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

[grantr22]

Hello,

 

I have the idea of using three 4.5m lengths of scaf joined with two solid tube joiners (e.g. Flints SCF114) to make a single 13.5m length.

 

I'm exploring the possibility of using such a bar to support a mirrorball, which would be positioned centrally.

 

Due to the logistics of the building this bar could only be supported at either end, by being clamped to existing IWBs which are mounted vertically on the walls at either side. These have recently been load-tested for 100kg.

 

The 13.5m bar & joiners would weigh roughly 25kg, the mirrorball, motor and chains together weigh 7.6kg.

 

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?

 

But overall is using 13.5 metres without extra support along the length a viable option or are the mounting points simply too far apart?

 

Thanks

[Seano]

But overall is using 13.5 metres without extra support along the length a viable option or are the mounting points simply too far apart?

Eek. Gut feeling says yes - way too far - it'll be horribly smelly.

You should prolly be looking at a light duty truss, eg: Astralite.

[grantr22]

Thanks Seano, and I found this which answers all further questions:

 

http://www.litestructures.co.uk/documents/...teLoadChart.PDF

[david.elsbury]

Answer probably dependant on the wall thickness of the tube as well.

[kerry davies]

Seano's gut feeling and my sad experience both come to the same twanging, clattering sound. Even thickwall tube with two couplers flexes enough without a load to make it unwise. What you gain in rigidity is lost by the weight difference with three tubes.

 

Trussing would do it at an expense but I would personally discover creative reasons to strike the mirror-ball. Or maybe reposition it on a stand onstage etc. You would be hard pushed to justify loading that length truss on IWB anyway.

[adamharman]

Agree that truss is the way to go if you absolutely have to have the mirror ball in the middle of the room.

 

Although the load is tiny, the span is huge for a single tube. An informed guess would be that it's borderline on the bending capacity of the tube, probably too much for the couplers, and definately a fail in terms of how much it will bend.

 

Alternative thought which would give a reasonable coverage of the room, how about a mirror ball each side of the room? The extra ball and spots will be a lot cheaper than the truss, either to hire or buy.

[henny]

why not string a lenth of 4mm wire rope across the room, yes it will have some sag in the middle but it will take the weight of a mirror ball

[Brian]

why not string a lenth of 4mm wire rope across the room, yes it will have some sag in the middle but it will take the weight of a mirror ball

Are you sure? Try doing the sums to see what the tension load in the cable is with a 7.6kg load at the centre.

[adamharman]

Brian beat me to it.

The tension in the wire will vary with how much it sags. At 45 degrees you'll have 7.6kg (or nothing if the ball is on the floor by then!), the closer you get to a straight wire the closer the tension gets to infinite. It's also exponential, so anything even close to straight will need a huge tension

The 100kg rating on the fixing points will be for vertical load. Pull-out load will be something different altogether.

[Brian]

I don't have hours to spend on this but a quick BOFP calculation shows as an example...

 

A 7.5kg load in the middle of a 13.5m cable tensioned so that the sag in the middle is 10cm will exert a force in each leg of the cable of 253kg. Each end-of-cable fixing point will carry a shear force of only 3.75kg but a pull out force of nearly the whole 253kg.

 

This would suggest that a rope with a diameter greatly in excess of 4mm is required.

 

If this rope was to be fixed to the existing IWBs then they would need to be retested as to what horizontal load they can take. Their 100kg figure will be for vertically applied loads.

[grantr22]

why not string a lenth of 4mm wire rope across the room, yes it will have some sag in the middle but it will take the weight of a mirror ball

Are you sure? Try doing the sums to see what the tension load in the cable is with a 7.6kg load at the centre.

 

Thanks guys, already done this before and from what people have said here it's probably a better alternative to scaff or truss after all. I like the idea of having a MB either side instead though, think I'll look into that.

 

Given some deliberate sag to reduce tension (the central load being 1.3m below the mounting points) the horizontal load at either side calculates to 20kg, which seems a lot on paper when I could pull this up myself with one arm and minimal assistance (and I'm certainly no body-builder!)

 

A concurrent post has been automatically merged from this point on.

 

This would suggest that a rope with a diameter greatly in excess of 4mm is required.

 

I agree that tightening the rope to keep sag to a minimum would need much thicker wire rope not to mention near bomb-proof mountings for the IWBs. Though the 4mm wire I've previously used has a Minimum Breaking Load of 960kg, so with 40kg tension overall as above this is a considerable safety factor?

[Seano]

Thanks guys, already done this before and from what people have said here it's probably a better alternative to scaff or truss after all.

Yep - if you have the height available a bridle does seem like a good option...

I like the idea of having a MB either side instead though, think I'll look into that.

.. and remains a good option for that. With an MB either side you'd effectively have what is known as an 'H' bridle (for fairly obvious reasons if you think about the shape of the letter 'H').

 

which seems a lot on paper

Yep. And a wee bit more in the real world, because the simple calculation (I think) you're doing neglects the self-weight of the wire rope.

 

Does the wire rope have to be made off to these IWBs you mention? It would prolly be better practice, if its an option to give them their own anchors. Eye-bolts, chem-fix, that kind of thing. Which needn't be hellishly expensive to get done, could that be an option?

[paulears]

Wire rope wise, then top left to bottom right, and then another bottom left to top rightwith a dog, or two at the centre point would seem a workable solution, and be more rigid too?

[grantr22]

Thanks guys, already done this before and from what people have said here it's probably a better alternative to scaff or truss after all.

Yep - if you have the height available a bridle does seem like a good option...

I like the idea of having a MB either side instead though, think I'll look into that.

.. and remains a good option for that. With an MB either side you'd effectively have what is known as an 'H' bridle (for fairly obvious reasons if you think about the shape of the letter 'H').

 

which seems a lot on paper

Yep. And a wee bit more in the real world, because the simple calculation (I think) you're doing neglects the self-weight of the wire rope.

 

Does the wire rope have to be made off to these IWBs you mention? It would prolly be better practice, if its an option to give them their own anchors. Eye-bolts, chem-fix, that kind of thing. Which needn't be hellishly expensive to get done, could that be an option?

 

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.

 

Unfortunately we can't make any improvements to the building (i.e. drill holes in it or anything) mainly because it's listed so this presents a lot of hurdles and red tape.

 

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?

[david.elsbury]

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?