Temporary Works and EuroCodes
I noticed an interesting article in the NCE mag this week (kind of rare I know!) about the relevance of EuroCodes to Temporary Works design and the use of the term ‘Safe Working Load’ (SWLs).
The key issue the author brings up is how do you use a prop with a ‘Safe Working Load’ in a EuroCode compliant design? Was the prop originally designed to EuroCode? Is it ok to assume the ULS capacity is 1.5 times the manufacturers stated SWL?
Having done a few temporary works designs now using just SWL (with no factors at all), as well as with ‘old BS’ verification and new EuroCode verification it certainly is a bit of a muddle. Proprietary equipment specifications can have a myriad of terms which can be confusing, so sticking with Safe Working Loads in my view is essential.
Has anyone come across this problem???
Holdfast Training Services Trial Blog 
It i It is indeed a bit of a muddle
In theory part of the limit state approach is that the partial factors can be used to vary the risk. What I mean by this is that if we take risk as a product of outcome and probability of outcome, then there would be a clear argument that temporary works partial factors on (say) actions being reduced to reflect the fact that the spectrum of actions in the life of a structure would be wider than the spectrum in any temporary state. We would therefore expect greater outliers over a longer period? However, there is a counter-balancing argument that the spectrum peaks in temporary works may be made worse by the possibility of a large ‘unusual’; this associated with some not necessarily foreseen temporary but high load.
What do we actually ‘see’ in the codes?
Well, I was using the scaffolding code BS EN 12811-1:2003 the other day and it has the same gamma factor on permanent and variable actions and it’s 1.5..so I can’t see either:
a) the philosophical logic in this or
b) any reflection on my first point I made above – especially sice the material factors recommended are ‘from BS EN 1993-1’ for steel scaffolding
In BS EN 1991 we have useful ‘eta’ factors These should be really useful in temporary works since they reduce the combination effect of high variable actions by permitting the design to apply a damping to combination variable action – so wind + storage load on a scaffold platform BUT whaddya know there’s no sign of this in the scaffold code ( unless it’s hidden in the action figures….so ..
So the mechanism to be more elegant in design appears to be on offer but is not translated into actualité
The problem then becomes what does a safe working load actually mean? One of the problems with the old permissible state codes was that the factors of safely on different elements ( columns and beams , in the old steel code for example) were not consistent.
Finally we come to a real snake bit. You mentions props…well consider the props used for temporary retaining wall support.
Traditionally wall stability has been calculated by using the limit state (active and passive pressures) We might then compute prop actions and be entirely happy that the prop forces were less than 1.5 x the SWL of the supplied equipment props.
But hold-yer-horses. This is a structural engineer approach to a far more subtle problem. The reason for the propping is almost certainly to arrest deformation. This will imply that the service stresses form soil to wall will be far higher than the ULS states and imply a working load in the propping system far higher than the ULS actions………. I’ve always said that if you understand this you understand limit state theory
It is my observation that the limit state codes tittilated by offering the prospect of closer control upon design, including temporary works design through stochastic methods…but often simply graft on complexity for no returns.
Like in Geotechnical Design, Temporary Works uses Categories of Temporary Works. 0, I, II and III.
For Category 0 and I then simple designs using SWL should be permissible.
For Cat II and III then more in depth analysis should be undertaken using load combinations and ultimate limit states to verify designs.
The question that I think is at the crux of the problem is “Should not structural engineers be able to size proprietary equipment using SWL, for Cat 0 and I temporary works”???
The answer I think should most certainly be yes. This is because proprietary equipment providers account for gamma “I”, whereas EuroCodes don’t.
Gamma “I” is Gamma Ignorance. Proprietary kit has, I believe, a greater than 1.5 factor of safety. This accounts for the ignorance factor. In EuroCodes we don’t apply Gamma I, but perhaps we should!!!