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Author Topic: Help! I have my union exam...  (Read 472 times)

Ezra Kire

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Help! I have my union exam...
« on: July 08, 2021, 03:59:21 PM »

HI everyone. I'm a grip in the film industry in NY. My Union exam only comes up once every three years and it's at the end of the month now. I have been studying for years and done a ton of research and but I can't find the answers to these two questions... Maybe someone can help?

1. If a truss has a UDL of 2000 lbs, and there is a 1000 lb load on it, how much weight can be added before it makes out the CPL?

(It's my understanding that the CPL is just under the UDL, but that they are roughly the same. So <1000 lbs. Pass or fail?)

This one is really difficult...

2. How much weight can be cantilevered 8' on a 10' truss?

(I looked everywhere for the answer to this and found something in the Arena Riggers Handbook which said this: "the rule is that you use the CPL of a span that is 4x the span of the cantilever". But that makes little sense. A span 4x greater than 8' would be the CPL of a truss that is 32'. Does that make sense? I will probably take a class in this eventually but for now I could just use the answer.)
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Mac Kerr

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« Reply #1 on: July 08, 2021, 04:07:25 PM »

HI everyone. I'm a grip in the film industry in NY.

Please go to your profile and change the "Name" field to your real first and last name as required by the posting rules displayed in the header at the top of the section, and in the Site Rules and Suggestions in the Forum Announcements section, and on the registration page when you registered.

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Peter Morris

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Re: Help! I have my union exam...
« Reply #2 on: July 09, 2021, 05:00:02 AM »

HI everyone. I'm a grip in the film industry in NY. My Union exam only comes up once every three years and it's at the end of the month now. I have been studying for years and done a ton of research and but I can't find the answers to these two questions... Maybe someone can help?

1. If a truss has a UDL of 2000 lbs, and there is a 1000 lb load on it, how much weight can be added before it makes out the CPL?

(It's my understanding that the CPL is just under the UDL, but that they are roughly the same. So <1000 lbs. Pass or fail?)

This one is really difficult...

2. How much weight can be cantilevered 8' on a 10' truss?

(I looked everywhere for the answer to this and found something in the Arena Riggers Handbook which said this: . But that makes little sense. A span 4x greater than 8' would be the CPL of a truss that is 32'. Does that make sense? I will probably take a class in this eventually but for now I could just use the answer.)

A truss or any beam has a maximum allowable stress.  The stress = bending moment / section modulus of the beam or truss.

The section modulus is a fix value for a particular truss section being bent around a particular axis.

So in all of these cases you can just look at the bending moment equations to see how they are related.

UDL  Moment                   =   (Length of truss x the total distributed load)/8
CPL  Moment                    =   (Length of truss x centre load)/4
Counter Leaver moment    =   Length of truss x load

There is also a maximum load the truss can take no matter show short it is -  the max load will be the same for all of these if its a very short truss. (think 5 feet or less)

If the truss is long  "the rule is that you use the CPL of a span that is 4x the span of the cantilever" is correct.

... and the CPL is half the UDL - so the answer is 500lbs in your case. (technically its the sum of the moments i.e. 2000 lb UDL moment = 1000 lb UDL Moment +  500 lb CLP moment)

you can see how some of this works in these tables https://www.xsftruss.com/load-table-and-ratings-guide/#center-point-load
« Last Edit: July 09, 2021, 05:18:33 AM by Peter Morris »
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Ezra Kire

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Re: Help! I have my union exam...
« Reply #3 on: July 09, 2021, 10:31:36 AM »

A truss or any beam has a maximum allowable stress.  The stress = bending moment / section modulus of the beam or truss.

The section modulus is a fix value for a particular truss section being bent around a particular axis.

So in all of these cases you can just look at the bending moment equations to see how they are related.

UDL  Moment                   =   (Length of truss x the total distributed load)/8
CPL  Moment                    =   (Length of truss x centre load)/4
Counter Leaver moment    =   Length of truss x load

There is also a maximum load the truss can take no matter show short it is -  the max load will be the same for all of these if its a very short truss. (think 5 feet or less)

If the truss is long  "the rule is that you use the CPL of a span that is 4x the span of the cantilever" is correct.

... and the CPL is half the UDL - so the answer is 500lbs in your case. (technically its the sum of the moments i.e. 2000 lb UDL moment = 1000 lb UDL Moment +  500 lb CLP moment)

you can see how some of this works in these tables https://www.xsftruss.com/load-table-and-ratings-guide/#center-point-load

Thank you so much, Peter, for your response. It's much appreciated. Having no formal rigging or engineering training (just on-the-job learning) I had to watch a few videos to understand more of what you are saying. The chart you sent is very helpful and I can see the relationship of Length to Load. By the time you get to a 50' span the CPL is just above half the UDL.

So it's now my understanding that if the max cantilever load is equal to the CPL of a span that is 4x the length of the cantilever (8') then I would have to look up the CPL of a span that is (4x8=) 32'? Interpolating from the chart you sent, the CPL of a truss at 32' is about 1600 lbs, so then the (Length of truss x center load)/4 would be: (10x1600)/4 =4000 lbs? seems excessive, no?  Or would it simply be 1/4 of the Center Point Load (400 lbs)? which seems to make more sense.

Thanks for your time! This is very interesting stuff
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ProSoundWeb Community

Re: Help! I have my union exam...
« Reply #3 on: July 09, 2021, 10:31:36 AM »


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