What it costs: Tensile fabric
- Date: 02/10/2009
Building Magazine: Fabric roofs are becoming the topping of choice for everything from bike sheds to concert venues. Peter Mayer of BLP Insurance looks through the available options.
The millennium dome – now known as the O2 – is still the largest tensile fabric structure in the world, but such roofs are no longer the preserve of landmark buildings. Increasingly, they are being specified in public spaces, schools, as cycle shelters, or for temporary outdoor events.
Two fabrics are used for external coverings: glass fibre cloth and polyester. The most common and least expensive of these is polyester, coated with PVC, which is available in a wide choice of colours. The PVC usually incorporates ultraviolet stabilisers, fire retardants and anti-fungicides and has an expected service life of 15 to 20 years.
The polyester PVC composite can be improved with further coatings. A polyvinylidene fluoride (PVDF) lacquer can be applied over the fabric to improve cleaning properties – it becomes self cleaning to some degree. The expected functional service life of the lacquered membrane would be longer than for PVC alone. But service lives tend to be based on appearance; the service life of PVDF finishes is 15–20 years for non-weldable types and 10–15 years for weldable types. Other top coatings include polyvinyl fluoride (PVF) film, typically for aggressive environments with a service life of 25 years, and acrylic coatings, which have a shorter life.
Glass fibre cloth is usually coated with polytetrafluoroethylene (PTFE) or Teflon for external applications. Life expectancy is high – from 20 to more than 30 years. The non-stick properties of PTFE enhance its weather proofing properties and provide a degree of self cleaning. UV radiation bleaches the PTFE coating white. Glass fibre fabric needs to be manufactured to tight tolerances and installation is exacting to ensure there is no creasing and correct tensioning is achieved.
Physical properties of the fabric such as tensile and tear strength are specified to ISO 1421 and ISO 4674 and are related to the composition of the fabric and its thickness. Thicker, heavier grade fabrics generally are stronger, but have lower light transmission.
Fabrics can be treated to achieve flame resistance. PTFE is non-flammable. Where fabric is used as a cover for a building, a sprinkler or mechanical extraction system may needed to manage potential smoke production from fabric.
Single layers of fabric only achieve a U-value of 4–5 W/m2K. Some manufacturers have developed composite sandwich constructions, claiming U-values as low as 0.2-2.6W/m2K using mineral fibres and multi-layer foil membranes.
BS 3424 is the British standard for testing coated fabrics. International and European standards have replaced withdrawn parts of BS 3424, for example BS EN ISO 2411 for adhesion of coatings.
Design and supporting structure
Designing fabric structures is a specialist engineering discipline using powerful 3D CAD and finite element analysis software. The fabric shape, support and cable structures are modelled so that expected wind-induced stresses and deflections can be accommodated in the final design and the risk of ponding is minimised.
The final shape depends on the cut of the fabric, positioning of supporting structures and whether the fabric is pretensioned. Edge detailing may be scalloped with wire in an edge cuff or incorporate a metal rope for bolting to a clamp plate system.
Supporting structures may comprise columns, masts, cables or ringbeams. Material choices include galvanized low carbon steel (polyester powder coated or painted), aluminium (anodized or polyester powder coated) or stainless steel.
Condensation is a risk on the underside of fabric structures used as buildings. A risk analysis should be carried out to minimise this and inform a strategy to manage the risk.
Other cost issues
Cost benefits may be made by specifying off–the–peg service rather than bespoke design. Printing on the fabric and non-standard colours will increase costs.
In-use costs include external and internal cleaning, regular inspection and adjustment of tension. Outbreaks of fungal or algae growth would need specialist cleaning.
Small cuts can be patched up on site, larger repairs will require the fabric to be dismantled and taken to a workshop. Costs associated with risk of damage and vandalism can be designed out by keeping the fabric out of reach and using supports that are not easy to climb.
BLP Insurance provides latent defect insurance for building www.blpinsurance.com