Sandbags + Hemp Counterweighted System

[cybergibbons]

Well, we've (well, not me, other people) decided to do a show that requires flying a fair bit of set. This would be a great idea, but...

 

Out entire system is a dead hung hemp rope one. Excepting the house tab which is on a counterweight (which is going to be used for something heavy....). So I need to get some largish sandbags for use as counterweights, or find some very strong people.

 

Flints do very small (4kg) sandbags - I was thinking more of the 20kg - 50kg range. Does anyone know where to get them from, or possibly even borrow/hire in the London area?

 

One other question - what is the accepted practice for trimming the 3 hemp ropes and attaching the sandbag? At school we had some things called clews the took 3 ropes into one, and we attached the counterweight and haul line to that. We also had some trim clamps, which simply held the ropes together and made keeping the load trim easier, but this wouldn't be any use in this application.

 

Would a large master link do the job?

 

Andrew

[robloxley]

You probably need to read some American rigging books for info - I'd recommend the Stage Rigging Handbook by Jay O. Glerum.

 

Other things to consider are the load ratings of your hemp blocks and tie-off cleats and fixings, as they were probably only designed for man-liftable loads.

 

As for sandbags, Flints might be able to make you some larger ones, or get their supplier to.

[cybergibbons]

I'm trying to source that book right now - amazon won't have it for over a month though, which is a shame.

 

I know that our blocks (and grid) are rated much higher than a sensible man liftable load. The strength of the cleats is unknown - mounted onto a timber frame bolted through into a concrete block wall which runs round the fly. The same way 5 manual winches rated to 250kg each are attached to on the opposite side, so I should imagine they are strong.

 

Still, I'm not talking huge loads - we've had to haul heavier things before, but only once during the performance - and in a blackout. To make the set fly effortlessly and fast, we'll need some form of counterweight.

 

It's my understanding that the tie off point will never bear the full load - only the weight which is unbalanced (to the live side to allow the set to drop), in much the same way a rope lock does on a counterbalanced system.

[Kevin Robertson]

Why use sandbags?

Have you considered any other forms of assisted lifts?

The easiest would be a 'billy block'. This is a double purchase assist to the line.

Attach a single sheave pulley block to your hand line using a prussic loop at the grid. Tie one end of an other line to the flyrail, run through the block and haul on the other end. *note* you need to pull the line double the distance of the moving bar.

There are other methods. Look in a rope access book for running assists to enable the full height of the flytower to be used.

Always remember that if your loft blocks are rated to 100kg (for instance) then you can only hang half that weight (50kg) on the bar. e.g 50kg on bar, 50 kg force required to lift load (we will forget about friction for the moment) = 100kg applied to the loft block.

Also, find out what your Alloy bars are rated to, taking into account the number of joints etc. I think you will be surprised at how little they are allowed to lift.

If you are going the way of counterweighting the hemp sets, Why not use stage weights instead of sand bags?

An excellent stage rigging hand book is a book by Chris Higgs who is a member of the blue room. (He has two books and are excellent books for all riggers to have handy)

 

Good luck.

[Chris Higgs]

The billy block will carry twice the load that is applied to the assist line (less the allowance for friction, and assuming the two billy block lines are parallel and vertical).

If you have a 100kg load on the bar, the head block will carry 141 kgs at an angle of 45 degrees to the horizontal towards the stage (the resultant force).

The billy block will not change any of the forces in the installed blocks.

The assist line will carry 50 kgs in each part and as Kevin says, you will need a line length equal to twice the travel height.

The cleat and tie off (once made) will carry 50 kgs each as an uplift force.

The loop used for the prussik knot must be of adequate strength and not damage the hemps.

LOLER applies, of course, so make sure you have someone competent to inspect, test and sign off the installation before you use it.

I have tried to make this as clear as I can, but without drawing a picture, it is difficult.

As always, if you don't quite get it, don't do it.

[cybergibbons]

There are several reasons that I was going to go for sandbags:

 

1. The play is set, for part of it, "backstage" - and the sandbags are required as part of the script anyway. I was going to use them for both roles.

2. We don't own any blocks or pulleys that I'm satisifed are safe for anything but decorative use, so I discounted these as options, thinking that the sandbags would be cheaper, having free access to sand here.

 

Our fly gallery is very close to the grid, and would cause problems if we used a double purchase system unless there was some diversion of ropes going on.

 

Is it actually safe to use a prusik in a system on stage? I use them for climbing, but honestly didn't think that it was an approved method for stage rigging. Surely they are unpredictable at best on hemp ropes?

 

Nonetheless, of all the hoist systems, I feel sandbags are the most appropriate, giving smooth travel for the entire distance of the fly. Stage weights are a possibility - though some sort of mount would need to be fabricated.

 

I'm moving all the ropes around on the grid and using shorter barrel, it's well withing the loading limits over the span.

 

Kevin - when you say rope access, do you mean in the sense of SRT? I can't think of any practical system that's used in SRT that could be used in flying.

 

Chris - does your new book talk about theatrical rigging much? We have a copy of the first one but there isn't much in it on that side of things.

 

Thanks,

 

Andrew

[Kevin Robertson]

The Rope access I was talking about is not really a SRT (Static rope technique), It involves using an ascender on your hauling line in a double purchase assist that enables the line to travel full distance. But if you don't have any blocks that are rated then this would really be out.

I have always considered a prussic loop to be safe on a hemp line, if used correctly.

How are you going to attach your sand bags to your line? If you don't use a prussic you could use an alpine butterfly knot.

Please check if your alloy bars are up to the load. These have recently been found to have a much lower SWL than was originally thought, and they also degrade over time.

[Chris Higgs]

The best book I have come across is the Stage Rigging Handbook by Jay Glerum, credited and referred to in my book (and why neither of mine have much to do with installed theatrical rigging - the information already exists) but it is American and needs a degree of competence to 'translate' into UK units/practice. For example, don't copy the wire rope or wire rope grip stuff, it is not relevant in Europe.

 

Sandbags need safe attachment, ascenders are not appropriate for lifting.

Use the ABTT code of practice 'Flying', the requirements of BS 7905 and you won't go far wrong.

 

Unfortunately, you may find that you are limited to what you can achieve.

But as we all should know by now, the show might not go on (as planned).

[Chris Higgs]

........oh yes, I nearly forgot. One could interpret LOLER as specifically excluding using a prussic knot as the use of knots is not permitted in the make up of a lifting assembly. A bit picky, I know, but if you were the enforcing officer, what would you say?