[SML] Rigging calculation question...

[Bill Sapsis]

With all due respect to the previous commenters, working with an indeterminate structure is not something that should be taken lightly.  There are some very serious risks with a set up like this and, while I am not suggesting that you abandon the project, I am suggesting you approach it cautiously.

You might want to look up the definition of a statically indeterminate structure.  Wikipedia has something, of course, <https://en.wikipedia.org/wiki/Statically_indeterminate <https://en.wikipedia.org/wiki/Statically_indeterminate>>   and there are other sites.

There’s also a video on this website http://www.eilon-engineering.com/code/stage-safety.html <http://www.eilon-engineering.com/code/stage-safety.html> that discusses indeterminate structures pretty clearly.  Yes, it’s a commercial video put together by Eilon Engineering used to market load cells.  I have a great respect for Eilon Engineering and their products, but the reason for my suggesting this video is to promote the information it contains.

Rigging equipment that is stamped with a Working Load Limit (WLL) already has a Design Factors* (DF)  built in to it.  That DF is typically 5 or better.  Adding a second DF would not be all that helpful and would require the use of equipment that is significantly larger than you want.  i.e.:  Instead of a 5/8” Ø shackle you would end up with a 1”Ø or larger.  It’s a lot heavier and it wouldn’t fit into anything.

I strongly suggest the use of a load monitoring system when dealing with heavy loads and multiple hoist points on a single structure.  Load cells are becoming more and more common and many places (including SRI) have them available for rent.

*(Breaking Strength / DF = WLL where DF is an arbitrary number assigned by the manufacturer) 

Zat help?

Bill S.

bill at sapsis-rigging.com
http://www.sapsis-rigging.com
+1.267.278.4561   mobile
+1.215.228.0888   x206


> On Oct 6, 2016, at 12:44 AM, Kristi R-C via Stagecraft <stagecraft at theatrical.net> wrote:
> 
> This is a great engineer vs mathematician question1
> 
> Yes, we could be a mathematician and figure out the exact weight - and if it's critical to know, load cells would confirm or adjust the answer, but the person asking just needs to be close enough to engineer it to make it work safely. We always use a safety factor to derate the stated capacity of each item in the rig and chain motors only come in certain sizes, so it's fairly easy to assume a constant load (which we know it isn't but for the purpose of our task, it's good enough) and provide an abundant safety factor to cover the fact the weight calculation is not exact. And as you noted, there will be some actual differences in what the motor is carrying vs. what the math says it is because of the slight differences in how each motor runs, so a more generous safety factor, again, covers the inaccuracies in the calculations. 
> 
> If the question was "how much can my roof/grid support?" then a phone call to a structural engineer is certainly in order. I don't think we have one here.
> 
> "Do I need half ton, one ton, or two ton motors if I'm hanging X pounds over Y equal points on Z kind of truss?" is something many of us can do the math to figure out. I'd hate for someone to not ask a question because they are "not qualified." Being smart enough to ask when they are not 100% confident puts them into the qualified category in my book. 
> 
> Kristi R-C
> 
> Bill Conner said:
> Well, not trying to be mean but if you have to ask, you're not qualified to do it.  If the screen is wider than the middle 4 points, then the center two are carrying a majority and the next one each side less, and the long and short nearly nothing. That all assumes the controls are perfect.
> A licensed structural engineer would be a good idea.
> 
> 
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