Safety Factors for scaffold ties

I recently came across CFA Guidance: Anchorage Systems for scaffolding "NASC TG4:11". On page 13 it gives some interesting food for thought.

It states Nylon anchors should be tested to 3 times the working load but the interesting bit is other statement "all anchor types other than Nylon shall be loaded in tension to a load of 2 times the working load.

In addition to this TG20:13, 4.2.3.4; Table 4.4 page 97 from EN 12811 states Cf = Aerodynamic force coefficient 1.30 which is up from 0.7 from TG20:08.

My understanding from this is all sheeted scaffolds since TG20:13 tie loads are to be significantly higher than before. If we are now to add a safety factor of 2 to this then it makes the guidance given earlier on in the code of maximum 16 square meters per tie all most impossible to achieve in practice as the loads to be tested would be more than Heavy duty ties, not to mention the concern for the facade of the building or structure to be able to take the applied horizontal point loads which would in effect work out to be more than suction.

I am waiting to hear from manufacturers such as Apollo to hear what they recommend as I have read some literature that they recommend a minimum safety factor of 1.9.

What do others think or am I going mad?

some manufacturers supply the loads with the safety factor already applied to the SWL and some don't so you need to watch the small print on the data sheet.

the 0.7 sounds more like the temporary works factor to obtain modified qs

Nick-CDS said:

the 0.7 sounds more like the temporary works factor to obtain modified qs

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Thanks for that.

Yes I thought the same until I had a long chat with one chap who explained that now with the Eurocodes we should consider a combined case of suction and pressure where as the old code said we could get away with just suction.

Since then I have been using 1.3 instead of 0.7 which meant larger tie loads for sheeted scaffolds and more kentlidge on temporary roofs.

What factors have you been applying to obtain +Cpe and -Cpe? You'd have to work really hard to get a structure in both tension and compression both at the same time and in the same area.

---------- Post added at 09:23 PM ---------- Previous post was at 09:10 PM ----------

The cf number is more like a correction factor for how wind loading acts on the structure in relation to the shape. I'll have a better explanation when I get to a computer; doing this on my phone which is a bit of a wind so sorry for the short posts.

The 2 x working load on the ties is only for preliminary tests if "there is any doubt about the suitability or recommended load capacity of proposed anchors for a particular base material". If the strength of the base material is known, you don't need to perform this test. This is also only for testing so doesn't make the design tie loads higher. If the scaffold is butted to the wall you don't need to consider pressure (compression) acting on the ties and you can use the "site coefficient" from TG20:13 or EN 12811-1 which reduces the tension loads on the ties considerably (for an impermeable facade).

suction calcs. Whatever next:amazed:

Nick-CDS said:

some manufacturers supply the loads with the safety factor already applied to the SWL and some don't so you need to watch the small print on the data sheet.

the 0.7 sounds more like the temporary works factor to obtain modified qs

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I completely agree and must say I have been using the same modification factor for the last decade or so. My understanding of the reading on page 97 of the code is that it is taken directly from the eurocodes. After digging around in the eurocodes my understanding now is that now we are to consider suction and pressure. I wasn't 100% sure if I understood this correctly but thought worst case scenario would be that I would end up over designing due to higher q figures.

I must say it did baffle me a bit to allow for both suction and pressure at the same time and am starting to regret taking the "computer says no" approach to reading the Eurocodes.

I think that the need to consider suction and pressure has always been there. You don't take them at the same time though. Pressure occurs when the wind is blowing towards the face of the building which the scaffold is fixed to. Suction occurs when the wind is blowing parallel to the scaffold or when it is blowing from the other side of the building to which the scaffold is fixed. You have the suction force coefficient for parallel wind and the pressure force coefficient for wind blowing towards the building in the reference you quoted. The force coefficient for suction needs to be modified by a factor Cs depending upon how porous the building facade is. If it is completely open, it is the same as for pressure (1.3) but if the building is completely closed it drops to about 0.4.
For a building with a closed facade, the tension tie load is nowhere near as big as the compression tie loading.
We don't specify or test the ties for compression but rely on butting tubes against the building so the end result is that tie loads are not as big as you suggest for closed facades.
With open facades the force coefficient will give a larger tie loading but usually the other sides of the building shelter the back face of the facade so the wind speed and dynamic pressure will be lower and the tie loads will be lower.
Does that all make sense?

It's quite complicated this old scaffolding malarkey :worried:

philliosmaximus said:

It's quite complicated this old scaffolding malarkey :worried:

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TG6 said:

I think that the need to consider suction and pressure has always been there. You don't take them at the same time though. Pressure occurs when the wind is blowing towards the face of the building which the scaffold is fixed to. Suction occurs when the wind is blowing parallel to the scaffold or when it is blowing from the other side of the building to which the scaffold is fixed. You have the suction force coefficient for parallel wind and the pressure force coefficient for wind blowing towards the building in the reference you quoted. The force coefficient for suction needs to be modified by a factor Cs depending upon how porous the building facade is. If it is completely open, it is the same as for pressure (1.3) but if the building is completely closed it drops to about 0.4.
For a building with a closed facade, the tension tie load is nowhere near as big as the compression tie loading.
We don't specify or test the ties for compression but rely on butting tubes against the building so the end result is that tie loads are not as big as you suggest for closed facades.
With open facades the force coefficient will give a larger tie loading but usually the other sides of the building shelter the back face of the facade so the wind speed and dynamic pressure will be lower and the tie loads will be lower.
Does that all make sense?

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Thanks for your input. It is exactly the approach I took until the unveiling of TG20:13. It did seem a bit excessive to take into account both suction and pressure together instead of separately.

I think I may have misunderstood the code and as a result may have over designed my wind (Q for sheeted scaffolds) value.

Before I re visit the Eurocodes would you say its still common practice to use the old modification factor?

philliosmaximus said:

It's quite complicated this old scaffolding malarkey :worried:

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Seems to be nowdays Phil. How many scaffolds have you had sucked over lately mate
Time for a meet up Phil. We can discuss the ergonomics of sucking a beer or 2

Q.Silver said:

Before I re visit the Eurocodes would you say its still common practice to use the old modification factor?

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As I am old myself, I think that you are asking the wrong person. I think that as Phil alluded, design of scaffolds has got to the point where the tail wags the dog. From their experience, scaffolders, like all skilled people in the industry, know what will work and what won't without needing to have it all designed or written down. They may come to a point where they aren't sure whether they need beams in pairs or if a single one will do and as a designer, I hope that I can help. There may be other things like balancing cantilevers and similar situations where my input can be useful but mostly I am kept busy producing paperwork for the sake of a box ticker who never comes out of the office.

Back to the point - I learnt structural design around 40 years ago when limit state design was just being introduced and to be honest, most of the the principles from then are just as valid now. Wind loadings have changed as more research has been done and I follow the changes but I am sure that I could get out a copy of CP3 Chapter V and get almost the same result as I now get using BREVe, EN 1991-1-4, TG20 and EN12811. I don't apply factors unless I feel that I understand what I am doing with them and if the end result looks wrong, I will go back and look at how I got there. If you want to learn about which factors to apply when designing for wind loads, going back to BS5973 and CP3 will give you a quick answer based on load factor design. This is a real figure which you can compare with safe capacities of components. If you get a significantly different answer using current codes, you may well have got something wrong.

Remember that limit state was based on load factor design, limit state factors being adjusted to give the same answer to a problem as would have been obtained using a method that had worked well since 1930 or so. Only recently have the factors been fiddled with to make design a bit more efficient and to reflect changes in material technology but what worked then can pretty much be said to work now. Ask an old scaffolder if you want to know what to put in a scaffold design (if you can find one)!!

Nice post TG6. I agree that if someone (especially a practising engineer) doesn't understand the wind code and doesn't know the difference between each of the factors, they should perhaps educate themselves through study, training, advice from experienced scaffolders and engineers or college/university, rather than on a Scaffolders Forum.

biffo0911 said:

Nice post TG6. I agree that if someone (especially a practising engineer) doesn't understand the wind code and doesn't know the difference between each of the factors, they should perhaps educate themselves through study, training, advice from experienced scaffolders and engineers or college/university, rather than on a Scaffolders Forum.

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Scaffolders forum is the best place for scaffolders opinions, as was asked for in the first post. I am sure biffo0911, you are not implying anything that would get a solicitor excited.

Not sure why a solicitor would get excited about some advice on how to calculate the load on a scaffold tie? Unless he is thinking of changing trades?

biffo0911 said:

Not sure why a solicitor would get excited about some advice on how to calculate the load on a scaffold tie? Unless he is thinking of changing trades?

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Nothing wrong with advice and even some criticism but the statement "that if someone (especially a practising engineer) doesn't understand the wind code and doesn't know the difference between each of the factors" is an assumption that I wouldn't want you to substantiate through solicitors.

It also seems to go against rule one of the forum.

"http://www.scaffoldersforum.com/scaffolders-forum-rules/2-scaffolders-forum-rules.html"

But since that is not what was meant no harm has been done. Unless if I am wrong?

with the TG20 being updated since 2010 does the tie loads still apply as suggested above ?