TG20:2013 Ladder beams - Low Shear (1 Viewer)

TG6

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If in TG20 they now quote a working shear resistance of only 8.3 kN, what has changed considering that we been using over 14 for many years?
 
They've probably tried to use the lowest figure they could find so all types of ladder beams can be used but I've never seen figures that low; 8.3 kN is very low - we have tested the ladder beams we use and they take over 40.0 kN in shear (with no safety factors applied) so we will continue to use our SWL figures for shear of 20.0 kN. The aluminium ladder beams manufactured by Appollo take over 10.0 kN SWL.
 
I have (in the past) analysed ladder beams as Vierendeel trusses and have been able to justify 14.7 at one rung out from a support based on the tube and weld strengths but wouldn't go above that because I have not got the sort of test experience (and data) that you have. Dropping it to 8.3 is going to give us loads of hassle when designs are checked and someone quotes TG20:2013 as the basis for rejecting a ladder beam in a design. Lucky that most bridges nowadays use Ali beams - pity that there is so much variation in the quoted strength of those!
 
TG20:13 does state that those are the minimum SWL's required for the beams meaning you can still use beams to a higher spec as long as you have manufacturers data to back up your figures.
I agree regarding the ali beams but luckily my company only use 2 types so we just use the lower figures for those.
 
We've always adopted the old SGB data but I know they were fabricated with full penetration welds, 4.4mm wall thickness and correct grade steel - trouble is there are loads of cheap copies on the market which use none of the above but look the part - I've heard rumours of testing which has demonstrated fails at much lower levels than should occur.
 
My view is that although tg20 stipulates the low capacity I think it based on a minimum requirement..but these are prefabricated components and as such these will be used as per the manufacturers specifications.
 
I agree with you OB, the figures quoted in TG20:13 are the minimum requirements for a beam using in a TG20:13 complaint bridged scaffold, it is still fine to use higher figures as specified by the manufacturer. I recently analysed some ladder beams with 3.2mm thick high yield (355N/mm²) tube and they are slightly stronger than the more traditional ladder beams fabricated from S235 4mm thick tube. There was a batch of 3mm low yield ladder beams sold in the UK a couple of years ago which lead to some companies downgrading their stocks.
 
I think your main problem with ladder beams (and most kit to be honest) will be trace-ability
There are so many types, 305 c/c 305 O/A Depth, full saddle welds, spade ends not to mention various crs for the rungs.
When you talk about manufacturers recommendations you may still have issues, not so long back I bought alloy beams from a reputable manufacturer only to find they had subbed out the manufacture of their beams to outside contractors one in the UK, a very reputable supplier and the other in Eastern Block Europe.
Weld tests failed 20% of the UK and 100% of the Eastern Block Beams.

The point being you know not from whence you beams came!!
 
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