Doubling Truss

I've often heard of double stacking truss on top of one another to add more capacity to the system. Usually just attaching two pieces of truss together with cheeseboroughs every so often and rigging from the bottom cord like usual, but is it possible/safe to do the same thing except attaching them together width wise instead of height wise? Does that make sense?

Basically, here's what I'm doing. I have a projector I'm flying and we usually fly it with 20.5" box truss. The projector has mounting points on the bottom that we screw eyebolts into (shouldered and rated of course) and then use spansets and shackles to hang it from just below the truss. The projector rigging points are 24" apart from each other so this gives a little bit of an angle to the spansets and subsequently puts an angle on the force of the eyebolts. I do know it's well within the limits of the eyebolts but I have other reasons for wanting to do this.

I currently don't have any 20.5" box truss but I have loads of 12" laying around. I was thinking maybe if I could "stack" two horizontally it would give me 24" of width which would help with the rigging of the projector. I just don't think that's even feasible since I feel like I'd be putting the cheeseboroughs in a tension scenario.

If I used the 12" truss, the angle on the eyebolts would be 45 degrees. I'm unsure if that is acceptable, and I don't think I'd even feel safe with that based solely on looks.

Thoughts? What would you do? Just find some 20.5"? Cause that's what I'm trying to get my hands on.
 
I'm no rigger so take that for what it's worth. What I would do is get 90 degree double cheese. One goes to the truss cord, the other allows you to hang a piece of pipe perpendicular to the truss. You'll need 2 pipe and 4 cheese. From there you should be able to mount the projector. Just make sure that everything is centered on the truss so the truss doesn't twist.
 
Stacking truss can be done but you really have to tie the pieces together to get the full effect... idealy you would have a cheesborough every foot.

I would also suggest the pipe 90 degrees to the truss route as long as you are looking to just get more points to hang from vs making the truss stronger. Connecting 2 trusses horizontally does not make one truss in the system any stronger, it just makes the system wider.
 
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Would you have enough capacity if you rigged the 12" pieces individually one on each side of the projector? Instead of both points going to the same stick of 20.5" truss you could hang two pieces of 12" and hang half the load from each.
Most truss I have encountered is only engineered to deal with the load vertically so I wouldn't imagine attaching another piece to the side would gain you much in terms of capacity.
 
Perfect, thanks guys! That's essentially what I was thinking. No more capacity but wider. I'm trying to get my hands on some 20.5" truss, but if that doesn't come through I'm going to go with the pipe at a 90degree angle method. That seems to be the simplest solution.
 
I think you are over complicating this. How much does this projector weigh? Considering it does not have a real rigging mount I cannot imagine too much. Could you cheeseborough two pieces of sch40 pipe perpendicular to the truss in order to get your hanging points?
 
Would you have enough capacity if you rigged the 12" pieces individually one on each side of the projector? Instead of both points going to the same stick of 20.5" truss you could hang two pieces of 12" and hang half the load from each.
Most truss I have encountered is only engineered to deal with the load vertically so I wouldn't imagine attaching another piece to the side would gain you much in terms of capacity.
Two trusses next to each other can each support their rated loads. If you can ensure that the loading is distributed evenly between them, you should get exactly twice the capacity of one truss alone.

If you take the same two trusses and arange them vertically and tie them together sufficently along the entire length, you get quite a bit more strength. As much as 4x the bending strength and 1/16 the vertical deflection based on doubling the overall height. A vertical gap between the member trusses can increase the strength even more (essentialy the trusses become the top and bottom chords of a larger truss you build with them). The gain is limited by the quality of the horizontal and vertical shear connection between the member trusses.

The overall strength is also limited by the shear strength of the connection at the supports (each member truss can only provide their individual shear capacities - only bending strength and related deflections are improved by arranging the member trusses to increase the overall height). Typically the shear strength at connections only governs when the span is only a few times the overall depth.
 
If I used the 12" truss, the angle on the eyebolts would be 45 degrees. I'm unsure if that is acceptable, and I don't think I'd even feel safe with that based solely on looks.

It ultimately depends what eyebolts you have and what they're secured into.

Bolts generally have hardly any strength 'side on'. I mean - M10 stud, loaded in a straight line, can usually take half a ton. But you can snap it over your knee.

Some eyebolts - such as Rud pinks, and some collared eyebolts - can be loaded sideways because they're designed to deal with that force. But unless you have an eyebolt you know to be purpose designed for sideways loading, you should not do it. Some people think it's safe to assume that if an eyebolt is rated for a ton, and you're only loading it with 100kg, the fact that the load is sideways is not a problem because 'it's really over-rated anyway'. But this is not at all true. There is no direct correlation between the WLL of something being loaded in the manner it has been designed for, and the WLL of something being loaded in a manner it's not been designed for. I mean if you car can do 200km/h, it's not safe to assume that it can easily do 20km/h up a vertical slope, is it?
 
This is awesome information everyone! Thanks a ton! I swapped out projectors because the original one died. The new projector is smaller and the eyebolts are only 16" apart. Still not 90degrees so we are going with the cheeseborough/pipe idea.

One more quick thing. The pipes we will be using are 2' long. Sch40 1.25" pipe. We will be cheeseborough-ing those to the truss and then hanging the projector with 3' spansets, into shackles, into eyebolts, into projector. Usually when we hang from the truss, we just do a basket over the truss. However, if we do that this time the spansets could potentially slip off the end of the pipe and...well, that wouldn't be good. How would you guys recommend fixing that? Would it be enough to just choke the spansets instead of doing a basket? Could also stick an empty lighting clamp on the ends of each pipe to prevent the slipping, i guess.
 
Do you have one stick of truss with a pipe sticking out of either side and that's why there is the potential for the spansets to fall off? That sounds a bit sketchy to me, but I suppose you could to it safely. I would certainly have something on the end of the pipe to prevent it from sliding off. A big clamp would be an acceptable option in my mind. As far as the spanset goes you could choke it, or another option is to do a wrap around the pipe as you make your basket. The wrap will have a similar amount of friction on the pipe (probably more) to prevent sliding, but doesn't reduce your capacity (provided everything is at right angles and we don't have to consider lateral forces). It also means that the overall length or your spanset connection is a little shorter than you're used to rather than twice as long which may or may not be advantageous in your situation.
 
Out of interest why don't you change the eyebolts for half couplers and then clamp the projector frame straight to the pipe?

Are there half couplers out there with different size threads? The projector rigging uses size M10 if I remember correctly. I'd have to go check. That's a great idea if I could get that. Won't be able to get some in time for this production I don't think. I've got plenty of spansets.

Do you have one stick of truss with a pipe sticking out of either side and that's why there is the potential for the spansets to fall off? That sounds a bit sketchy to me, but I suppose you could to it safely.

That's kind of what I was thinking everyone above was suggesting. Instead of choking, I'm going to wrap the basket. I'd like the projector as close to the truss as possible. I'm definitely going to looking to the half coupler idea de27192 suggested. That will be very handy for the future as we will probably be doing this quite a lot.
 
Are there half couplers out there with different size threads? The projector rigging uses size M10 if I remember correctly. I'd have to go check. That's a great idea if I could get that. Won't be able to get some in time for this production I don't think. I've got plenty of spansets.

Normal Doughty half couplers will accept an M12 or an M10 bolt. It sits captive in the coupler so you can screw it in easily using the coupler as a spanner.

http://www.doughty-usa.com/cgi-bin/trolleyed_public.cgi?action=showprod_T57000

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My projector eyebolts are actually M8. :/ Will those couplers still work, or do I need to find something else?
 

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