Tie Testing (1 Viewer)

IDH

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Does anybody have any thoughts on this? Seems to be widely ignored in the industry the approach given in TG4:11. We have always typically given the 'proof load' required when testing but this has got nothing to do with the design load required?

From what I can gather the general approach taken by contractors is if they achieve the 'proof' load then all is good? My concern is though, if no other testing is undertaken we are not confident if when you achieve the 'prppf' load you are operating at a FofS of 1.01 or 3.0?

We've put a summary of the testing on our downloads section - thoughts welcome.

Blog - Preliminary & Proof Tie testing for scaffolds / Scaffold, Formwork & Temporary Works Design | Independent Design House
 
IDH, not sure what you mean by "this has got nothing to do with the design load required"? The proof test should be 1.25 x the working (design) load meaning you will always have a factor of safety of at least 1.25. For the preliminary test this would be 2 or 3 x the working (design) load, giving a factor of safety of 2 or 3.
 
Yes I know - that is what we have summarised.

My point is, if no formal Preliminary Test is undertaken, which is often the case, then relying on the 'Proof' test could result in achieving a maximum FofS of 1.25 and nowhere near the 2 / 3 required.

When we identify an anchor load of 5 kN it means we should be identifying a proof load of 6.25kN for QC but actually the contractor should be able to demonstrate 10 kN (or 15 if Nylon sleeves) without movement. This just doesn't seem to be happening.
 
It may be worth editing the blog to include the 1.25 factor for proof load testing as you have quoted the failure test load factors.
Having said that you are probably asking the question in the wrong place and a poll in the general forum would go some way towards an answer to your question.
The problem that we suffer from in the industry is that we are very frequently presented with an unsuitable material and very variable for tying to and testing to failure has three critical factors which potentially preclude the use of a sensible number of ties (from a commercial point of view).
If you are fixing into a brick you have the weakness of the brick itself added to the weakness of the bond of the brick to other bricks added to the weakness of the bond/tie to the rest of the structure. It is not possible to sensibly test the last of these and the second can be misrepresented by poor testing so it is probable that any testing, proof or failure, will give optimistic figures. There is a lot of pressure to get satisfactory test results to be able to erect and tie the scaffold so the tests are probably manipulated to achieve the right answer rather than being properly evaluated.
If you were to analyse the ability of a typical brick wall in a big house to carry lateral loads from wind, it will probably fail on upper storeys. If you want to fix another 5m height of sheeted scaffold to it to carry a temporary roof, the increase in load only makes it much more certain to fail. The fact that you have properly tested ties to establish a low but adequately factored working load and installed a suitably high number of them does not mean that the whole lot may not fall down.
It is certainly a valid point to question the understanding of tie test loads at site level but if you think too much about it, you will probably be able to convince yourself that a worryingly high number of scaffolds cannot be built to acceptable safety factors.
The next question is 'do you accept what you can't control as being a roughly known and reasonable risk or do you try to control the risk down to unrealistically low levels?'
The answer from an engineer to that question may not be the same as from an insurer or a lawyer and in court you can bet that it will be very black & white.
 
It may be worth editing the blog to include the 1.25 factor for proof load testing as you have quoted the failure test load factors.
Having said that you are probably asking the question in the wrong place and a poll in the general forum would go some way towards an answer to your question.
The problem that we suffer from in the industry is that we are very frequently presented with an unsuitable material and very variable for tying to and testing to failure has three critical factors which potentially preclude the use of a sensible number of ties (from a commercial point of view).
If you are fixing into a brick you have the weakness of the brick itself added to the weakness of the bond of the brick to other bricks added to the weakness of the bond/tie to the rest of the structure. It is not possible to sensibly test the last of these and the second can be misrepresented by poor testing so it is probable that any testing, proof or failure, will give optimistic figures. There is a lot of pressure to get satisfactory test results to be able to erect and tie the scaffold so the tests are probably manipulated to achieve the right answer rather than being properly evaluated.
If you were to analyse the ability of a typical brick wall in a big house to carry lateral loads from wind, it will probably fail on upper storeys. If you want to fix another 5m height of sheeted scaffold to it to carry a temporary roof, the increase in load only makes it much more certain to fail. The fact that you have properly tested ties to establish a low but adequately factored working load and installed a suitably high number of them does not mean that the whole lot may not fall down.
It is certainly a valid point to question the understanding of tie test loads at site level but if you think too much about it, you will probably be able to convince yourself that a worryingly high number of scaffolds cannot be built to acceptable safety factors.
The next question is 'do you accept what you can't control as being a roughly known and reasonable risk or do you try to control the risk down to unrealistically low levels?'
The answer from an engineer to that question may not be the same as from an insurer or a lawyer and in court you can bet that it will be very black & white.

A point well stated TG6,
the tests performed are local and not global, if half the test loads were applied to the building structure as they should there is a massive chance that walls would be laying down everywhere.
For that reason the foundation clause was critical to quotes and drawings alike.
It is the responsibility of the customer to ensure that the foundations provided, whatever and/or building are capable of withstanding all extra imposed loads resulting from our structure in use.
 
That's good point TG6. A question a few SE's have asked me is "How do you know all these ties won't pull the facade (or a particular wall panel) down?". This question pops up regardless of how many tie tests are done; local test Vs global loading. Not always an easy answer, especially on historic or listed fabrics where tie points are limited.

---------- Post added at 12:49 PM ---------- Previous post was at 12:48 PM ----------

Alan beat me to it. ********.
 
That's good point TG6. A question a few SE's have asked me is "How do you know all these ties won't pull the facade (or a particular wall panel) down?". This question pops up regardless of how many tie tests are done; local test Vs global loading. Not always an easy answer, especially on historic or listed fabrics where tie points are limited.

---------- Post added at 12:49 PM ---------- Previous post was at 12:48 PM ----------

Alan beat me to it. ********.


Don't worry Nick, it's not a competition.........
but if it was I'd have won :)
 
Some good info above. Just to throw another spanner in the works, do you agree that the factor of safety for the proof tests should be at least 1.65 to make it comparable with everything else on the scaffold, rather than 1.25?
 
Some good info above. Just to throw another spanner in the works, do you agree that the factor of safety for the proof tests should be at least 1.65 to make it comparable with everything else on the scaffold, rather than 1.25?

Nope. The factors and testing frequency already increase should any fail at the 25% extra mark. If we up the initial proof load, we'll have to up the rest.
 
Some good info above. Just to throw another spanner in the works, do you agree that the factor of safety for the proof tests should be at least 1.65 to make it comparable with everything else on the scaffold, rather than 1.25?

Double no! Proof loading proves that should the actual load get to the design load (serviceability limit state for the youngsters), the tie will not come out of the wall. It can't really be much bigger than 25% over the design load or else you run the risk of damaging the fixing so that it could fail at less than the service loading. Proof loading is an old established method of checking things without breaking them - if a floor slab is suspected to have been under reinforced for instance, you can proof load it and if it doesn't collapse, it passes. The army had an important role to play in testing bridges that way in the past - apparently the ranks are expendable enough to be used as easily obtained kentledge - if you need more weight get them to bring their tanks along! I think that we may have gone away from that approach nowadays and use more sophisticated methods and monitoring to avoid accidentally killing people.
It's interesting to think about rule of thumb percentages like 25% for proof load and 70% for failure loads, 10% for continuity, 25% for wind load stresses etc. Despite the advances in analysis and the development of modern codes, I think that you can still apply them and get an economical and 'safe' design.
 
Haha. Some good replies above and I agree - 25% over the design load should never occur in practice if we have got our calculations right so there is no need to go any higher. Obviously in some safety critical installations or where there is a distinct chance that the anchor may be overloaded the designer may specify a higher factor of safety for the test/design load.
 
Haha. Some good replies above and I agree - 25% over the design load should never occur in practice if we have got our calculations right so there is no need to go any higher. Obviously in some safety critical installations or where there is a distinct chance that the anchor may be overloaded the designer may specify a higher factor of safety for the test/design load.

as you say when we come to "critical installations" I would be inclined to install additional anchors before over testing anchors which may leave them in a weakened condition.
I may be inclined to do a random testing of "test anchors" (not to be used) and crank up the number to get a feel for what the local elements of the structure will withstand and go from there.
 
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