Max length of horizontal Alu scaf supported only at either end?

[Brian]

... I'd be interested to know the answer too please :-)

There's far too little information to work it out. You'd need to know exactly how the couplers behaved.

[Seano]

Sorry I omitted to mention, the calculations made do take into account the weight of the rope, clamps, safety bonds, dogs - everything inc. the TRS for the motor too. I put the whole lot onto a weighing scale, it comes to 5.62kg.

But not, I'm guessing, the distributed nature of the self-weight. ie: when you look at the 'depth' of the sag and the length of the wire you're essentially assuming the shape of the wire is a straight line? This assumption is good enough if the self weight of the wire is pretty much negligible compared to the load. (As it is in all but the more extreme cases when you're constructing bridles to hang motors.)

 

Actually the shape of the wire is a catenary curve, and the assumption that its a straight line becomes less good as the weight of the wire (and any cables etc. run along it) becomes a more significant part of the overall load. More exact calculations take you into calculus, differential equations and hyperbolic functions - maybe that is what you've done when you say you did allow for the weight of the cable, but whenever I've tried it it just made my head hurt. :** laughs out loud **:

Harry Donovan's excellent book 'Entertainment Rigging' has tables in it for working out how cable sag will affect bridles. I've never used them, but they certainly look very impressive.

 

None of which means your calculations are wrong as such, but its always good to be aware when one is making assumptions.

 

I've come to the conclusion that this wire rope system is effectively lifting equipment so it would have to be inspected to satisfy LOLER. Is this right?

Yep, prolly. Inspecting a steel wire rope isn't at all difficult, but if you didn't want to do it yourself - for 4mm wire used infrequently it'd almost certainly be more cost effective to replace the wire each time than re-use the old one.

[madorangepanda]

I'd be interested if someone much more knowledgeable than I could address the following question from the OP, I'd be interested to know the answer too please :-)

 

Obviously there would be some sag/flex in a bar this length. Would altering the lengths of the sections reduce the amount of flex e.g. using a 6m in the middle with two 3.75m sections at either side, or two 6m sections at either side and a 1.5m in the middle?

 

Purely from experience, more couplers = more sag. They are an additional weight on top of the weight of the tube. Also, very little data exists on side loading a coupler of this type, it's not really what they are designed for.

 

 

 

As others have said, truss is your best bet, for that kind of load a wire really does need some solid fixing points which are seemingly not possible in your venue.

[Biskit]

I think my main wory with the scaf idea would be that the sag would cause the scaff bars to 'pull' on the joiners and potentially come right out, as one would inevitably be the 'weakest link' and not done up quite as tightly as the other.

 

Regarding the 4mm wire - when a more generous 'sag' is allowed (of the order of 1.5 to 2m) the calculations of 'pull out' force and also wire tension do become much more reasonable (of the order of 20-30kg by my very quick calculations). If this is the venue I strongly suspect it is :** laughs out loud **: , such a sag could easily be allowed without the ball being anywhere near the ground!

 

Though as has been said, technically if the points don't have a stated rating for pull-out force, no-one will 'advise' this method. And yes you're probably correct regarding the LOLER requirements, although I have seen different 'interpretations' of the regs over what is included and what isn't in the grey areas between installed parts of a building structure and actual lifting gear.

 

Edited to add... don't forget to consider the actual installation of whatever solution you come up with, and whether it can be done safely. I can see installing a solid bar involving lots of wobbling on ladders...

 

Ben.

[paulears]

Doing it with the couplers apart from anything else would look ridiculous. Even with just one coupler, the amount of 'wobble' is considerable, and three tubes and two couplers means that even supporting it's own weight, it will look as bent as a banana. At least you could thread a safety rope through the middle in case the couplers crack (joke please, not serious .... but thinking about it? .....)

[Seano]

(joke please, not serious .... but thinking about it? .....)

You could combine the two approaches and make a suspension bridge. :** laughs out loud **:

Someday, when I have a *lot* of time (and a big box of gripples) on my hands I might have to give that a try...

[Giles Favell]

Its worth having a look at this for general information........

 

BS 7905-1 Lifting Equipment (which covers our theatre world) says....

5.5.1 General All anchorage points from which a load is to be suspended, raised or lowered, which are part of the fabric of the building or internal structure, shall have their strength determined and certified by a competent person

 

BS 7906 -1

8.17 Bars ...... Joints in aluminium bars should be avoided.

and table 13 gives a MAXIMUM span between lines of 4m with a continuous bar...

 

It goes on to say..... "Spans between suspensions of 3m or less are generally recommended". Bridles may be used to avoid spans of more than 3m. Specialist advice should be sought when dealing with larger spans or unusually heavy loads."

 

Just out of interest I ran the figures into a structural program, and hanging 5kg in the centre of this 13m Aluminium bar (assuming it was one continuous bar...) it would deflect approximately 215mm - more than 3 times the maximum 65mm that would be allowed (1/200th span).

 

Be very wary of Pull-Out loads on wires strung side-to-side as well - although the mirror ball won't do so very much, if anything should ever snag it, it can rip out surprisingly easily.

 

I trust this may be useful!

 

Happy Christmas!

 

Giles

[grantr22]

Some more good info there, thanks Giles.

[c.cam108]

(joke please, not serious .... but thinking about it? .....)

You could combine the two approaches and make a suspension bridge. B-)

Someday, when I have a *lot* of time (and a big box of gripples) on my hands I might have to give that a try...

I'm reminded of the emergency exit lighting solution for one exit in the town centre here. The span of around 3m is made in steel conduit, with t-boxes connected to the two signs about 600mm from each end. There is a steel rope anchored around 1m above and clamped to the conduit in the centre. A very odd one, but it seems to work (and has done for about 20 years I think).

 

Colin C