UDL<CPL<QPL

[motorcrazy]

Hello motor and rigging freaks, and yes I have had a few coffee's.

 

Just wanted to see what you guy's out there did for calculating UDL's and CPL's.

 

IF YOU DON"T UNDERSTAND THIS DON"T EVEN THINK ABOUT USING IT.

 

I was going to put the equations for what I use for UDL,CPL.

But the thought of someone using these with out understanding them perfectly is scarey.

 

If you want just email me.

 

later

 

Rick

 

I am curious has any of you ever walked away from a gig becuase of concern for equipment or the building was sketchy?

[Ike]

I'm not certain I'm sure what your actually asking! While a certain amount of caution is a good thing when it comes to posting information such as this on a public forum a simple disclaimer (similar to the one you posted) would do fine. At the end of the day the forums value comes from people sharing information publicly.

 

Don't a lot of manufacturers supply data for quarter (and even fifth) point loading as well as centre and universally distributed loadings?

 

I am curious has any of you ever walked away from a gig becuase of concern for equipment or the building was sketchy?

Yes but not a particularly interesting story. Went to venue, didn't like the look of the mother grid, didn't believe the marked SWL, paperwork wasn't in order, went home. The venue has since had a complete refit.

 

Some of Europe's most respected riggers post on here and a few more lurk, I'm sure if you post a specific question they will help when they can. You never know, depending on what truss you use you could well get a reply from the designer!

[Roderick]

Maybe you should consider posting here before you had some coffee's, they don't seem to agree with you :huh:

 

If you are not going to post anything that may start an interesting discussion or don't want to share your information, just don't post - not too hard to understand?

If you are trying to accumulate email addresses for your new business, just say so.

 

I am curious has any of you ever walked away from a gig becuase of concern for equipment or the building was sketchy?

 

No, I have never walked away from a gig with dodgy rigging details, but I have often insisted that rigging plans were re-calculated and adjusted the rig accordingly if I felt it didn't add up.

[motorcrazy]

Wow this room is a tough crowd, I was trying to say When I took rigging training I was given a formula for doing UDL and CPL's. I was talking to another rigger and he gave me a different formula for doing Load calculations.

 

 

 

Later

 

Rick

[mumbles]

AND UNLESS YOU HAVE RIGGING TRAINING, DON't EVEN THINK OF USING THIS............

And so realising that this wouldn't exactly be sensible information to make available to untrained people, you posted it on a public forum because.

 

You have to divide by 2 to add half of the total weight of the truss to each motor.

Well, only sort of, as I'm sure you're aware, though others may not be. The above is correct, assuming the truss is hung symettrically on the motors, so in many cases wouldn't be correct.

 

 

But as my main argument,

R1 = P1b1 + P2b2 + P3b3 + etc....

R1,R2 = are the loads on the motors

No.

 

The load multiplied by the lever arm would the moment, however as the motor is effectively a pin joint, this moment is 0.

 

So, what I think you mean is:

 

M about R1 = 0

 

so,

 

0 = P1a1 + P2a2 + etc + wLL'/2 - R2L'

 

Where P1 and P2 are the loads, with positive as acting downwards.

 

a1 and a2 are the distances from motor 1.

 

L is the length of the truss, and L' the distance between the motors.

 

W is the weight of the truss per metre

 

And R2 is taken with acting upwards as positive.

 

This means;

 

R2 = (P1a1 + P2a2 + etc +wLL'/2)/L'

 

And then that R1 = P1 + P2 + etc wL - R2

 

Hope you follow.

 

Edit purely to say that I am in no way qualified as a rigger, but am on a Civ Eng course, hence noticed the discrepancy. If you're reading this and you don't know what you're doing, please seek professional help before relying on the sayso of someone behind an IP address, possibly a hell of a long way away.

[motorcrazy]

O.k

 

 

If your doing a truss line of 40 or longer all the motors are in the same line, the motors are hung upside down along the trust. The chain end goes to the stingers or beam clamps. When moving the truss to working hieght, you bump each motor so they are inline with each other. You move the truss to about 3 ft in the air, Once at working height you add your cabling and lights.

 

 

In the formula The p1 and p2 are the weights, that formula as it is written is for 2 weights along the truss. And the formula came from Tomcat. So send your complaints to them, copied from my cheatsheet's.

 

Sound like you lady's are scared that a friendly canadian is smarter then you........that was a joke...................

As per showing off, I have said it before that I am trying to talk shop with any rigger that might be on here. Sometimes I like to pick peoples brains.

 

PLASE READ ALL THE FORMS BEFORE REPLYING.

 

O and gareth, I have posted a question and hopefully you can find some time with all that typing you like to do.

[mumbles]

As I said, I am not trained as a riger, so if Tomcat have told you something, I wouldn't dream of correcting it.

 

However, whichever way you look at it, the formula you wrote give R1 units in kNm, which means that in terms of a reaction it is wrong. Wither there should be some cunningly placed brackets to change how this works (though I still think the answer would come out as kNm), or there are some characters missing.

 

Hopefully someone else can come along and give an opinion.

[Ike]

As mumbles says that formula is incorrect. As you have now removed it from the original post its hard to say where it got misquoted.

 

I take it the Tomcat notes aren't online anywhere for us to see for ourselves?

[ed_]

Yes and no:

 

AND UNLESS YOU HAVE RIGGING TRAINING, DON't EVEN THINK OF USING THIS............

And so realising that this wouldn't exactly be sensible information to make available to untrained people, you posted it on a public forum because.

 

First point: unless you understand all the issues involved, only one of which is being addressed here, don't engage in this activity. Fair enough.

 

Second point: don't make information available to untrained people? No way. It is the untrained people that, by definition, need as much information as they can get!

 

Yes - be aware of your limitations - but let's not hoard knowledge in the belief that doing so somehow discourages muppetry.

 

In any case, am I alone in noticing a discontinuity between mumbles' explanation of how to calculate the load on each motor on a straight two-point truss (let's ignore situations with more points) and the OP which seems to allude to a method of working out the allowable CPL of any truss given the UDL? Assuming that working up from the mechanical properties of the truss is not an option, I would find any such method rather suspect.

 

Short lengths of larger truss can to have their CPL limited not due to the bending moment of the truss being the limiting factor but due to a point load exceeding the maximum shear capacity of the truss. Does the formula you propose take account of this? If so surely it would require a knowledge of this value for any truss the formula was to be used for, a value which would require reference to the engineering report of the truss - where the loading chart showing UDL, CPL, TPL, QPL etc. for various lengths of truss would be found and render such a formula wasted effort...

[mumbles]

Second point: don't make information available to untrained people? No way. It is the untrained people that, by definition, need as much information as they can get!

 

Yes - be aware of your limitations - but let's not hoard knowledge in the belief that doing so somehow discourages muppetry.

You seem to have missed my point. I agree, and over the last 10 months, I have learn't alot from other members on here, but something as critical as this, I think word of mouth over an internet forum shouldn't be used as a method of teaching. If you haven't been taught, learning from certified trainers, is the only really suitable route. I mean, if little Johny Smith came and read something written on here, and then assumed from reading this, he would be able to hang a large truss, then did so for an event he was working on, would you be happy standing underneath it.

 

In any case, am I alone in noticing a discontinuity between mumbles' explanation of how to calculate the load on each motor on a straight two-point truss (let's ignore situations with more points) and the OP which seems to allude to a method of working out the allowable CPL of any truss given the UDL? Assuming that working up from the mechanical properties of the truss is not an option, I would find any such method rather suspect.

 

Short lengths of larger truss can to have their CPL limited not due to the bending moment of the truss being the limiting factor but due to a point load exceeding the maximum shear capacity of the truss. Does the formula you propose take account of this? If so surely it would require a knowledge of this value for any truss the formula was to be used for, a value which would require reference to the engineering report of the truss - where the loading chart showing UDL, CPL, TPL, QPL etc. for various lengths of truss would be found and render such a formula wasted effort...

No the formula wouldn't get you anywhere near this, but the OPs was no closer.

 

Anyhow, why would you want to calculate the maximum permissible CPL, UDL etc when as you say they are available in tables. This can only be worked out from the properties of the truss (again as you say), and as such the formulas are only to use to establish the reactions/loads in the chains, from the loads that are going to be applied ot the truss.

 

With regards to examples with more points, half one this morning was no time to start thinking about applying virtual work and the such to a truss system. :P

[ed_]

Mumbles - I think you've missed my point!

 

I do disagree with you on how knowledge should be made available. Of course we must all be wary of posts here that say something like 'is XYZ truss suitable for a 12m span?'. Reasoning: this is nowhere near enough information to make an informed decision, and someone may read 'yes you can use a 12m span of XYZ truss to fly some pinspots' and then use it as a 12m spreader truss for a 2 ton centre point. Those are situations where we should be very careful what we say.

 

However, most issues that we discuss on this forum are those which will have catastrophic effects if not treated with respect. Should we then close the rigging forum and the electrical forum? Of course not. We need to be careful, we need to qualify our statements appropriately, but let's not allow the presence of muppets on the Internet hamper our discussions. If either of us really believed that, surely we wouldn't be having this discussion?

 

However, I was not taking issue with your general reply to this topic - what you say in your posts is true. I was taking issue how the OP seemed to put forward a manner of deriving the maximum CPL of a span of truss given its maximum UDL - something I don't think would be possible without a high degree of complication. One would have to take many variables into account, and the hassle of doing so would render the formula impractical. From reading your last post again, I think we agree - even to a degree that we're making the very same point.

[mumbles]

Mumbles - I think you've missed my point!.....

Sorry, as I was writing my last post, I was wondering how we seemed to be arguing exactly the same points with a different conclusion. Whoops,...sorry, I'll try to wake up properly next time :P .