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Topic Title: Cable ladder
Topic Summary: Loading/recommendations of heavy duty cable ladder
Created On: 08 November 2017 12:40 AM
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 11 November 2017 11:20 AM
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basil.wallace

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Corrected as kg/km, Andy
 11 November 2017 07:38 PM
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leckie

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Hmm, well can anyone give me a bit of help with my question my post, like, how do you actually make the calculation? I am assuming that it could be done with the weight of the sum of a metre of cables, add additional required capacity ( say 25%), then add a safety factor, I think OMS said a factor of 3x, and would that be about it?

How to add an additional safety factor arithmetically for a grown man swing, I have no idea - I might need to ask for a clarification on that!

Marvellous isn't it
 11 November 2017 10:00 PM
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mapj1

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so, there is a pair of fixings every 'x' say 2m, usually more to do with the spacing if the girders above. So, 2m of tray/ladder or basket and its fixings weighs perhaps 4 kg, 2m of cables weight 4 kilos per cable, perhaps 5 cables in the bundle so 20kilos of cables per pair of supports, and one Jaymack weights 70kg. So, each pair of hangers where it is impossible for a person to hang on it needs rating for 24kg times safety factor, and each section where you think it possible there may be a body, needs rating for 90kg times safety factor.
This sets the type of suspension you may use or it may make you decide if moving things closer, or further apart, is desirable.
Clamps onto I beams and hangers and so on are usually easy to work out a rating. The big unknown is if you have something like rawlbolts going vertically into a concrete ceiling of uncertain provenance.

-------------------------
regards Mike
 12 November 2017 01:11 PM
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Jaymack

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Originally posted by: mapj1
The big unknown is if you have something like rawlbolts going vertically into a concrete ceiling of uncertain provenance.

Or providence.

Regards
 12 November 2017 02:13 PM
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Fm

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Are you using drop in anchors?
 12 November 2017 11:57 PM
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Fm

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Anchors,10mm screwed rod drops and 41x41mm uni with ladder hold down brackets on zebs and bolts
 13 November 2017 06:35 PM
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OMS

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Originally posted by: leckie

Hmm, well can anyone give me a bit of help with my question my post, like, how do you actually make the calculation? I am assuming that it could be done with the weight of the sum of a metre of cables, add additional required capacity ( say 25%), then add a safety factor, I think OMS said a factor of 3x, and would that be about it?

How to add an additional safety factor arithmetically for a grown man swing, I have no idea - I might need to ask for a clarification on that!

Marvellous isn't it


If we assume that you are putting 4 x 4C x 185mm2 cables on medium duty tray

1 - Decide on the deflection and thus the support spacings

Tray/m = 8Kg, cable/m = 4 x 6Kg - total = 32Kg

Assume something for extra capacity, etc so work on say 40Kg/m

40Kg/m = 400Newtons

Typical medium duty tray with a deflection of span/100 at 400N = 1.5M spacing

Taking the hangers at each end plus mid span these will be exposed to 40Kg x 3 m run = 120Kg and the load imposed on 6 hangers = 120/6 = 20Kg

Add a safety factor of say x 3 = 60Kg per fixing

For information, a 10mm2 threaded road would do about 175Kg (so you have this misuse margin here) normally and about 500Kg to fail

Looking at rawl bolts into reasonable concrete, the pull out of a M10 is about 600Kg - so just above the predicted fail of the rod

It's as easy, or as complicated as that

Most cable suspension systems are significantly over engineered based on industry custom and practice - or significantly under engineered down to cost - can we change the M10 to gripple wire sort of thing

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 13 November 2017 08:38 PM
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leckie

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Ah well, that's interesting, on bigger tray, say 300mm plus, we space at 1500mm on 10mm rod into 10mm anchors, with addional support at all ends, angles, tees of crossovers. So probably OK in most cases. Thanks for that OMS, I will put that calculation methodology onto my contract spreadsheet.

Praise the nerd!
 15 November 2017 05:40 AM
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basil.wallace

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Looks good to me, OMS

I'll definitely will adopt your process for cable ladder.

Basil Wallace PgDip MIET EngTech
 27 November 2017 12:39 PM
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leckie

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Originally posted by: OMS

Originally posted by: leckie



Hmm, well can anyone give me a bit of help with my question my post, like, how do you actually make the calculation? I am assuming that it could be done with the weight of the sum of a metre of cables, add additional required capacity ( say 25%), then add a safety factor, I think OMS said a factor of 3x, and would that be about it?



How to add an additional safety factor arithmetically for a grown man swing, I have no idea - I might need to ask for a clarification on that!



Marvellous isn't it [IMG][/IMG]




If we assume that you are putting 4 x 4C x 185mm2 cables on medium duty tray



1 - Decide on the deflection and thus the support spacings



Tray/m = 8Kg, cable/m = 4 x 6Kg - total = 32Kg



Assume something for extra capacity, etc so work on say 40Kg/m



40Kg/m = 400Newtons



Typical medium duty tray with a deflection of span/100 at 400N = 1.5M spacing



Taking the hangers at each end plus mid span these will be exposed to 40Kg x 3 m run = 120Kg and the load imposed on 6 hangers = 120/6 = 20Kg



Add a safety factor of say x 3 = 60Kg per fixing



For information, a 10mm2 threaded road would do about 175Kg (so you have this misuse margin here) normally and about 500Kg to fail



Looking at rawl bolts into reasonable concrete, the pull out of a M10 is about 600Kg - so just above the predicted fail of the rod



It's as easy, or as complicated as that



Most cable suspension systems are significantly over engineered based on industry custom and practice - or significantly under engineered down to cost - can we change the M10 to gripple wire sort of thing



Regards



OMS


OK, I have had a look through this and just want to make sure I have understood it.

In the example OMS gave he assumed 4 x 185mm 4 core SWA at an assumed weight of 6Kg/m/cable if I am understanding correctly. My information (BATT & Eland) say its 10.3Kg/m - so is that just a look up misread from OMS or am I missing something? So if I am right that would be 4 x 10.3 = 41.2Kg/m.

If we add additional capacity for future cables to be added, say 30%, that would be approx 54Kg. I am not sure if OMS added this to his calculation, it looks like he just added the weight of a length of tray? I am probably getting this wrong though!

So in OMS example for tray/m he says 8Kg, so I assume that is an estimate for a 3m length? I have a figure of 9.2Kg for a 3m length of 300mm swifts tray, so I assume that is what I should allow.

so this gives me 54Kg x 3 for a 3m span plus the weight of the tray itself at 9.2 Kg, so approx 171Kg in total. So if the span is 3m and there is a hanger either end plus one at mid point (1.5m spacing) for mcompliance with deflection requirements, that is a load of 171/6 per fixing - 28.5Kg. So if we add a safety factor of x 3, that equals 28.5 x 3 = approx 86 Kg per fixing.

As my calculation is coming out rather higher than OMS I wondered if I had made a mistake or misunderstood something. I am knocking up a little spreadsheet to check loads for different fixing type so would appreciate any advice.
 27 November 2017 12:51 PM
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OMS

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I was making it up as I went along Leckie - just to give you an idea of methodology

Basically, ascertain your actual and predicted loads, determine the allowable deflection and thus the centres of the fixing points - add in your margins, and decide what you need to hang it

If you put 4 x 185mm2 on a tray (and certainly it'll be bigger than 300mm) you won't have any spare room to add any spare capacity - work out the bending radii - it just about fits on a 600mm2 tray (with a few associated 95mm earths)

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 27 November 2017 01:16 PM
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leckie

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Ah, thanks, not a bad estimate then!

For what ! am looking at today its only a 10mm and a 50mm SWA so a bit of 300mm should just be OK, but its quite close on the bending radius. Cable diameter of the 50mm 4 core is about 32mm x 8 = 256mm, so it just goes.

Thanks
 07 December 2017 10:04 PM
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basil.wallace

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Originally posted by: leckie

Ah well, that's interesting, on bigger tray, say 300mm plus, we space at 1500mm on 10mm rod into 10mm anchors, with addional support at all ends, angles, tees of crossovers. So probably OK in most cases. Thanks for that OMS, I will put that calculation methodology onto my contract spreadsheet.



Praise the nerd!


Leckie, could you email me a copy of your contract spreadsheet. I'm doing a similar exercise. My email address is basilwallace@yahoo.co.uk

Basil Wallace PgDip MIET EngTech
 08 December 2017 08:51 AM
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davezawadi

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I am sure that some of you are thinking that this all sounds a bit complicated, because you are not used to the accessories! As OMS says a safety factor of 3 is suitable for tray installations, and if you use this minimum figure you are well in safe territory. The biggest risk comes from the material you are using as the support, because it is much less well defined than the steelwork. If you have any choice you should use structural steelwork first, then RC structures of significant dimension (floor slab, beams of 300mm size or more etc). Next comes significant sized timber, say 150 mm or more both ways. You do not use sheet timber, plaster board, small planks as all of these cannot provide a fixing which can support proper loads safely, although they may hold up light fittings etc. I would always suggest that support of your installation and a person (70 kg) with a safety factor of 3 is the minimum requirement, unless it a bit of 100mm lightweight tray with a fire alarm cable fixed to it.

Should a structure be overloaded the following happen in order:
1. Over deflection of horizontal members due to excess bending moment.
2. Overstress of the bolts holding the structure together (particularly the 6mm roofing bolts so popular with electricians!)
3. Possible failure of bolts reducing the overall structural integrity, leading to a chain reaction of minor failures as loads are redistributed to already overstressed parts.
4. Failure of the fixings to the support structure, for example breakout of the concrete around rawlbolts, or failure of the bolt "leaves" and consequent loss of support.
5. This is the point at which collapse is likely, again as loads are redistributed of other overstressed fixings etc.

From the above you will see that failure to collapse is quite complex, and it is unlikely to be failure of the added support structure as a whole, but how it is fixed to the building structure. Of course you could cause a bit of 10mm threaded rod to fail in tension, but this takes several tons and so is way past where you should be with loadings. It could be faulty materials but again this is unlikely. Nuts are much stronger than bolts, so any thread failure will be the "bolt" part, and you can beef this up if necessary by using "high tensile" threaded bar (more expensive but much stronger), or M12 or M16 fixings and support rods.

If you want to hang large loads from a building structure you should certainly consult a Structural Engineer, but up to a tonne or two this should not be necessary. Larger jobs always have such a chap available and a phone call will give you much better sleep! Cable tray with large SWA on it is pretty lightly loaded between supports as the cables are very stiff, so do not press on the tray as hard as one might expect along the length, and when you use ladder the cable between supports is stiff enough not to need the tray at all.The largest loading comes from large quantities of thin cables like CAT5 etc.

Manufacturers of all the fixings (and chemical ones are particularly good) give safe loading information, this should be followed to the letter. Back to the bolts, there is no point in not using high tensile bolts everywhere, they are cheap (less than roofing bolts from the electrical wholesaler) and much safer. They are quicker to fit, and less likely to have sharp edges to damage cables. I suggest an electric impact wrench for tightening, you will probably not even need a spanner on the other end. If you want bolts and nuts, I can supply reasonable quantities at the right price give me a ring! All the other accessories (girder clips, clamps, chemical fixings, rawlbolts, unistruct, rods, etc are best bought from a specialist supplier, again overpriced at the electrical wholesaler because of quantities sold and huge range available.

I hope that helps, it is not rocket science, just engineering! Ask away with questions, I do this every day.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 08 December 2017 08:57 AM
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davezawadi

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600 mm2 tray is a new component OMS! This will be heavy return edge tray 600 wide, and you may well have to increase the width at bends because it will reduce the installation cost and effort.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 08 December 2017 09:25 AM
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OMS

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My bad - typo

Typically you can put 4 x 4C x 185mm2 with a separate 95mm2 supplementary earth on 600mm medium duty return flange cable tray - the cables snake from side to side depending on the side the bend points to (snaking is good practice as it allows a degree of movement if you have to contend with seismic or blast considerations

Again typically, 10mm dia rod and unistrut bars at 1.5m centres is adequate for both suspended load and minimises tray deflection to not more than span/100 - typically around 0.4kN/m

More generally, it would be usual to give the structural engineer an area weighted load to make allowances for services - anywhere from 0.2kN/m2 to 0.5kN/m2 depending on the services strategy - and from there, define the point loads within that allowable envelope for specific secondary or tertiary steelwork etc

For example, you could allow say 0.25kN/m2 for the occupied areas, but increase to 0.4kN/m2 for the identified primary services routes and then add the point loads in for say 300kg per fan coil unit

Same sort of strategy for floor loadings in plant rooms, data halls etc and for roof decks where there is external plant proposed

It's not difficult for the building services engineer to provide this information in a format the structural engineer can deal with - or do it directly - the maths isn't difficult once you get to grips with first and second moments of area

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 08 December 2017 12:14 PM
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davezawadi

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Yes OMS I agree with you entirely. It is slightly more difficult for the electrician trying to add more circuits or changing existing circuits in older buildings. We get a lot of this when people want to hang up loads of kit in an existing building, which is where my comments were primarily aimed. If there is any doubt it is easy to add support at closer intervals, and of course one can then use lighter tray. The comment about snaking the cables is also very useful to many, as some think it is untidy rather than useful which it is!

Regards

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
IET » Wiring and the regulations » Cable ladder

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