Scaffolding check according to EN 12811-1 (1 Viewer)

Hatter..

I've not used it but I have had the guy from CADS in the office to demonstrate it's capability.

There's no question that its a powerful tool and if doing 2nd order analysis on scaffolding is a requirement you have then it's probably the best on the market...

However, one of the main functions of TG20 is to remove the need for second order analysis by allowing you to analyse the scaffold by consideration of effective lengths and overall capacity checks (for combined axial and bending).

We have considered it and if there was a general requirement over and above TG20 to use second order anaylsis on scaffolding then we'd look at buying it. Until that happens and a justified reason for doing so occurs then we won't get it... afterall, its pretty darn expensive!!

Ben.
 
Hatter..

I've not used it but I have had the guy from CADS in the office to demonstrate it's capability.

There's no question that its a powerful tool and if doing 2nd order analysis on scaffolding is a requirement you have then it's probably the best on the market...

However, one of the main functions of TG20 is to remove the need for second order analysis by allowing you to analyse the scaffold by consideration of effective lengths and overall capacity checks (for combined axial and bending).

We have considered it and if there was a general requirement over and above TG20 to use second order anaylsis on scaffolding then we'd look at buying it. Until that happens and a justified reason for doing so occurs then we won't get it... afterall, its pretty darn expensive!!

Ben.

Ben,

Which method do you use to determine the effective length of a standard? Tables 22 and 23 in TG20:08 Vol 2 contradicts the information (taken from BS 5973:1993) given in Appendix D, particularly that for a scaffold tied at 2m vertical intervals (ie every lift), the effective length ≥ 2.7m rather than 1.0L = 2.0m. I am inclined to follow the information given in Appendix D (and therefore 5973) as it ties in compression member design from BS 5950-1:2000, and would like to understand how the NASC arrived at the values in their tables.
 
Mark,

Tables 22 & 23 are for 'basic' independent scaffolds only. As TG20:08 states that you can have a pavement lift of 2.7m in a basic independent scaffold, I believe this is partly where they have arrived at this figure. The independents are also constructed using transoms on singles only at most bays so you are relying on the strength of the ledger in bending to transfer the buckling load to the adjacent ledger braced frames and tie points to justify a 2.0m effective length. If you can prove that the effective length of the standard is 2.0m, which it very often is in designed scaffolds, you can use the 29.1 kN load for 'as new' tubes from table 20.
 
Mark,

Tables 22 & 23 are for 'basic' independent scaffolds only. As TG20:08 states that you can have a pavement lift of 2.7m in a basic independent scaffold, I believe this is partly where they have arrived at this figure. The independents are also constructed using transoms on singles only at most bays so you are relying on the strength of the ledger in bending to transfer the buckling load to the adjacent ledger braced frames and tie points to justify a 2.0m effective length. If you can prove that the effective length of the standard is 2.0m, which it very often is in designed scaffolds, you can use the 29.1 kN load for 'as new' tubes from table 20.

Cheers Biffo, I should have realised that :embarrest:
 
Mark..

Biffo is spot on. Its very rare that we actually design an independent so in the majority of designs we are actually using the lift height as Le. One point to note, structural transoms should be provided at every ledger braced bay on a sheeted independent and the same on non-sheeted independents above 15m high (see 10.2 page 51). So only the non-ledger braced bays are fixed with singles... and as biffo says, its then down to the ledger to effectively restrain the standard.

You can use App. E reasoning if you like for independents but you just need to make sure you face brace up the inside row of standards and ground all the braces in the first lift.

B.
 
Mark..

Biffo is spot on. Its very rare that we actually design an independent so in the majority of designs we are actually using the lift height as Le. One point to note, structural transoms should be provided at every ledger braced bay on a sheeted independent and the same on non-sheeted independents above 15m high (see 10.2 page 51). So only the non-ledger braced bays are fixed with singles... and as biffo says, its then down to the ledger to effectively restrain the standard.

You can use App. E reasoning if you like for independents but you just need to make sure you face brace up the inside row of standards and ground all the braces in the first lift.

B.

Cheers, I don't do alot of 'street work' independents so I've always specified 'Aberdeen Transoms' at all node points with doubles or band & plates with these often acting as positive ties so my thinking has always been as per 5973 effective lengths. Was just querying as I couldn't find reference to the calculation method used to arrive at the figures in the table but it seems obvious now that Biffo has pointed out the allowance for a pavement lift. Personally prefer working with the engineering formulae rather than tables and tables of figures which often require interpolation to apply. Using excel or similar software makes it a doddle to work out Safe Axial loads on effective lengths, maximum heights, wind loads etc without referring to the tables or graphs by calculation and trends.
 
Hey up chaps,

I would go careful on using 2.0m as the effective length on an independent or similar (checked some designs where engineers have used 2.0m as the Le where it is basically an independent but has a roof on) as this is not the case from reading "Analysis of large proprietary access scaffold structures, M. H. R. Godley and R. G. Beale". I would only tend to use 2.0m as the effective length where there is lateral restraint at the top and bottom of the standard i.e. a birdcage. From reading the report, even when a standard is fixed with load bearing fittings p-delta effects and buckling effects are present due to the adjacent non-ledger braced frames and ties every 4m etc. This makes it extremely difficult to prove a 2m effective length on a independent scaffold. How have you guys proven this before? It would be very interesting to see the calcs.

Hope you have fun reading the report by Godley, it is very heavy!

R
 
Its hard to justify 2.0m Le unless you are bracing each frame of the scaffold in each horizontal direction.. But in most cases, if you ground the braces and arrange your tie pattern in 2.0m vertical intervals you can use 2.7m which gives you 17.85kN. Not many scaffolds exceed this load in the standards...

Beam support standards spring to mind, they have to be fully braced, but generally are anyway. Alternatively you can adopt the approach shown on page 238 of TG20 for freestanding scaffolds.

The thing that gets me most about this issue is that, if it was such a big problem, how come I never heard of a single scaffold failing due to inside standard buckling in all the time scaffolds were designed to 5973??

Tie removal: yes. Overloading: yes. But not specifically standard buckling if none of those two hasn't occurred already.

anyway.. on another subject, what do you think about having to putting a transom on load bearing fittings at every node? I totally understand why, but are people actually doing it?

Ben.
 
Have pondered Le for some time - I'm not totally convinced but reckon there are two conditions. The Le on the ledger braced standards with ties every lift is 2.0m. However, adjacent bay is generally not tied / braced and when they refer to 2.7m le it is this condition - the worst of teh two - they are refering to in TG20. If yoiu tied / braced every bay then Le would be 2.0m all over.

I think.

Also - defo doubles on transoms at ledger braced standards unless tie tube is there.
 
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