Out riggers

[rmarchand]

Hello,

 

at present in my venue we have a 32ft x 16 ft sqaure tri-light 10" truss on CN Loadstar winches, hanging roughly 16 ft above the stage . There is a gap of approcimately 1.5metres behind the truss and the back wall (the back of the stage). I am looking to buy "outriggers" that are scaff/truss clamped onto the present truss, which would then support a 16ft scaff bar, to hang drapes/band backdrops (we do a number of SJM productions here) from. Could anyone advise me as to the best value for money suppliers of such equipment, and also if this is the best way of achieving the goal of hanging the drape flush to the back wall at the lowest possible expense.

 

When the NME tour came through our venue they used short scaff bars, similar to scaff tower bars with the ends designed to clamp to other bars (as with a scaff tower). Loadstrapped roughly 6 or 8 along the truss than put two 8ft scaff bars into the u clamps of the scaff bars, and joined the two sections together with a scaff joiner. Is this the best way?

 

Cheers

 

Rob

[Wilf dLampy]

For this kind of thing we sometimes use outriggers (we call them "snap braces" in house), a short, 2/3 foot aluminium scaff tube with a clamp at each end like this-

http://www.scaffolding.co.nz/site/pacificscaff/images/originals/CirclePic2b_83x96.jpg

It then attaches under a top** chord of the truss and lies over the other top** chord so it sticks out (no loadstrapping required*)

 

Once at least two of these are in place a normal aluminium scaff bar can be clipped into the overhanging ends.

 

Obviously things to watch out for include excessive weight being hung from this bar which may either deform the braces or (more likely) unbalance your truss- make sure you know what is being hung on it rather than letting a band or promoter hang their banner with lead weight attachment on it themselves.

 

 

*Under normal circumstances, IMHO, if in doubt ask etc. etc.

 

**edit- just noticed the mention of trilite (rather than square truss), therefore if the truss is apex up, replace the occurrences of top chord with bottom chord :P

[rmarchand]

Cheers Wilf. Could you reccomend any suppliers for the components?

[robloxley]

You might want to check the truss is designed for such off-axis, torsional loads!

[mumbles]

or this kind of thing we sometimes use outriggers (we call them "snap braces" in house), a short, 2/3 foot aluminium scaff tube with a clamp at each end like this-

IPB Image

It then attaches under a top** chord of the truss and lies over the other top** chord so it sticks out (no loadstrapping required*)

 

Once at least two of these are in place a normal aluminium scaff bar can be clipped into the overhanging ends.

 

Obviously things to watch out for include excessive weight being hung from this bar which may either deform the braces or (more likely) unbalance your truss- make sure you know what is being hung on it rather than letting a band or promoter hang their banner with lead weight attachment on it themselves.

Having recently done a similar thing to wilf, this can work depending on the truss for upto about .5m (as wilf said) but not much further, at least not in my experience. Also, 16ft of drape weighs a lot, so the moment about the chord where the bar is attached is substantial, so the force on the chord the bar rests on is much larger. For each kilo of mass applied downwards on the scaff bar that is cantilevered out at 40" (ie about .5m from the back of the truss), there will be 4 kilos of force on the chord at 10". I don't know how much 16ft by 6ft of drape will weigh, but multiply this by four to see the force on the chord, and know see whether this is ok with the point loading limit for the truss chord. This is without taking torsion into account as rob said, so get someone qualified to have a look at it before you do anything, please!

[Peter Tovey]

Actually, given the clamp in question has negligible resistance to rotation, there is no moment about the chord where the bar is attached, only a reaction force. The 2 potential structural failures are due to 1) the force applied to the chord where the bar is attached (which is larger than that applied to the chord the bar rests on, and is applied upwards), or 2) the moment in the jerry-rigged bar at the point where it rests on the chord (where the moment is largest). This is probably the more likely failure given that the type of snap hook we're looking at (such as is often seen on QA towers) is almost invariably attached to very thin-walled alloy tubes designed for use primarily in tension, and which has about as much resistance to bending as a wet poster tube...

 

The torsion induced in the truss structure as a whole might also be an issue. This is quite easily calculated and checked (along with point loads, overall load, etc etc). If OK then the idea above is fine, but make sure you use a sensible tube type and check the moment.

 

Pete

[Pete]

There's some really cool snap baces with a half coupler one end for conection of the drape bar!

[mumbles]

Actually, given the clamp in question has negligible resistance to rotation, there is no moment about the chord where the bar is attached, only a reaction force. The 2 potential structural failures are due to 1) the force applied to the chord where the bar is attached (which is larger than that applied to the chord the bar rests on, and is applied upwards),

Whichever way you look at it the largest force will be downwards on the chord the bar rests on. Using the dimensions I used before, the force downwards on the rested upon chord is 4 times the downward force of the drape. The force upwards on the attached chord is 3 times the applied force by the drape.

 

 

2) the moment in the jerry-rigged bar at the point where it rests on the chord (where the moment is largest). This is probably the more likely failure given that the type of snap hook we're looking at (such as is often seen on QA towers) is almost invariably attached to very thin-walled alloy tubes designed for use primarily in tension, and which has about as much resistance to bending as a wet poster tube...

 

The torsion induced in the truss structure as a whole might also be an issue. This is quite easily calculated and checked (along with point loads, overall load, etc etc). If OK then the idea above is fine, but make sure you use a sensible tube type and check the moment.

 

Pete

Agree with point 2.

 

But with the torsion, depending on the amount of torsion that is applied, the weight loading limits will change. It is as you said an easy enough calculation, but does require a knowledge of how structures work, going as far as second moment of area, in order to understand the stiffness of the tube structures in x and y directions. Many people out there are capable of doing this, but as soon as someone jumps in in a school theatre, and does this as someone on here has said it should work, then...

[paulears]

would it not be better to use some of the truss friendly truss clamps, like the Doughty ones with a swivel to allow 90 degree rotation, and attach 1 to each top chord and sit the lightweight scaff tube into this. This removes the need to really clamp hard the thin wall tube to stop rotation, and puts less stress on the truss.

[Seano]

...and also if this is the best way of achieving the goal of hanging the drape flush to the back wall at the lowest possible expense.

 

I think everyone is so focussed on the cantilevered outrigger idea that this bit of the OP may have been missed. I'd say the answer to that question is probably no. Its a way of achieving that goal when visiting your venue for a one-off gig, but as its your venue you have an opportunity to install something better.

 

I'm not familiar with your venue, but the best option may be a hemp-set. (Not really hemp.) It may be the cheapest option too. (It certainly will be if you try the cantilever option, get it wrong and bend your truss!) This would require a set of suitable pulleys, a couple of hundred feet of suitable rope, a cleat on the back wall and someone who knows what they're doing to install it. The installation might be easy-peasy (and cheap) or quite tricky (and less cheap) depending on the architecture of your venue. Since you're looking to hang close to the back wall, dont forget that wall brackets are an option if the roof is unfriendly.

 

You'd not be messing with the rig, the backdrop could be rigged/derigged independently of the lighting, and you could have it hard up against the back wall (a recipe for disaster with a bar cantilevered off your lighting truss).

 

Sean

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