Tensile Fabric Structures

Tensile fabric structures have made a dynamic contribution to contemporary architecture, but due to the nature of these structures, some care is required to ensure their longevity. Left to their own devices such structures may deteriorate visually as well as structurally. However, if properly installed, and regularly maintained, fabric structures will maintain both their form and looks for up to 25 to 35 years.

Tensile fabric structures are dynamic, in that the fabric skin moves and undulates as forces, such as wind and snow are applied to it. In turn many fixings and boundary cables will also be prone to this flexing movement. However well built the structure, because of this dynamic nature, wear and tear will be created and regular maintenance is essential.

Maintenance, and indeed construction, of tensile fabric structures require many elements of ‘work at height' such as powered access and tower scaffolds. The configuration of steel spars and cables required for such structures often necessitates access into the tightest of corners. This is particularly the case once the fabric ‘skin' is in place. The use of rope access and fall protection or restraint is therefore an essential element throughout the lifespan of these structures, as other access is simply not possible. IRATA provides an essential professional body, providing training and best practice on which to base a safe approach to working on such structures. Nevertheless it should be clear that work on a tensile fabric structure may not just involve working directly in suspension. A range of techniques are often employed using the core skills provided by IRATA rope access training:

• Fall restraint: Work on the top of a fabric ‘skin' may require a technician to be attached to a rope in such a manner as to prevent them from falling over the unprotected edge, yet not provide direct support. This may be achieved using a rope adjustment device such a ‘shunt' on a working line, or a lanyard of pre-determined length fixed to a suitable anchor point. Careful management of a mobile rope device must be maintained to ensure that a technician is restrained a suitable distance from an edge.
• Horizontal lifelines: Moving along un-protected edges between trusses or through steel ‘space frame' structures may be protected by use of horizontal lifelines. These lines facilitate regular and easy ‘point' access along structures. However a full understanding of system and anchor loadings is essential. If there is any doubt about the suitability of anchor points, an engineer should be consulted to advise.
• Aid Climbing: This is often required to provide point access by climbing through the structure. When using rope positioning lanyards care has to be taken to minimise potential fall factors and consequent shock loads, if directly climbing through the structure. As a rule of thumb, if the fall potential is greater than 0.5m then the technician is strictly in a fall arrest situation and suitable equipment should be selected.

Technicians will often be required to combine a number of these key access methods whilst working at height: For instance first operating a powered platform to provide point access, and then aid climbing through the structure, before finally setting up and transferring to ropes. A diverse range of working at height skills is therefore required. As previously described technicians may also find themselves in the ‘grey area' between rope positioning and situations that require ‘fall arrest' techniques. Careful management, planning and supervision is required to ensure that appropriate measures are in place to mitigate such risks.

Tensile structures (and all buildings) are designed in accordance with BS6399 part 2 for the wind loading. This typically means the structures are designed for gust speeds of 80 to 100 mph. These wind speeds apply a wind loading of between 0.4 & 1.0 kN/m2 (kilonewtons per metre squared). Knowledge of the construction dynamics of tensile fabric structures is an obvious essential prerequisite for a technician undertaking inspection and/or maintenance, as the potential for major structural damage to the fabric structure, and any adjoining building, is therefore considerable, both in the event of poor initial construction or inadequate maintenance.

In the engineering of tensile fabric structures a pre-stress is built into the fabric ‘skin'. This pre-stress can be created in a combination of ways, depending on the structure:

•  Headrings: These provide the fabric attachment point for conical structures and can be used to tension the fabric by altering the fabric height by use of sand pots at the base of a mast or by the use of ‘jacking' points for the headring itself.

•  Boundary cables: Fitted into pockets on the perimeter edges of the fabric skin, these are tensioned between masts or spars by means of swaged stud tensioners.

•  Rigging/Bottle screws: May be used on fixed masts/spars to achieve suitable pre-stress loads.

•  Membrane plates: These are the corner fixings bolted onto the fabric using a simple clamp plate. They provide for the attachment of the fabric to masts or spars and also take the forces from the adjustable boundary cables.

Technicians must have an understanding of how all the above elements work together to tension and shape the fabric. The combination of the ‘pre-stress' and the double curvature this builds into the fabric skin prevents flapping and the potential for the fabric to invert. If the fabric inverted a balloon shape would be created and the fabric would act like a sail, the potential flapping exerting enormous shock loads onto fixings. When inspecting the structure particular attention must therefore be paid to these fixings and fabric tensions. This knowledge of material and elements must also be applied to careful cleaning of the fabric. UV resistance is often built into materials by using an external lacquer coating. The correct cleaning methods must be carried out when cleaning fabric ‘skins' to ensure that these lacquers are not damaged.

Access work within the field of tensile fabric structures therefore presents itself as a challenge for technicians, encompassing as it does, a wide range of core construction skills; from steel erection in the installation stages to careful maintenance inspections essential to maintain the maximum lifespan. What is more the technician has the opportunity to practise these diverse access techniques on some pretty amazing structures.

Author - Jason Porter