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Temporary Roofs

The Story

Environmental Considerations

One of the most fundamental aspects of the design of temporary roof and temporary enclosure scaffolds is the calculation of environmental loads on the scaffold. The main considerations for the scaffold design engineer are to ensure sufficient strength, stability and rigidity under wind loading, and either to mitigate or to support the calculated snow loading.

Wind loads in scaffolding design are of course calculated in accordance with the requirements of Eurocode 1 (specifically BS EN 1991-1-4), since the old BS 6399 codes of practice were superseded and withdrawn in March of 2010. There is, however, one complication to this. In an advisory note to the U.K. National Annex of Eurocode EN 1991-1-4, the BSI states:

“Calibration of BS EN 1991-1-4 against BS 6399-2 has shown that there are differences in the values of pressure coefficients and in some cases the EN values are significantly different to those currently used in the UK. National choice is not allowed for the external pressure coefficients. It is therefore recommended that the external pressure coefficients in BS 6399-2 continue to be used to maintain the current levels of safety and economy of construction.”

So only the external pressure coefficients are still calculated using BS 6399, all other aspects of the wind load in scaffold design calculations for temporary roof scaffolds are carried out using the Eurocode. The method for calculating the wind pressure (now called peak velocity pressure, rather than dynamic pressure as before) is substantially different when compared to BS 6399. The underlying principles, however, have not changed. After all, wind is the same physical phenomenon as it always was! The different coefficients and factors do result in different wind loads though, often significantly lower (10-15% based on our own comparisons) than what would have been calculated using the old methods. These reductions seem to be, at least in-part, due to the more in-depth approach of the Eurocode producing a less conservative estimate of wind loading. This less conservative design can of course lead to significant cost savings, due to the resultant reduction in materials and erection time, for those contractors whose scaffold designers are using the latest codes of practice!

Snow loads for scaffolding design calculations are determined entirely by using the Eurocode (in this case BS EN 1991-1-3). The method for calculation of snow loading does not differ greatly from that of the old BS 6399, although the results can vary between the two methods. Regardless of which code is used, snow loading has always presented a potential problem for scaffold designers in temporary roof scaffold design, especially for larger span temporary roofs. As previously mentioned in part 1 of this article, tension bars/cables or pick-up braces are often specified on the scaffold design drawings to increase the snow load bearing capacity of a temporary roof. In many cases though, even these measures are not sufficient (or not practicable) and so the scaffold design engineer must consider implementing snow management / mitigation systems.

A snow management system(s) within the scaffolding design is something that is rarely considered in U.K. compared with Northern Europe and Scandinavia yet it can be very effective at reducing the loads imposed onto the temporary roof. The two most common forms of snow management system are to spread grit salt on the roof at regular intervals or to maintain a temperature above freezing underneath the roof.

Spreading grit is simple and cost effective, however it poses two problems to the design engineer that must be overcome. Firstly, is there a suitable access point(s) from which salt can be spread to achieve full coverage? Secondly, if the temperature drops significantly, will the grit salt be effective in melting snow? The practical implications of spreading salt must be considered at the design stage and a safe means of access provided as necessary. Secondly, the effectiveness would need to be risk assessed by the scaffolding design engineer and an action plan devised for significantly low temperatures.

Maintaining a higher temperature below the roof is the most effective way of mitigating snow build-up that is available to the scaffold design engineer. It poses no access problems and can be engaged as and when required based on weather forecasts and/or site monitoring. The most common way of doing this is to install industrial capacity mobile fan heaters, similar to those used to maintain temperature when fresh concrete is curing. Specifying a minimum temperature of, for example, five degrees celsius would ensure that snowfall on the roof would always melt.

Another factor to be considered in the design of temporary roofs is rain water dispersion. Even in cases where dispersion of water off the sides of the roof is not an issue, the roof itself must still ensure that water always runs-off, and is not allowed to ‘pool’, creating additional unnecessary loads on the temporary roof.

Roofs that use rigid sheeting methods (e.g. corrugated sheeting, plastic panels) do not generally have any problems with water dispersion, provided that the roof is not flat - a minimum pitch angle of 5 degrees is generally specified on scaffold design drawings. Monarflex type sheeting is more likely to have problems with ‘pooling’ and care must be taken in the erection of these roofs to ensure that sheeting is kept taut at all times, with a minimum pitch angle of around 15 degrees being recommended.

Keder type sheeting is easily tensioned, as the end of each sheet forms a pocket through which a ‘tensioning tube’ can be fitted during erection. It is still recommended that roofs with Keder sheeting be installed at a minimum pitch angle of around 10-15 degrees, to allow for the elasticity of the sheeting even when properly tensioned.