Rigging a Truss Grid

[larkie88]

So, a little background information first. I am a lighting designer at a community theatre. This theatre is unique as it is outdoors in a tent. The space is also set up in a ¾ round. Every show is a new challenge, but most I am able to work through. Except this one.

Our rigging system is set up like a grid. We use 1-1/2” aluminum triangle truss. The whole system is held with 10 – ¼ ton chain motors. I am not able to provide a better picture at this time, but here is how the grid is set up:



Each large segment is 15’, while the shorter segments are only 5’. No truss section is completely continuous. At each joint, there is a connector. I have roughly indicated the location of each motor. Unfortunately, the motors cannot be moved.

Recently, we upgraded our sound system, adding more weight to the grid with the new speakers. Also, we have been using moving lights for some of our shows. My biggest concern is obviously the load limits. With all of this heavy equipment, I am beginning to fear that the current setup will not hold.

I am just trying to determine how to calculate the loads at each pick point. I am an engineer, so the math is not a problem for me. My first approach at this particular problem involved my knowledge of statics, dynamics, and mechanics of materials. Would this be appropriate? Using moment and force diagrams? Or is there an easier way? This seems to be a complex problem since I am having trouble finding information freely available online. What I was originally looking for is a good book on rigging that went into detail the calculations for such a problem. Like I said, the math is not an issue for me, but I just need to know the calculations for the loads. Does anyone have any recommendations?

[rochem]

If you're trying to calculate the structural integrity of your truss, the math is actually not terribly complex. Do you know who manufactured the truss you have? All of the major manufacturers produce a cut sheet for their products that tells you the loading limits for their truss. One such example is this cut sheet from Tomcat: http://www.tomcatglobal.com/files/14/download.html. You just look at a given span length (say 10' for example), and the info will tell you the maximum allowable uniformly distributed load (UDL), maximum load per foot, maximum point loads, maximum deflection, and more such information.

This will give you the maximum load that the truss can support, but also keep in mind the maximum capacity of the chain hoists that are holding it up. If you're less concerned about the structural integrity of the truss and more about the integrity of the structure supporting the hoists, then force diagrams are the way to go. The Stage Rigging Handbook by Jay Glerum goes fairly in depth into the math of rigging, and that would be a great resource to get a strong handle on doing more complex rigging calculations like you're looking at. Another good book to pick up would be Entertainment Rigging by Harry Donovan, although finding a sub-$100 copy of that book can be a challenge. Hope that helps.

[ruinexplorer]

While I love Jay Glerum's book (and have taken his class three times), I would say that Harry Donovan's book is more in line with what you are doing.

[larkie88]

Thanks! I actually have Jay Glerum's book. It doesn't go into more detail about rigging battens, which is what I think I need. I do like that book though. It is an excellent resource. I will look into Donovan's book. Does it get into three moment theorem?

[Footer]

Thanks! I actually have Jay Glerum's book. It doesn't go into more detail about rigging battens, which is what I think I need. I do like that book though. It is an excellent resource. I will look into Donovan's book. Does it get into three moment theorem?

Your going to want Structural Design for the Stage. It is the engineers book for technical direction.

[tprewitt]

...Donovan's book. Does it get into three moment theorem?

Yes.

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