top by the roof, but the roof also has to move in every direction in relation to them.

At the Renault Parts Distribution Centre in Swindon (AR July 1983), Foster solved this problem by means of an all-too-visible flexible skirt borrowed from hovercraft technology. At Stansted, the solution is less crude. A complicated hinged linkage device welded to the inside of the perimeter roof beam is free to rock up and down while simultaneously sliding from side to side along a horizontal stainless steel pin attached to the top of the wall frame. The weather is kept out by a toughened glass downstand connected at its bottom edge to a discreet flexible strip, tucked away behind an aluminium upstand.

Permanent travelling ladders are provided for internal cleaning of the glass. External cleaning is from an ordinary mobile ‘cherry picker’.

Undercroft walls

The external wall of the undercroft, below concourse floor level, is very different from the glass walls above, but no less sophisticated. Most panels are of rigidised aluminium on a plastic frame, mineral-insulated and with a steel liner. They are only half the height of their counterparts at concourse level, but the full 3.6 metre width has been maintained, mainly to allow access to the undercroft for vehicles during construction.

A standard louvre panel and a standard window - this time using structural silicon glazing - complete the kit of parts. Panels are supported on an internal framework of aluminium extrusions, steel channels and steel universal columns. Joints are pointed in silicon. Though this is an in-situ material, it is nevertheless easy to take out and replace should panels need to be re-arranged. Such flexibility was considered essential since it was likely that the wall would be erected before the detailed planning of the undercroft had been finalised.

Concourse cabins

The Stansted concourse would be wonderful as a completely uninterrupted space, like a cathedral or perhaps an aircraft hangar. But it has a job to do and that job involves more than just processing air travellers. As well as check-in desks, baggage conveyors and security screens, there are shops, restaurants and offices to be accommodated.

The expectation of the client for a modern air terminal is normally that the building will be interior designed, probably not by the architect. This is what happened, for example, at Heathrow Terminal 4, where Fitch were commissioned to fit out Scott Brownrigg & Turner’s basic shell. One does not, however, commission Foster Associates to design just a basic shell. On the other hand, as Associate John Small puts it, the practice ‘does not do interiors’. Shell and interior are one as far as Foster Associates are concerned.

But still the shops had to be somehow accommodated and in such a way that the architectural unity of the concourse space was not destroyed. The answer was to enclose them within the strict confines of single-storey cabins standing on the concourse floor as buildings within a building. The cabins have separate structures and are services separately, upwards from the undercroft, of course.

The basic cabin structure is a miniature version of the main structure. Steel trees, with branches cantilevered from a cluster of four columns, support a flat roof of light steel sections and woodwool slabs. Supply air ducts rise between the columns. There are suspended ceilings of two types: a flat chequerboard of lights, grilles and plain panels for the offices, and a perforated metal ceiling following the tapered profile of the structure in the public retail and restaurant areas.

Concessionaires have almost complete design freedom within the confines of the cabin, but at the interface with the main concourse space there are strict rules to govern, for example, the size and position of signs. Most important of all, nothing is allowed to project above the 3.375 metre overall height of the cabin. This is crucial to allow free air movement from the structural tree pods and to preserve the unity of the concourse space and the orientating long views to the perimeter.

So the cabins have an aesthetic function, drawing a frame around the work of the retail designers, but they also have an important practical function - to isolate these high fire-risk areas from the main concourse space. Fireproof ‘Gyproc’ and metal stud walls enclose nonpublic areas like kitchens and stores. But the shops themselves of course have to remain open. They are therefore provided with a downstand fascia which creates a 700mm deep smoke reservoir below the ceiling. Smoke is then extracted via return-air ducts at the perimeter. When everyone has escaped, automatic rolling security and fire shutters descend from behind the fascia.

Check-in desks

Another major element of Foster’s design of the interior was the standard check-in desk. As always, the design started with a rigorous analysis of the problem and a refusal to accept conventional solutions without first questioning and testing them. The main problems with existing desks seemed to be the unpredictablility of the lessees’ requirements for equipment such as computer monitors and telephones, and the difficulty of maintenance. Often dozens of different desks had to be specially made, usually in hand-laid grp.

They were wasteful to manufacture and difficult to modify or replace. What was needed, Foster decided, was a kit of parts: a basic and fairly crude steel frame to which a range of panels and surfaces could be attached. A full-size mock-up was set up, tested, modified and ergonomically adjusted. The range of panels in the final kit includes injected grp, stainless steel, black granite and lino covered plywood with a special moulded lip for desktops. The kit system makes maintenance much easier. If a desk is damaged, there is no need to rip the whole thing out. The damaged panel can be removed and a replacement ordered from the suppliers.

Behind the check-in desks, the baggage conveyors are concealed by screens assembled from another kit of parts consisting of a galvanised steel skeleton and panels of powder-finished ‘Zintec’ steel bonded to a ply core.

The same rigorous design approach has been applied to every detail of the concourse interior. Foster has retained total control, but not to the exclusion of other designers. There are demountable screens designed by Giusseppe Boscherini, a carpet for the airside floors designed by Ron Nixon and a signage system designed by Alan Fletcher of Pentagram. Furniture is based on standard models, but even here there were comparative tests, field trials, mock-ups and modifications. It is interior design of a kind, but it has very little to do with the tacky, temporary, ‘themed’ interiors of other airport terminals.

British Rail station

In conceptual planning terms, the British Rail station is part of the services undercroft. Its main technical feature is civil engineering rather than architecture: a 600 metre long, 8.5 metre high concrete retaining wall forming the southern boundary of the levelled terminal site. This was and the parallel in-situ concrete wall of the linear undercroft plant room form the basic enclosure for the twin tracks and central platform. The trench is roofed over by the concrete slab at concourse level, carrying the main access road. Concourse and station overlap, so that the trunks of the structural trees forming the canopy over the forecourt stand in the middle of the platform. The trunks rise through glass floors in the forecourt so you can see the whole tree from the platform.

The design is the result of a collaboration between Foster Associates and British Rail architects. It is envisaged that there will always be at least one train waiting, so there are no enclosed waiting rooms, only a small cabin containing a ticket office, supervisor’s lavatories and plant. There was some concern about wind conditions in what is effectively an open-ended tunnel. On the advice of Tom Lawson of Bristol University, some modifications were made to the ground profiles, and a simple glass screen was provided at the buffer end.

Enclosures for the ramps and escalators that rise up from the platform to concourse level could hardly be simpler. Walls and roofs are made of butt-jointed structural toughened glass, bolted to a tubular steel frame using an Austrian version of Pilkington’s ‘Planar’ system. For the designers at Foster Associates even this extreme minimalism is apparently over fussy. There are, they say, too many bolts.

Signs, advertisements and lighting are what give the station its architectural character. As for the former, Foster Associates found themselves at loggerheads with BR, who refused to reduce the amount of illuminated advertising. But it is the background lighting, designed in collaboration with Claude Engle, that is the real success of this space. The master stroke was to concentrate on reflected rather than direct light, illuminating the whole space, including the tracks, not just the platform. The natural concrete walls could so easily have been cold and dismal but they are transformed by warm, orange floodlighting at low level. The only other major lighting element is a suspended strip at the edge of the platform, lighting up the trains themselves. Directional lights on top of this suspended strip shed a carefully graded light on the concrete ceiling.

Satellite building

For security reasons there must be a strict segregation of arriving and departing passengers in an airport terminal. In the main building at Stansted segregation is horizontal so that all passengers can share the same concourse space. In the satellite building out on the apron, however, segregation is vertical.

This long, narrow, three-storey structure is basically a built circulation diagram. The track transit vehicle from the main terminal arrives at a station at basement level. Departing passengers are channelled via escalators up to a single second-floor departure lounge. From there they descend to one of nine gates on the level below, giving access via the air-bridges to the aeroplanes. Arriving passengers use the same gates, but at different times, walking straight through to the arrivals corridor on the same, first-floor, level. From there they descend to the other platform of the track transit station without crossing the path of departing passengers. Linked opening of automatic doors and escalators in the gate areas ensures that it is impossible to move directly between departure lounge and arrivals corridor.

The ground floor of the building is given over to plant and ground crew accommodation. The only place, therefore, where passengers are not on the move, is the top floor departure lounge. This is conceived as a linear observation platform, 250m by 30m, cantilevered out all round and fully glazed. As in the main concourse, passenger orientation is the priority. Travellers have a panoramic view of aircraft movements and are never out of visual touch with their departure gate and the aeroplane itself.

The structure of the satellite building is, for Foster Associates, uncharacteristically elaborate. It was originally conceived as two rows of structurally separate double cantilevered frames, each supported on a pair of columns - a linear, concrete version of the treelike structure of the main concourse. This was designed to allow an uninterrupted glazed slot in the ribbed concrete floor of the departure lounge, providing borrowed daylight in the arrivals corridor below.

In the event, as the design developed, the slot disappeared and the frames were linked, compromising the original concept and calling into question the necessity for the columns to be paired. Above the departure lounge floor, concrete columns give way to steel, with circular hollow section columns supporting cantilevered trusses. Here, though again the frames are linked, the slot has survived in the form of a continuous rooflight.

It is obvious from the general levels of finish and workmanship that the budget for the satellite building was much tighter than that for the main concourse, and presumably this was a deliberate policy. The usual rigorous Foster design development method was replaced by a more economical performance specification and subcontractor design procedure.

The cladding of the lower floors, for example, is ordinary industrial quality profiled metal, and the departure lounge is glazed in an off-the-peg curtain wall system. The glass itself, however, is not ordinary. It is a complicated laminate with a high shading co-efficient to prevent solar heat gain and a carefully gauged level of acoustic insulation - high enough to keep out intrusive noise, but low enough to allow jet engines to be heard. (Passenger orientation depends on hearing the aeroplanes as well as seeing them.)

The ceilings are rather disappointing. Large areas of uninterrupted perforated metal have overtaxed the capabilities of the subcontractor and the level of accuracy is well below normal Foster standards. With hindsight, the architects admit that it might have been better to have divided the ceiling into smaller areas by expressing the structure. The lesson has been learnt, however, and the problems will no doubt be ironed out when the next satellite building is built.

Client Stansted Airport
Lead designer Foster Associates
Quantity surveying services BAACL
Structural engineering Ove Arup & Partners

 

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