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The sky's the limit

TECHNICAL & PRACTICE: Inflatable roofs have finally come of age. Now Texlon provides energy- and material-efficient coverage at very low cost

The image of the lone abseiler lowering himself down the biomes at the Eden Project (right), is a dramatic picture which emphasises the scale of the engineering technology involved in the project.

The abseiler is an employee of Vector Special Projects, the designer, manufacturer, supplier and installer of the ETFE inflatable roofing solution, carrying out the final checks on the foil roof. As far as Ben Morris, the founder of the company, is concerned, the project could not have happened without the material.

Modesty aside, the material, known as Texlon, certainly has the potential to revolutionise the way large-span roofs are put together. The sizes of the structural members at Eden, for example, would have been prohibitively large to support a traditional glazing solution and transparent plastics would have degraded and could not have supported the geometries involved. It is its geometric flexibility and lightness which are just some of the keys to the material's future.

Foiled again

Traditional rigid-edge roofing or cladding has its weak points at the edges. Whether it is patent glazing systems at its frames, or composite panels and their gaskets, the movement inherent in any building causes these boundary edges to fail over time. In essence, the structure moves, the glass panels expand and contract within the frame, and this differential movement is experienced as a frictional force at the interface between the two. The unique feature of the Texlon panel system is that the movement is taken up within the ETFE material itself.Morris calls this a 'soft-tech', rather than hi-tech, solution to movement joints.

Each panel comprises three layers of ETFE (although higher specifications can include up to five layers), an extruded film of about 10 thickness. The three layers are not bonded together but are wrapped around a kader (rod) which then slots loosely into an aluminium extrusion. When the capping extrusion is fitted, it clamps down, compressing the three layers between gaskets to make an airtight seal and the composite aluminium extrusion thus forms a frame around the threelayer foil panel. A plastic plenum feed from a small generator or similar, delivers air to inflate the panel, hence the description of the panels as 'cushions'.

The system can be designed to be anything up to 35m in any one direction, spanning 3.5m in the other (although Eden used 11.5m-diameter panels so many variations on a theme are possible). The aluminium frame is fixed to the structure of the building and because the cushion is built up of a flexible material under a small amount of pressure (220Pa), any movement in the frame or subframe can be accommodated in the inherent flex of the material and the 'give' in its inflated curvature.Morris states that 'no differential movement is taken up in the frame'.

Wind cheater

A three-layer cushion has a U-value of 0.96W/m 2K (five-layer cushions have a U-value of 0.18W/m 2K) and together, the three layers have a light transmission of 97 per cent through the material. The taut material is designed to have a structural stability sufficient to be self-supporting and to accommodate snow and wind loads.

However, given the unique nature of the product, there are no relevant regulations to apply to this product and engineering methods and design concepts are having to be reinvented to deal with the novel circumstances embodied in the cushions.

For instance, calculations for wind loads are totally inappropriate for Texlon, given that, as Morris says, we are 'moving into the realms of symbiotic structures', whereby the wind energy is taken up in the skin, and only a proportion is transferred to the aluminium frame. In fact, in tests, the viscosity of the air-filled cushions actually dampens the wind effect. 'This is a new language, ' says Morris, 'it is totally different from the physics and appearance of tensile structures and we can only learn by listening to the material.'

There is nominal diffusion of air through the material and at the welded seal between the plenum and the cushion, such that the top-up air pump requires only 100W/1,000m 2running for 50 per cent of the day.

Also, it is recommended that in extraordinarily humid conditions, the supply air be de-humidified.

The other significant advance offered by Texlon is a reduction in the size and quantity of material used for the roof support structure. Commonly, about 10 times the amount of steel is used in columns and beams than is needed for support purposes, primarily just to provide stiffness. With Texlon, however, given that the cushions have inherent flexibility to cope with structural movement, and that their weight is nominally 3kg/m 2, then the requirement for selfsupport effectively becomes the only criterion for structural members.

Thus a completed Texlon roof costs about 30 to 50 per cent less than traditional steel and glazing systems.

As an example, Morris says the British Museum's roof cost in the region of £2,000/m 2, whereas he could have done it for around £600/m 2(including design fees). In this way, structure can become light in all senses of the word and materials can be used to their optimum effect.

Now you see it

The requirement for atria to be selfventing in the event of fire inspired Morris to create new styles of openable roofscapes. On one scheme, the entire 100m 2roof structure hinges open at one edge; on another, the roof is centrally pivoted and both sides release and drop down in a fire.

Morris is proud of the fact that these release mechanisms are straightforward pulley and weight technologies - combining the soft-tech of Texlon cushions with low-tech movable parts.

The latest development to come out of Vector's office is screen printing and body tint foils. These can vary from the opaque to translucent, and printed screens can accommodate almost any pattern. By printing alternating chequerboard patterns on the outer and inner foils in a three-layer system, the light passing through (and solar gain) can be reduced.

By the use of intelligent pneumatics, different layers of the foil can be closed up, and in this example, the alternating chequerboard patterns come together to block off 90 per cent of the light. By reducing the air on one side of the central foil layer, the U-value also changes. This ability to alter the climatic conditions at the flick of a switch, using simple technological devices, will undoubtedly prove to be a major advance for building management systems.

And the prestige projects keep coming. Rem Koolhaas' winning scheme for Los Angeles County Museum of Art incorporates Texlon over tension wires, a new design concept from Vector which incorporates an arrangement of steel arches held up by tension wires. The wires fall to nodes which are held up by wires passing over the arches and held down by wires from the ground. A structure inspired by the spokes of a bicycle wheel.

Morris is not happy to stand still, and his office is constantly challenging the material to do bigger and better things. Lateral thinking is as good a generator of ideas as any other for such a challenging material, and Morris tells of how one of his staff noticed that, at home, his baby was banging a toy drum, but when jelly was dropped on the drum surface, the irritating noise was muffled. The next day, Morris and his staff were in the workshop exploring the acoustic merits of jelly on the ETFE panels. They are now developing gel dampers.

There are many more weird and wonderful ideas in the pipeline, some I am not allowed to talk about here, but which take the idea of selfsupporting structure and energy efficiency to even higher levels. Two more embryonic lateral ideas - that are just part of Morris blue-sky thinking at the moment - include running heat recovery pipes through the cushions (Morris sees the day when there will be goldfish swimming about in the roof structure) and a development inspired by the fluorescent tube - filling the cushions with gas and applying a charge, to create a totally fluorescent roof.

As the knowledge of the material's possibilities improves, the scope for creating fun, efficient, flexible structures seem to be endless. It would seem that with transparent roofing, the sky's the limit.

Vector Special Projects can be contacted on 020 7375 2756

TECHNICAL INFORMATION

ETFE stands for Ethylene Tetra Fluoro Ethylene, referred to as in the text, as 'foil'.

Texlon comprises pneumatic cushions of modified co-polymer ETFE, contained by an aluminium extrusion and gasket rigidly fixed to the main building frame.

A three-layer cushion system has a U-value of 1.96 W/m 2K, which is better than horizontal triple glazing.

The cushions have low flammability and are self-extinguishing.They self-vent in the event of a fire and do not produce molten drips or give off quantities of dangerous fumes.

Texlon is an extruded product, so the ETFE surface is extremely smooth and is said to be self-cleaning in wet weather.

The material is 90-150 microns thick and a standard cushion weighs approximately 2-3.5kg/m

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