A–tension grabbing structures

NBS published a work section for tensile structures in February 2005 (H90). Technical author John Murray introduces the subject.

In February 2005, the RIBA presented Frei Otto, the German architect, engineer and pioneer of tensile structures, with the 2004 RIBA Royal Gold Medal. Born in 1925, we may speculate that it was during his time as a young World War II pilot that he was inspired by aerodynamics and lightweight stretched membranes which subsequently lead him to develop the canopies and tented structures that have been favoured by architects and designers ever since.

But of course tensile architecture is probably one of the oldest methods devised to provide shelter, predating basic wood or brick and block compression structures. The humble 'conic' tent is the simplest form of tensile structure, and excelled where two conditions prevailed: a shortage of building material, and a need for mobility. Evidence has been found which confirms that we have been making tents for at least 40,000 years, initially using animal skins hung over large bones and, some 30,000 years later, incorporating woven fabrics. One of the first applications of tensile technology came from transferring sailing principles. The crowds at Roman amphitheatres (e.g. the Colosseum) were protected from the weather by retractable fabric roofs, supported by masts and cables and operated by sailors.

Despite these precedents, the benefits of tensile architecture have only recently begun to be fully explored, thanks to the post-war work of Frei Otto and subsequently, Horst Berger, a civil engineer who put Otto's theories into practice. Today, designers benefit from a range of synthetic woven fabrics which act as both structure and cladding, reducing weight, cost and environmental impact and providing attractive, efficient and economical solutions where large open spans are required.

The form of a fabric membrane is fundamental to its strength and stiffness. To resist both uplift and down-forces, typically due to wind and snow respectively, the surface must be double curved and anticlastic (centres of curvature on opposite sides of the surface). Prestressing is introduced to minimize deflections and ensure that the fabric remains in tension under all load conditions.

Fabrics

Woven fabrics consist of systems of threads which are perpendicular to each other (warp and weft). Their visual appearance, texture and mechanical properties depend on the type of weave, usually plain or hopsack, which in turn leads to specific requirements for shape and orientation of the cutting patterns. Their surface texture increases seam strength and, where required, coating adhesion.

Depending on the particular application and required durability, the fabric may be either uncoated or coated. Used externally, uncoated fabrics may be impregnated to improve their weathering capabilities. The benefits are short term however, and uncoated fabrics are generally only suitable for temporary or internal structures.

The two main woven fabrics suitable for external tensile coverings are polyester and glass fibre. Coatings are applied to both sides of the fabric and improve stability, UV resistance (and hence tensile strength by preventing the degradation of the fabric) and cleanability. Polyvinylchloride (PVC) coatings are compatible with polyester fabrics, and polytetrafluoroethylene (PTFE) with glass fibre. PVC-coated polyester fabrics are relatively inexpensive and, because of their flexibility, are better able to resist damage during fabrication, transportation and erection. The PVC coating offers good water resistance and the serviceable life of the fabric is expected to be in excess of 15 years. However, compared with PTFE-coated glass fibre fabrics, PVC-coated polyester fabrics exhibit relatively high levels of creep, necessitating re-tensioning, get dirty more easily and have a reduced life span. PTFE-coated glass fibre, being a much stiffer material, is less susceptible to deflection under load but prone to folding damage.

There are no British Standards for tensile fabric coverings or systems. However, the members of Tensinet, a thematic network for upgrading the built environment in Europe through tensile structures, have produced a design guide collating current good practice and recommendations. Because comparatively little is known about the behaviour of tensile fabrics and because there is little structural testing of fabrics being undertaken, current tensile architecture design practice is to use high factors of safety. On-going research at the University of Newcastle upon Tyne aims to remedy this so that the acquired knowledge can be used to improve design practice for fabric structures. A better understanding of fabric behaviour will have significant benefits for the design, manufacture, installation and maintenance of tensile fabric structures.

NBS subscribers wishing to specify a tensile fabric covering system should specify the covering, its fabrication and attachment in work section H90, and the supporting structure in work section G10. Those wishing to subscribe to NBS should contact the NBS Customer Centre on 0191 244 5500.

Written May 2005. Confirmed current August 2008