Difference in Factors of Safety

[top-cat]

I'm interested that I gave pretty valid reasoning in post 2 (everything which is lifted requires double the factor of safety - we just come to that fact in different ways) and everything since has been a mixture of assumption and speculation. Any thoughts on it?

 

If you're not already wearing one David, you should get yourself one of these.

 

I bet you have one in all the different colours.

 

But really... for a thread that should invoke a lot of numbers and standards... there is a lot of "I reckon this is more dangerous than this" which is not really how it works.

[Seano]

I bet you have one in all the different colours.

 

But really... for a thread that should invoke a lot of numbers and standards... there is a lot of "I reckon this is more dangerous than this" which is not really how it works.

 

Ha ha. Nah mate, I don't think everyone is an arse.

 

Actually that kinda sorta is exactly how it works. Chosen by an individual, specified by a committee or written in a standard, like a lot of engineering trade-offs ultimately it is all somewhat arbitrary.

[kerry davies]

Spot on Seano. Sat in our lovely design offices with our coffee and slide rules one of the first things discussed on safety factors was; "Just what sort of dummies will be using this kit? Double it." It really can be as arbitrary as two engineers disagreeing and settling the matter on the toss of a coin.

 

Been there, done that, got a slightly different tee-shirt.

 

A question for those who don't think this is true. When was the last time you saw a safety factor of 11.63:1? If it were anything but arbitrary surely all those even numbers are a bit of a fudge?

[Brian]

It really can be as arbitrary as two engineers disagreeing and settling the matter on the toss of a coin.

 

And once it gets to standards committees writing international standards it gets worse.

 

Your typical committee mix is made up of people who work for manufacturers and people from official bodies. It's not a cheap business being on a committee and the whole effort is effectively subsidised by manufacturers who give their staff time off to travel around Europe and who subsidise their travel and subsistence.

 

Imagine you work for a (say) German manufacturer who has traditionally made things to a German standard which likely has very high safety factors. Someone on the committee from (say) the Far East proposes a lesser standard which will give their lesser designed products a commercial edge. Will your employer allow you to vote in favour of the proposal?

 

Or you are representing a country who has a certain way of doing things. Are you going to vote in a proposal that outlaws that method?

 

Or someone proposes a whole new independent test regime that will cost your employer 1,000s for every product they make.

 

It is indeed very very rare to find a standard which is based purely on good science.

[librarian28]

The consequences of a full lighting bar dropping on a chorus line are, of course, more serious than an aerialist meeting the floor quickly. Though I guess I can see why a committee might feel more twitchy about pontificating on the latter.

A 'live' load isn't live because it is breathing (is it?). We've all hand winched an LX bar down only to notice that the wires are getting slack and then noticing that the far end has snagged on the opposite fly floor, only to then watch helpless as it clears itself and drops 3 feet. A live load is something that could (or does) exert a force greater than its weight (isn't it?). In olden days, we used to call it a snatch load.

 

In a previous thread, Tom suggested that a 75 kg aerialist might drop down a silk and exert a force of 200 kg. This sounds right because, I guess, as well as exerting this 200kg force downwards (against the shackle they are hanging from) there is also some part of their body (hands, shoulders, waist) that is experiencing the same force and I've a sense that human flesh couldn't take much more than that twice nightly.

 

Much of this discussion has been about what safety factor to adopt but, if we did stop tossing coins and came up with (for example) the definitive answer of Kerry's suggestion of 11.63 : 1, I still don't know what that means or what shackle I should buy for a 75 kg aerialist:

Should it be one that breaks at 75 x 11.63 i.e. 872.25 kg?

Or should I get one with a SWL of 75 kg x 11.63 i.e. a SWL of 872.25kg?

Or should Tom's 200 kg be fed into those two equations - giving:

One that breaks at 200 x 11.63 i.e. 2326 kg?

Or one with a SWL of 200 kg x 11.63 i.e. a SWL of 2326kg?

[dbuckley]

And once it gets to standards committees writing international standards it gets worse.

I have had many discussions over the years with people who do this.

 

When it's to do with safety, they often assume that the people implementing whatever the standard requires are retards, and thus need to have chapter and verse giving them no wiggle room for error, or, indeed, common sense. Added to that, the committee often do retarded things, but usually for the right reasons.

 

Despite my reservations and frustrations with the whole thing, everyone I've come across doing this work is doing it with good intentions, trying to make for a better outcome.

[top-cat]

Should it be one that breaks at 75 x 11.63 i.e. 872.25 kg?

Or should I get one with a SWL of 75 kg x 11.63 i.e. a SWL of 872.25kg?

 

Or should Tom's 200 kg be fed into those two equations - giving:

One that breaks at 200 x 11.63 i.e. 2326 kg?

Or one with a SWL of 200 kg x 11.63 i.e. a SWL of 2326kg?

 

I understand your direction and I'm not picky but the WHOLE thing which defines a SAFE working load is the SAFEty factor.

 

The SWL has absolutely no bearing on the total load tolerances of an object in it's own right. It's a useful statistic to have because it gives people a clear understanding of how much weight you can reasonably put on it before it could become dangerous.

 

So a shackle with a SWL of 3.25T based on a 5:1 safety factor, when used in an environment operating a 10:1 safety factor (IE BGV-C1 lifting system, performer flying system), would only have an actual SWL of 1.625T because the safety factor is 10:1 so the increase in safety factor would decrease your SWL but the MBS (minimum breaking strength) would remain constant because that is a mechanical thing.

 

Also the reason that we repeatedly go on about "rigging should be done by riggers" is because you need to factor the potential maximum loads into your equipment choice, and not rely on your safety factor to take up the slack in your knowledge. What I mean by this is that somebody who is a competent with deciding how much force could be put on the lifting equipment needs to choose their ratings around this. The safety factor is NOT there so that you can disregard the extra possible loading safe in the knowledge that there is enough redundancy in the system to take it.

 

So a system designed to generally take 75kg but where dynamic loads of up to 200kg could be predicted as being very possible, you would absolutely design the system based on that, albeit occasional, 200kg loading. The same way that if you install a rigging point for a fall arrest inertia reel, it will hopefully take the weight of one person throughout it's existence -so maybe 100kg. But you still need to install all the rigging based on the predicted loading of a fallen climber, because that is the maximum predicted load that the point might take.

 

If for some strange reason the a point ends up taking a greater load than could have been reasonably predicted, THEN that is where the redundancy from your safety factor is reasonable to come into place.

 

So the correct answer is (3) - one with a SWL of 200kg because this is the greatest force that you can reasonably predict it being subjected to when used in the manner intended. Which with a safety factor of 10:1 would have a minimum breaking strength of 2000kg. It's quite simple really.

[OllieDuff]

1412653431[/url]' post='508657']

1412625862[/url]' post='508638']

Should it be one that breaks at 75 x 11.63 i.e. 872.25 kg?

Or should I get one with a SWL of 75 kg x 11.63 i.e. a SWL of 872.25kg?

 

Or should Tom's 200 kg be fed into those two equations - giving:

One that breaks at 200 x 11.63 i.e. 2326 kg?

Or one with a SWL of 200 kg x 11.63 i.e. a SWL of 2326kg?

 

<snip>

 

So a system designed to generally take 75kg but where dynamic loads of up to 200kg could be predicted as being very possible, you would absolutely design the system based on that, albeit occasional, 200kg loading. The same way that if you install a rigging point for a fall arrest inertia reel, it will hopefully take the weight of one person throughout it's existence -so maybe 100kg. But you still need to install all the rigging based on the predicted loading of a fallen climber, because that is the maximum predicted load that the point might take.

 

So the correct answer is (3) - one with a SWL of 200kg because this is the greatest force that you can reasonably predict it being subjected to when used in the manner intended. Which with a safety factor of 10:1 would have a minimum breaking strength of 2000kg. It's quite simple really.

 

NOW we're getting somewhere. As stated above, and as I alluded to in the OP, where the load is live/dynamic/snatch (according to your preferred nomenclature), one already factors in the maximum foreseeable instantaneous load when specifying the SWL of the system. Therefore, an increased safety factor for dynamic loads isn't justified by the fact that it is dynamic.

The consensus seems to be that at some point, someone sat down and decided that flying a person is more dangerous than flying an equivalent mass of lights (questionably true), and that the way to mitigate the risk is to up the FoS (almost certainly not true). Intellectually unsatisfying, but consistent with my experience of committees in general I suppose...

[Brian]

The consensus seems to be that at some point, someone sat down and decided that flying a person is more dangerous than flying an equivalent mass of lights...

 

I think you'll find that two different committees were given two different tasks, at two different times, in two different meeting rooms, in two different hotels, in two different countries, and came up with two different answers to what is, essentially, the same basic question.

[librarian28]

So the correct answer is (3) - one with a SWL of 200kg because this is the greatest force that you can reasonably predict it being subjected to when used in the manner intended. Which with a safety factor of 10:1 would have a minimum breaking strength of 2000kg. It's quite simple really.

(I don't think you are being picky). But...... it's not that simple for me - in fact it's proper doing my head in.

So, I go to Flints and ask for a shackle with a SWL of 200kg. And they say "Did sir want one that has a SWL based on a safety factor of 5 or 10 or what? (Sir needs to be a bit more specific)". Because they have never asked me that and it's not shown on the shackle's marking. So,I am now thinking that I may have in my collection (it's a fire bucket, actually) two similar shackles both marked with a SWL of 200 kg but one may be half as strong as the other.

 

Referring to the sentence that I've quoted, perhaps I should be predicting the greatest force, multiplying it by 10 and then buying a shackle with a MBS of 2000 kg. Implying that, at least in the rigging world, the term SWL has no useful meaning.

[top-cat]

So the correct answer is (3) - one with a SWL of 200kg because this is the greatest force that you can reasonably predict it being subjected to when used in the manner intended. Which with a safety factor of 10:1 would have a minimum breaking strength of 2000kg. It's quite simple really.

(I don't think you are being picky). But...... it's not that simple for me - in fact it's proper doing my head in.

So, I go to Flints and ask for a shackle with a SWL of 200kg. And they say "Did sir want one that has a SWL based on a safety factor of 5 or 10 or what? (Sir needs to be a bit more specific)". Because they have never asked me that and it's not shown on the shackle's marking. So,I am now thinking that I may have in my collection (it's a fire bucket, actually) two similar shackles both marked with a SWL of 200 kg but one may be half as strong as the other.

 

Referring to the sentence that I've quoted, perhaps I should be predicting the greatest force, multiplying it by 10 and then buying a shackle with a MBS of 2000 kg. Implying that, at least in the rigging world, the term SWL has no useful meaning.

 

 

This is generally because it's a given with industrial lifting equipment to have a safety factor of 5:1. So asking for a 200kg SWL shackle in Flints will likely get you a shackle with a 200kg SWL at 5:1 FoS.

 

Whenever you buy any lifting equipment - from Flints or otherwise - you should be able to ask for a manufacturers datasheet / manufacturers instructions that outline the useful information properly.

 

In any case shackles are probably a bad example, they are very strong for their size and in 90% of rigging applications you end up choosing your shackle because of it's physical dimensions before you do for it's SWL. Arena rigging mainly uses 3.25T and 4.75T... the reason you use a 4.75T for 2 ton points is nothing to do with the increased SWL whatsoever, it's just that a 2T motor hook doesn't fit into the 3.25T shackle.

[ImagineerTom]

Well firstly whenever I'm rigging and presented with 2 "identical" rated shackles I'll pick whichever one looks to be in better condition, followed by whichever one looks to be better made/stronger FWIW you shouldn't generally keep shackles in a metal bucket since there's all sorts of corrosion, minor damage and scrapes that can happen in there - We use the green "really useful box" stacking crates, 30 x 40 x 10cm size.

 

Ultimately there is no "law" about what safety factor you should use in shackles for rigging, just the requirement for "appropriate" decisions to be made by "competent" people as to which standard or ratio to use. When I design / install rigging for a flyer performer I have to include a report / summary of what I've done, what equipment and tollerances I've selected and why the combination I've chosen is appropriate for this particular job. There's times I've installed stuff that's rated well below the 10:1 ratio, other times I've created stuff well above this ratio (typically because I know the performer concerned is actually working on a fancy new trick that involves lifting 3 people up and although it's not in this show I know they will be sneaking in out-of-hours to practice it) so whilst there are hard and fast rules within the "stuff" rigging world (usually based on no more than the guestimate policies of the venue / insurance / rigging engineer) there really is no hard and fast rule about the safety factors in performer rigging simply because the number of variable combinations (with multiplying effects) is infinities and unpredictable to a committee sitting in a boardroom.

[MarkPAman]

Probably not the answer, but, unless you inhabit he world of Douglas Adams, it's not possible to throw yourself at the ground and miss! Whereas if an object is dropped from above, it has at least a chance of not hitting anybody.

[Seano]

I think you'll find that two different committees were given two different tasks, at two different times, in two different meeting rooms, in two different hotels, in two different countries, and came up with two different answers to what is, essentially, the same basic question.

 

More than two. ;)

 

There was a very interesting session on this topic at the Plasa rigging conference yesterday.

 

Abigail Matthews from Momentum Engineering gave a presentation of factors of safety used in various different areas, construction, temporary structures, rigging etc. Followed by a discussion with a ridiculously well qualified panel of structural engineers. There was certainly no shortage of standards cited, European standards, American standards, Australian ones. Standards for construction, tents, fairground rides, temporary structures other than tents, gymnastic equipment, truss... Situations where several different inconsistent standards apply, situations where there isn't any standard that quite fits. All very fuzzy.

 

4:1 was often used in 19th century British ironwork construction apparently, but the origin of the 4:1, 5:1 etc (that are arbitrarirly doubled to 8:1, 10:1 etc) in the Machinery Directive and various standards seems to be lost in time.

 

I think all of the engineers present at some point described moments while designing buildings, structures and wotnot where they chose a standard to work to, or chose a factor of safety to work to in a pretty arbitrary kind of a way. Sensible, conservative, educated guesswork.

 

One thing was very clear though: a 10:1 factor of safety is way way higher than was used to spec the beam, the building or the stage it's hanging from. (Quite likely to be as low as 1.2:1)

 

Oh, and relating back to the OP: hanging people and hanging loads over people, from that PoV, are the exact same thing.

 

(I don't think you are being picky). But...... it's not that simple for me - in fact it's proper doing my head in.

So, I go to Flints and ask for a shackle with a SWL of 200kg. And they say "Did sir want one that has a SWL based on a safety factor of 5 or 10 or what? (Sir needs to be a bit more specific)".

 

Are they really so unhelpful? Their catalogue is wonderfully clear. If you're having that conversation a lot and it's doing your head in maybe you could specify your shackles by their nominal size instead.

[librarian28]

Are they really so unhelpful? Their catalogue is wonderfully clear. If you're having that conversation a lot and it's doing your head in maybe you could specify your shackles by their nominal size instead.

(It was just an imaginary conversation) but I think that such a conversation would have been far from unhelpful. (i.e. helpful). Surely they would be being very helpful to point out that two shackles each marked with a SWL of 75 kg might, in fact, be two different things. And, perhaps, the best info they can give is solely the MBS and invite you to pick your committee and use that suggestion for what weight you can actually hang from it.

 

I fear that it would put the cat among the pigeons good & proper if a punter asked me the SWL of a rig that I'd installed and I said that I didn't know how strong it was but the shackles were well big and chunky.

 

(I am imagining Tom lovingly wrapping each shackle in tissue paper as he places them in his crate - like Conference pears).