The worldwide market for temporary enclosures has steadily increased over the last two decades as facility owners, engineers and contractors recognise the safety benefits and cost-effectiveness of installing temporary enclosures. Apart from minimising weather-induced delays to a project, scaffold sheeting enhances site safety by preventing debris and small objects from falling and it reduces the risk of people falling from scaffold platforms. Furthermore, scaffold sheeting improves the working area by reducing wind-chill factor, increasing heat retention and containing dust and pollutants. Environmental impact is thus minimised by improved containment.
When evaluating temporary enclosures, particularly in safety critical installations such as in the marine, offshore and petrochemical sectors, the main considerations are the material’s performance characteristics and Flame Retardancy.
Recognised worldwide, the first technical standard for temporary enclosures is BS 7955:1999, produced by the British Standards Institute (BSI) in consultation with the Health & Safety Executive (HSE), manufacturers and industry representatives including the NASC (National Access and Scaffolding Confederation). One of the members of the BSI working group committee and a major contributor to the development of the Standard was Industrial Textiles and Plastics Ltd, the manufacturers of the market-leading scaffold sheeting product, Powerclad.
Within BS 7955, test methods and minimum tensile strengths of the material, its attachment points and the fixings themselves are specified. The material must have a minimum tensile strength of 630 Newtons in each direction (machine and cross direction), the eyelets should withstand a minimum of 500 Newtons and the fasteners should withstand a minimum of 500 Newtons. Together, these three elements determine the performance of the system and should be referred to when specifying and designing enclosures or when comparing different materials.
Temperature range is also important since cold-crack is an issue at low temperatures. Thickness of material is ambiguous since it depends on where the measurement is taken (coating layer, intersection of the weave, reinforcement band). A better comparison is the weight of material as this provides a measure of polymer content. Sheeting is supplied in different widths to fit tube-and-clamp or modular scaffolds. Modern scaffold sheeting designs incorporate eyelets at regular 10cm intervals on each reinforcement band which provides a very versatile attachment system. Wherever the scaffold intersects a reinforcement band, an eyelet is within 5cm reach. This has a further benefit whereby the support density (the number of fastening ties per square metre of sheeting) can be varied to increase or decrease the resistance to wind forces, much like the bed-of-nails principle. As with any non-conforming product, non-compliant materials inevitably compromise site safety and put operatives at risk. In addition to complying with BS 7955, materials should be produced to ISO 9001 Quality Assurance standards, preferably with batch traceability.
Powerclad scaffold sheeting is designed with additional safety features. Engineers can alter the method of attachment in order to vary the wind resistance of the system. This ensures that in gale force winds, sheeting can be designed to detach to reduce the risk of damage or collapse of the scaffold structure. In windy conditions, an inadequate number of ties will inevitably result in the sheeting detaching prematurely and too many ties may put excessive forces on the scaffold structure leading to its collapse.
In high winds, the sheeting should be regarded as sacrificial in order to maintain the integrity of the scaffold structure. Sheeting begins to detach as soon as the wind forces exceed the breaking strength of the ties and/or the eyelets and/or the sheeting material. Another method to reduce the wind force on the scaffold is the use of an air-permeable material with a
smaller aerodynamic co-efficient. One such product is Powerclad Filter Sheeting, a closely woven material which, in addition to reducing wind forces, has the added advantage of providing much needed ventilation in hot climates and in painting programmes where improved dispersion of solvents is desirable.
Clearly, all structures require individual design interpretation depending upon site location, elevation and shape, the period of installation, the variability of the wind speed factors and whether the sides of the structure are sheeted or open.
Flame Retardant Standards
Powerclad scaffold sheeting is generally available in standard and self-extinguishing Flame Retardant grades. Specifying which Flame Retardant Standards is also essential as small-scale laboratory test are only indicative and are not necessarily representative of full-size installations in the field. A Flame Retardant material is one that self-extinguishes. It does not mean flame proof. Small-scale laboratory tests generally involve removing the flame after a set period and then observing whether or not the material self extinguishes. Different test methods have varying pass or fail criteria such as, char length, flaming molten drips, time to extinguish etc.
In order to replicate full-size installations, two large scale tests were developed by the Loss Prevention Certification Board, Flame Retardant Standards LPS 1215 and LPS 1207. The minimum specification for temporary protective materials outdoors (tarpaulins and scaffold sheeting) is LPS1215 and for indoor applications (temporary screens and enclosures) the minimum specification is LPS 1207. Powerclad FR complies with both.
Compliant scaffold sheeting is printed at regular intervals along the edge of the scaffold sheeting with the manufacturer’s details, compliance with BS 7955 and compliance with each Flame Retardant standard. This enables safety inspectors to quickly identify the material and confirm verification of compliance. Occasionally some products are offered as compliant to BS 8410 or BS 8093. This is in fact misleading. BS8093 has been withdrawn and replaced by BS8410. This standard refers to a method of installation rather than a measure of performance.
Scaffold sheeting should be installed according to the manufacturer’s instructions and to BS8410. Prior to installation, it is essential that a thorough safety review and risk assessment of the scaffold design is undertaken to ensure that the structure will withstand the additional forces caused by sheeting the scaffold. The safety review and risk assessment should be far reaching and include consequences of installing sheeting close to unusual suction forces (such as motorways, bridges, tunnels & railway tracks), to surrounding buildings and areas, the public, pedestrian and vehicular traffic and the consequences of accidental collapse. BS8410 assists with both the risk assessments for scaffold designers and installation for contractors.
Technical brief - FR standards for weather protection & containment enclosures