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[Philip Roberts]

An organization I volunteer for has some Truss (Thomas 16" I think) and a pair Genie ST-25 lifts. We typically have put the forks of the genie in each end of the truss to support the weight and then have used ratchet straps to keep the forks in the truss.

Wondering if there is a better/safer/more standard way of doing this?

Thanks

Philip

[Rob Timmerman]

Start here:
http://www.jthomaseng.com/instructions/Truss%20User%20Instru ctions01.pdf

[Philip Roberts]

I'm not sure why I didn't think to look at that.

Looking at that document and the other drawings on the Thomas Truss website the truss I have isn't quite the same design.

I've attached a picture, wondering if any one can ID the manufacturer from that. It also show how we have been doing supporting the truss from the lifts other than the ratchet strap hadn't been put on yet.

Thanks

Philip

[John Birchman, CTS]

I know several of the production companies in Orlando have these sleeves that slide onto the legs of the lift, that have 1/2 cheeseborough attached to the top of them and a bolt on the side that tightens the sleeve to the leg. They all look like they have been hand made in a machine shop, I don't think I have seen any thing like that for sale from anyone.

John

[James Feenstra]

that's a really bad way to lift truss, based on the picture

it'd be better if the truss was sitting on top of the forks

[Phillip_Graham]

James Feenstra wrote on Thu, 01 October 2009 19:01

that's a really bad way to lift truss, based on the picture it'd be better if the truss was sitting on top of the forks

+0.5

The preferred method is the one outlined in Colin's post below.  I will repost this link to a previous post I wrote about the statics of the situation.

 http://srforums.prosoundweb.com/index.php/m/447621/182/?srch =statics#msg_447621

If the truss was placed on top of the lifts, it would at least have the proper ability to bear the bending moment on the truss.  Suspending the truss closer to the second moment (i.e. inside the "box" of the truss) reduces the bending moment resistance capability of the truss.

[Collin Donohue]

Perhaps some of the smarter fellas can chime in on this, but my typical practice is to hang the truss from the forks with spansets.  Reason being... the masts of a Genie lift don't go perfectly straight up when the lift is properly leveled.  It seems that they lean back just a bit.  So when you're on the ground, the distance between the two opposing forks is (for example) 30'.  But when you crank it to height, it ends up being 30'-4".  If the truss is rigidly secured to the forks, you've got some funky forces pulling the Genie's together when they shouldn't be.

Hanging the truss via spansets gives you a secure connection, but also allows for that variance in the gap when lowered versus raised.

Thoughts?

Or are my bubble levels both off??  

[Phillip_Graham]

Collin Donohue wrote on Fri, 02 October 2009 09:06

Perhaps some of the smarter fellas can chime in on this, but my typical practice is to hang the truss from the forks with spansets.  Reason being... the masts of a Genie lift don't go perfectly straight up when the lift is properly leveled.  It seems that they lean back just a bit.  So when you're on the ground, the distance between the two opposing forks is (for example) 30'.  But when you crank it to height, it ends up being 30'-4".  If the truss is rigidly secured to the forks, you've got some funky forces pulling the Genie's together when they shouldn't be. Hanging the truss via spansets gives you a secure connection, but also allows for that variance in the gap when lowered versus raised. Thoughts? Or are my bubble levels both off??

Colin,

From a statics standpoint, this (the hanging spanset method) is the preferred way to hang the truss.  Since the truss is freely supported at both ends, the bending moments placed on the truss by the distributed load are not transferred to the lift.

For a discussion of the statics of this see:

 http://srforums.prosoundweb.com/index.php/m/447621/182/?srch =statics#msg_447621

[Collin Donohue]

Phillip Graham wrote on Fri, 02 October 2009 08:48

From a statics standpoint, this is the preferred way to hang the truss.  Since the truss is freely supported at both ends, the bending moments placed on the truss by the distributed load are not transferred to the lift.

Hey Phillip... just to be sure, were you referring to my method, or the OP's method being correct?

Thanks!

[Phillip_Graham]

Collin Donohue wrote on Fri, 02 October 2009 09:56

Phillip Graham wrote on Fri, 02 October 2009 08:48 From a statics standpoint, this is the preferred way to hang the truss.  Since the truss is freely supported at both ends, the bending moments placed on the truss by the distributed load are not transferred to the lift. Hey Phillip... just to be sure, were you referring to my method, or the OP's method being correct? Thanks!

Your method, and I edited the above post for clarity to that end.

The remaining caveat with spansets is their location on the truss matters.  The spanset exerts a compression force on the truss, and thus needs to be located in a location where the truss has a compression bearing member.  This is done properly in the stacked projector picture you posted, as the spanset is near the truss end termination.

In projector case you show, the spansets are going to have very little de-rating of their load capacity, as they are bearing a nearly vertical load.

The CG of the projectors, with respect to the truss, will determine how much of the load fraction each of the two forks of the lift bear, and the twisting moment on the lift head.  If the projector CG lies at the truss center the forks bear the load equally.

Am I making sense?