Sliding check for scaffold structures

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alltasker

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Fellow designers,

What method do you use to check a ground bearing, free standing, scaffold structure against sliding due to wind loads? The structure may be shrink wrapped.

I would assume that we must start by calculating the vertical "dead load only", or "the out of service" structures weight and then apply a coefficient of static friction to find the Resistance to sliding. Steel on Wet timber is typically taken as 0.2. This gives quite a low sliding Resistance for the structure sliding off the sole boards.

What coefficient of friction would best replicate a sole board sliding across compacted granular fill for example?

Any thoughts would be appreciated.
 
I've always found it to be a touch irrelevant, if there is a concern of slipping then the scaffold has to be anchored to the floor.

You either have two scenarios in my eyes;

The scaffold is erected upon any ground and there is a worry of slipping, now we can assume that the scaffold will be erected upon sole plates which will bear upon timber, The slipping concern between the timber and the base-plates would be higher in my view, due to the area being much smaller, and in previous projects where it has been calculated to be so, than between the timber and the ground it has been erected upon.

Second scenario is where a concern for slipping is raised and the scaffold in then either bolted to the ground using anchors suitable for shear, ground anchors installed at 45, or short standards concreted into ground.

But this is just the way i would see it.
 
For scaffolds situated on concrete we always use the coefficients provided in BS 5975:2008, table 24. Coefficient between galvanised steel (baseplate) and timber (soleboard) = 0.4. The coefficient between the soleboard and concrete is higher at 0.7 so we don't use this unless a higher coefficient is required, in which case you can nail the baseplate to the soleboard. There are also coefficients in there for soil and various other surfaces. We always calculate sliding for free-standing structures that are sheeted. There are a lot of situations where the scaffold cannot be anchored therefore the dead load needs to be increased by adding kentledge for example to ensure the scaffold cannot slide. BS 5975 also recommends a factor of safety of at least 2 against sliding although the NASC document for temporary roofs and buildings, TG9:12, recommends a minimum FOS of 1.3.
 
Thank you for the replies.

Yes Ashreactive, I agree that the slipping between sole plate and sole board would occur before slipping of the sole board across most surfaces and as you say, the sole plates can be nailed to the sole boards. If there is a risk of sliding on a concrete surface then anchors are indeed a suitable and easy solution.

Biffo0911, the version of BS5975 that I have only shows coefficients for steel, aluminium timber (soft and hard - parallel and perpendicular across grain) and concrete (cast and trowelled).

The specific structure i am concerned with is a 7.7m high shrink wrapped tower with plan dims of 6.2m x 8.5m. I have added kentledge to prevent overturning but using 0.4 as a sliding coefficient and 1.3 as Fos, i will need to greatly increase kentledge to prevent sliding. Not for granular fills or soils. I know that there are quick and easy ways to ensure connection between the sole plate and sole boards so i am more concerned with finding a suitable sliding coefficient for the sole boards sliding on the compacted stone fill (MOT type 1). Intuitively i would imagine that the resistance to sliding would be greater than 0.4 due to the rough nature of the stone fill.
 
The latest version of 5975, BS 5975:2008+A1:2011 gives the coefficient of friction for softwood timber to "granular soil" as 0.3 and softwood timber to trowelled face concrete as 1.1. Unfortunately it doesn't have type 1 compacted stone but i would assume it would lie between those 2.
All of the figures are derived from testing at the university of Birmingham, report can be found here:
http://www.hse.gov.uk/research/rrpdf/rr071.pdf
Table 11 on page 20 has the values quoted above.
For small structures of low weight the amount of kentledge required to resist sliding is often greater than that required to prevent overturning.
 
Yes, it's a pity that the research didn't extend to examining friction/sliding across granular fills but having that said I can understand that there could be difficulty arriving at an agreed figure due to the variability in stone type, level of compaction etc. I will use 0.4 for both the "steel on wood" and the "wood on stone fill" friction coefficients

Thanks again for your very detailed answers Biffo0911
 
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