The development of structural membranes

The earliest fabrics used to provide shelter were formed by simple membranes extracted from animals or vegetables. Later these membranes were cut into strips and interlaced to form larger, more practical textiles, and eventually these strips were twisted into circular sections allowing the manufacture of flexible, continuos fibres with enhanced strength.

The black tent was most commonly made of loose woven, spun goats hair, but regional variations used yak hair, sheep wool, camel hair, and even reed mats twined with wool. Other early membranes were constructed from cured leather, deer skin, seal skin, and even tree bark. Linen was used for the Roman velum and the first circus tents, however cotton was the first material to possess any significant structural strength.

Frei Otto's first structures were fabricated from the traditional cotton canvas with which his partner Peter Stromeyer was accustomed. These early canvas fabrics however were relatively weak and ineffective. Performance improved with the introduction of heavy cotton canvas, but its limited strength and poor UV resistance, meant that canvas structures had a maximum span of just 25m and were only expected to last for around three years.

Exhaustive research into membrane combinations during the fifties and sixties resulted in experimentation with a wide range of high performance materials intended to replace cotton. Possible alternatives included; coated glass fibre, steel meshes, aluminium meshes, acrylic sheets, coated synthetic fabrics, foam insulated fabrics, wire reinforced resins and so on. Otto also experimented with PVC coated polyester and nylon, although these early varieties had rather unpredictable properties.

Otto's Interbau Building at the City of Tomorrow exhibition in Berlin (1957) was his first attempt to use a non cotton based fabric on a real project. The building consisted of a highly elastic, flat, polyurethane membrane which was distorted into a doubly curved three dimensional form by an internal frame. Unfortunately this experimental form of polyurethane deteriorated very quickly, possibly due to an incorrect mix of the fibre additive titanium oxide, and had to be replaced by a heavy cotton fabric.

More complex membrane combinations were also tested. The Dortmund Ice rink (1963) for example was fabricated from a continuous filament polyester, coated with a rolled on aluminium foil to protect against UV degradation, which was in turn protected against corrosion with a polyacrylonitrile topcoat. This had a predicted life of 20 years but was opaque and cost about 4 times as much as canvas.

Many membrane combinations were experimented with during this period, but research gradually converged on a small number of practicable alternatives. The German Pavilion at Expo '67 in Montreal was one of the first buildings to use a PVC coated polyester membrane. It was dismantled after six years and showed little sign of degradation. Within a relatively short period of time PVC coated polyester and its sister membrane PVC coated nylon (used for air houses due to its high elasticity) became the industry standards.

As a demand for more permanent fabric structures emerged, so better performing alternatives to PVC coated polyester and nylon were sought. In 1972 PTFE coated glass fabrics were introduced following their development by NASA for the manufacture of space suits. PTFE coated glass is relatively inelastic, and so requires more accurate patterning than the more accommodating polyester or nylon based fabrics. PTFE coated glass is also more expensive, but tends to be longer life. The first application of PTFE coated glass was in the La Verne College Student Centre, in California (1973). In 1993, twenty years later, the original membrane was still in place, and was still capable of withstanding around 75% of its original design load.

More recently, high performance materials such as silicone coated glass have been developed. Whereas Teflon is translucent, silicone is transparent[7], and silicone coated glass has an anticipated life of up to 50 years. A combination of high cost, industry scepticism and early performance problems however, have meant that silicone coated fabrics have only been used to a limited extent so far.

Other developments have included woven PTFE fibre (hydrophobic, microporous, and extremely durable), laminated open weave grids, foils, multiple membranes and more theoretical active membranes such as those developed by Graham Stevens and Nickolaus Laing. Alternatives have also been sought to PVC which has been criticised by some environmental groups. However, alternatives such as polyolefin coated polyester have struggled with fire resistance issues, and fire retardants added to the coating have resulted in reduced adhesion at seams.

A more recent development involves the application of a further topcoat to the outside surface of the membrane which mimics the performance of PTFE. It is generally claimed that this topcoat makes membranes entirely self cleaning.

A huge range of membrane types have been made available over the last twenty years as manufacturers have become increasingly able to vary the way in which they select and combine the constituents of their products. A variety of base fabrics, coatings, topcoats, and colours have emerged resulting in a wide range of membrane strengths and translucencies.

[Millennium Dome, London. A double-layer PTFE coated glass fabric structure]

--Gregor Harvie 07:55, 9 June 2014 (BST)

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[edit] External references

Faegre, T; Tents: Architecture of the Nomads, John Murray (Publishers) Ltd, London, 1979, P130.
Vinzenzi, S; "Membrane Structures. The Design Process." RIBA Journal, March, 1985, P46.
Roland, C; Frei Otto: Structures, Longman Group Ltd, London, 1970, P66.
ibid. Roland, C; Frei Otto: Structures, P66.
Happold, T, "Chariots of Fire." Patterns, No.5, May 1989, P3.
The Institute for Lightweight Structures; "Tents", IL16, October, 1976, P135.
Cook, J; "Twenty Years and Counting." Fabrics and Architecture, V5 No. 4, July / August 1993, P18.
Davies, C, "Fabric Structures", RIBA Journal, October 1985, P59- 63.
Cowell, R; Forster, B; Jofeh, C; Law, M; "Lightweight Structures." Building, 10/6/1983, P28.
Bubner, Prof. E; "Roofing Over Large Areas with Technical Membranes." Proceedings of the International Techtextil Symposium, 4:13, 1993, P2.

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