19m Portland Braid

[nod]

Hey Nick

Sorry for the delay in posting, I was away for a week & forgot I'd asked a question of you!

 

I've been trying to get to the bottom of why a 10:1 safety factor is used on rigging ropes. There have been various discussions here about rigging kit compatibility which seems odd when there appears to be little availability of manufacturers data regarding this simple question.

 

Safety Factor - what is it & why/how is it found?

 

There are of course manufacturing standards for all sorts of products & textile slings are very specific in their requirements. Here a factor of 7:1 is appropriate, or 4:1 if combined with a steel component!? All very clear, or not?

 

The only reference I can find at 10:1 is for PPE. I'm not convinced that this is appropriate for a rigging scenario at this point in time. I am however looking forward to being convinced in one way or another, either by manufacturers, who I'd hope have a degree of experience from which to advise us, or from anyone with a relevant train of thought to add to the pot.

 

Many seem to agree that the rope should always be the weakest link in a rigging scenario so that if there is a failure, the trajectory of the log that caused the failure can be predicted to some degree, whereas any other failure point could lead to hardware flying in an unpredictable manner. On this basis the strength of our top anchor sling for the given tensile strength on 16mm double esterlon would need to be in the region of 14500kg. This is taking into account -35% rope strength for knots & -20% for sling configuration.

 

The easy answer that requires little thought is, DON'T OVERLOAD A SYSTEM!

 

So why is there any discussion?

Because failures do happen & therefore there is a need to understand why figures & factors are banded around like some kind of "Knowledge is Power" rap.

 

This information needs to be definitive; preferably from within the treework industry but maybe equally important from manufacturers.

 

I look forward to any discussion towards resolution so rigging can be removed from the arena of 'Dark Arts'.

 

Nod

[nod]

Dons book may be slighty aged now but I was under the impression it was still in line with currect UK safety guidance.

 

What do you work your rigging on Nod ? Maybe I am out of touch, would be interested to hear what you would recommend.

 

 

 

Hey Nick

 

Two points in question:

 

Can you direct me towards the current UK safety guidance as I'm unclear as to what it is or where it comes from?

 

I don't currently have a specific that I 'work my rigging on' as each scenario deserves to be taken on its own merits. If I feel it's appropriate to run a 2:1 SF & bin the rope after use that's what I'll do, sometimes it may be necessary after all nothing lasts for ever.

Until the jury returns any recommendation can only be based on what I think I might know at any point in time.

 

Nod

[ArbjobsNick]

Hi Nod,

I don’t think I am qualified or experienced enough to argue the semantics of rigging factors on the forum. If anyone is interested futher reading on this very specialist subject would be required and I would recommend the following:

 

International Society of Arboricultures book “Arborist Equipment” 2nd Edition

Written by Donald Blair available from the I.S.A.

 

Safety factors, cycles to failure are covered in chapter five “Arborist rope”. In short recommending the application of 10:1 or even 20:1 be adopted for rigging purposes. References for this section of the book include manufacturers and organisations including Sampson Ocean Systems, American National Standards Institute, The Cordage Institute, New England Ropes and Yale Cordage.

 

The 2008 HSE/Forestry Commission’s paper RR668 free to download on HSE website.

“Evaluation of current rigging and dismantling practices used in Arboriculture”

 

Also covers this subject and reference Mr Blairs work on page 174 they also suggest that with our ropes exposed to environmental influences, mechanical distortion, strength loss due to wear and other influences and that larger safety factors must be considered and refer to Blair (1999) recommendation to double the 5:1 design factor so that would take it to 10:1.

 

The Tree Care Industry Association’s

Best Practices for Rigging in Arboriculture 2011 available from TCIA or leading UK Arb retailers.

 

Cover and recommend 10:1 safety factors in relation to rigging ropes on Page 33 under the chapter titled “Rigging System Components”

 

I guess the information and recomendation on "safe practice" is out there you just need to know where to find it.

[nod]

Hey Nick

 

I have been reluctant to reply because the thread appears to become an argument, which serves no purpose for anyone, particularly those trying to make head & tail of the subject.

 

For all of the information that is out there, which you have cited a few examples, there is still a lack of cohesive clarity that bonds recommendations together. The 2008 HSE/Forestry Commission’s. “Evaluation of current rigging and dismantling practices used in Arboriculture” is the single document that has looked into the plethera of work that exists & still cannot come up with a definitive answer. Only that more research is required.

 

I think that if we believe that the word is 'cast in stone' it stops people from looking into the subject matter to the degree that is possible. We know that in misunderstood hands rigging can easily produce fatal errors in judgement. The more in depth we understand the subject the more likelhood there is that good decisions can be made.

 

I looked at the Yale website & they suggest 5:1 safety factors in rigging scenarios!!! Until we can firmly underline the subject who should/can we actually believe when there exist such vast differences in opinion? If, of course, we are turning to others for this knowledge!

 

For my part, the book is open & the pages are blank awaiting definitive answers. For my years of climbing there have been so many times when definitive answers have been proven otherwise that I now understand that there is always flux, the pages are not written with indelible ink any more. But this is how we learn, not by casting in stone!

 

We like to think that we know stuff, it makes us feel good, it makes us seem important. But that which is greater is to not know!

 

Regards

Nod

[Pete Bannister]

Nod, very interesting subject. In other fields of engineering, there are Phd s written on the matter! From an general engineering view, FofS range from around 1.2 to 30 or more. In rigging with fibre line or steel cable 3 to 10 is the usual range. In the case of repetitive cycles, varying environments, and very approximate estimates of forces or where failure cannot be tolerated then the global Fof S needs to be nearer 10 than 3. In broad terms, the more precisely the materials and forces in a system are known the lower the FofS can be.

[combined tree services]

after reading this thread i have come to the decision that i will be needing to replace my month old rigging rope fairly soon , i would have thought that with the forces applied by snatching heavy timber has pushed my safety factor to its limmits if the theory in 10:1 being a true calculation .

 

it is an interesting subject and something i hope continues , it is information and experience as detailed in your previous threads that is not readily available . thanks guys

[ArbjobsNick]

HI Nod

An argument is described as an attempt to persuade someone else of something, by giving reasons or evidence for accepting a particular conclusion. We don’t have to agree but we don’t have to fall out over it either.

 

With all your experience I’m pretty sure you are aware of the industries standard guideline to work on 10:1 safety factor ratios for rigging textiles. I have tried to give someone on here good advice based on what we both know is considered best practice. I can only draw my own conclusions as to why you would wish to debate best practice that has kept many arborist safe.

 

When crossing the road Best practice was taught as “Stop look and listen” of course nobody has to do this it’s your personal choice and you live with the consequences. In asking my friends and peers they all seem to agree that within our industry we have adopted certain safety factors to build in a safety margin / factor when using equipment. Cycles to failure also played a part in defining arboricultural safety factors the thinking being that despite a manufacturers safety factor (See Yale 5:1) so as well as a built in safety margin if as an industry we adopt a higher Safety Factor the more use (Cycles) we will get out of a rope before it fails examples of this are also quoted in the publications I have already recommended for reading.

 

Again nobody is forced anyone to follow industry best practice as you say it isn’t written in stone so if a rigger wants to break kit by overloading it that is of course their choice. Worst case scenario they kill someone in the process or damage property through their actions all they have to do is justify why they chose to ignore best practice. So I guess I am comfortable to quote 10:1 – 5:1 or 7:1 ratios for respective equipment, I may not be right in your opinion but I can live with that.

 

Not everyone may wish to conform to what many of us consider industry best practice but that doesn’t mean it’s a bad position to start from until someone gains a better understanding of the physics involved in rigging and how the equipment reacts to different forces.

 

Nick

[Andy Collins]

I think that two well-respected members of the Arb community discussing such an important issue on here proves the value of this site, things will never move forward without such discussion. Personally I lean toward the 10:1 SF, for no simpler reason than I heard the number, it stuck in my head, simples. As soon as people start bandying other numbers about, my head goes into overload. Confusion is dangerous, and the KISS system works best, and I know that in many arb companies there is usually only one person on the team who is likely to even start to understand the rudimentary theories of rigging!

[Mark Bolam]

I think that two well-respected members of the Arb community discussing such an important issue on here proves the value of this site, things will never move forward without such discussion. Personally I lean toward the 10:1 SF, for no simpler reason than I heard the number, it stuck in my head, simples. As soon as people start bandying other numbers about, my head goes into overload. Confusion is dangerous, and the KISS system works best, and I know that in many arb companies there is usually only one person on the team who is likely to even start to understand the rudimentary theories of rigging!

 

Or sometimes even less!