Sch. 40 for offsetting curtain from truss

ndp

Fun Director
Premium Member
We are finalizing the designs for a large robotics competition held in several weeks at our University's main auditorium/arena. We hang two 22' long black curtains to divide the arena floor in half, essentially. Usually, the curtains are tie-lined to a 50' span of 20.5" box truss. This year, the crew has proposed the following solution to "offset" this curtain from the truss by 3'-5'. They would do a few separate spans of the Sch. 40, with the pipes going perpendicular to the curtain pipe and truss at specific intervals.

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We are building a large truss grid this year with 12" box truss and corner blocks to connect two parallel runs of truss (separated by about 30'), so I'm not concerned about the truss being pulled down one way by the curtains. Just wondering what other folks think of the application here - specifically of the pipe and clamps. The curtain weigh 4.5lbs/ft, which seems like it would be in spec there.

Thanks.
 

soundman

Well-Known Member
I've seen it done before, it is labor intensive but fine. To cut down on the perpendiculer spans you could slightly stagger the parallel pipes.
 
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bobgaggle

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What's the point of offsetting from the truss? No good points on the steel to put the curtain where you want it?
 

brucek

Active Member
You probably want to put an extra wrap on the upstage side span-set to makeup for the upstage twist caused by the extra weight of the pipe & rag.
 

RonHebbard

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Premium Member
You probably want to put an extra wrap on the upstage side span-set to makeup for the upstage twist caused by the extra weight of the pipe & rag.
@brucek In plan view, I believe @ndp is hanging a truss across stage with welded corners at either end turning ninety degrees then running up stage; these latter two trusses ought to keep the cross stage truss from tilting / rotating.
Toodleoo!
Ron Hebbard
 

TimMc

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Adding multiple "levers" and creation of torsion in the truss (what's it's rating for that kind of load?)...

I guess I'm not clear on why the soft goods cannot be tied to the truss.

Exactly this, especially at 5' off the truss. That is a HUGE cantilever. Your moments at the truss are going to get really big. I'd want to see a plan view drawing of the whole setup before you start rigging it. That cantilever could cause some weird forces that will start getting multiplied. Added to that you are not keeping the truss in compression with how you have it rigged is also an issue. Finally, your pipe weighs 1.8# a foot... plus cheeseboroughs... plus the weight of the cantilevers. Thats a not zero amount of weight. If it was a traditional R&R upstage truss with a 1' cantever that was loaded down with pre-rig fixtures that would be one thing... but your not doing that.

What are you trying to solve here that a bridle or something like that could solve instead? Why aren't you just tying to the pipe.
 

bobgaggle

Well-Known Member
guess he didn't like the questions?

I reread the OP and he's got a big rectangle grid that's 30' wide by 50' long. 50' runs are 20.5" truss, 30' runs are 12". And he's not concerned with the weight of his cantilevered rig tipping the whole thing because its balanced by the other 50' run that's 30' on the other side of the pivot. This is so weird, why not put the truss where you want it to be and hang your soft goods from it?
 

egilson1

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This is so weird, why not put the truss where you want it to be and hang your soft goods from it?
I might be able to shed some light one this. Their space has installed rigging points at fixed intervals, and not a lot of options for bridling the points to be able to position the truss as needed for the drape line.

Now, as for what others have stated, on a strait run of truss, cantilevering anything out from the truss does some wonky things to the truss structure. For starters there is the already mentioned roll of the truss. And as stated, the common solution is to offset the truss wrap to counter act the roll. However this does not fix the issue of an unbalanced load on the truss cords, which should be of considerable concern. The load rating of the truss is based on both sides taking equal load, and in our cantilever example we are primarily loading only one side. So when you offset your pick you are now basically turning the box truss into a ladder beam. Point being, you are derating the truss. The engineering solution is to add load to the other side of your pipe to balance the truss.

Does having your truss in a 30'x30' box help? Yes and no. It certainly prevents the truss from rolling, but you still have the same fundamental issue of unequal load distribution. So what do you do? maintain some additional design factor in your hang. Don't load the truss to capacity, but to maybe 50% if the cantilever is heavy. Drape? maybe raise that to 75%. I don't have a specific answer, as I am still talking with the engineers about a common practice for this.

Clear as mud? You're welcome.
 

ndp

Fun Director
Premium Member
guess he didn't like the questions?
Been busy with school and show prep... I haven't been around here for a bit.

This was not my idea and I was always a bit skeptical, hence sourcing community knowledge so I could give the crew that was proposing it feedback. I'm never going to learn any younger, and when I give feedback and make decisions about things like this I like to do my homework. That's why I asked everyone here.

The question about hanging it elsewhere in the house - Ethan hit it on the head (and he knows because he installed our points :p). Specific ratings for the installed point locations, no bridling or additional points we could use.

And in the end, we decided against this idea for a variety of reasons, most of them noted here. We also had a good phone conversation with the engineers at Tomcat, and we got exactly what Ethan said. They don't have specs on torsional limits because it varies from application to application and would need to be analyzed by an engineer case-by-case. The 5' offset didn't matter enough to us, so we are just hanging the drape as we normally do, directly from the truss.

Thanks all for the advice.

-Nick
 

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