Tensile Membrane Roof

A tensile membrane roof is a roof whose covering carries load purely in tension: a thin fabric or plastic film is stretched between supports so that it stays taut under wind, rain and snow. Instead of resting on rafters or a deck, the membrane spans between masts, arches, cables, compression rings or edge beams. Membrane roofs are used for stadiums, airports, canopies, atria and shade structures, where long spans, low weight and daylight are wanted.

History

Tents have always worked on the same principle, with poles and guy ropes pretensioning the cover. The Russian engineer Vladimir Shukhov was among the first to calculate stresses in such structures, designing eight tensile and thin-shell pavilions covering about 27,000 m² (290,000 sq ft) for the 1896 Nizhny Novgorod fair. Frei Otto popularised modern tensile architecture with the West German pavilion at Expo 67 in Montreal and the roof of the 1972 Munich Olympic Stadium. Engineers including Ove Arup, Buro Happold and Horst Berger developed the field from the 1960s. Later landmarks include the terminal of Denver International Airport and the Millennium Dome (now The O2) in London. Until computer analysis matured in the 1990s, designers relied heavily on physical models and soap films to find shapes.

Shape and prestress

A flat membrane would flap and pond, so tensile roofs are given double curvature. The basic form is the saddle (anticlastic) surface, curving up in one direction and down in the other; conical and radial shapes are also common. Pretension in the fabric and supports keeps the surface stiff. The final shape depends on the cutting pattern, the support geometry and the pretension. Poor drainage is a key risk: water ponding in a hollow can deform the membrane progressively, and snow often does not slide off, so some roofs use heating to clear it.

Membrane materials

• PTFE-coated glass fibre: woven glass cloth coated with PTFE. Inherently non-combustible, largely self-cleaning in rain, and expected to last 30-50 years; the earliest projects outlived their 25-year design life. Heavier grades transmit about 9-12% of daylight and lighter grades about 14-16%. Single-skin weight is about 1.5 kg/m² (0.3 lb/sq ft).
• PVC-coated polyester: the more economical woven alternative, with a typical life of about 15-20 years.
• ETFE (ethylene tetrafluoroethylene): a fluoropolymer foil used as single layers or, more often, as inflated multi-layer cushions. A double-layer cushion weighs about 0.7 kg/m² (0.14 lb/sq ft), a small fraction of 6 mm glass, and transparency can reach about 90%. Cushions need a continuous low-pressure air supply. ETFE clads the biomes of the Eden Project in Cornwall (opened 2001), the Beijing National Aquatics Centre (2008) and the Allianz Arena in Munich.

In woven fabrics the warp threads are stronger than the weft, which affects how panels are cut and orientated.

Cables and supports

Membranes are usually edged or reinforced with cables of high-strength steel, stainless steel, polyester or aramid. Steel spiral strand is pre-stretched and load-cycled to remove construction stretch before use. Masts and arches carry the resulting compression into the foundations.

Maintenance

PTFE membranes are typically cleaned every few years with soft brushes and mild detergent. Tears in ETFE can be patched by welding from outside.