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Author Topic: Securing Truss to Genie Lifts  (Read 22729 times)

Collin Donohue

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Re: Securing Truss to Genie Lifts
« Reply #10 on: October 02, 2009, 11:37:08 am »

Phillip Graham wrote on Fri, 02 October 2009 09:14

Am I making sense?


Perfect sense.

Good to know that my gut feelings are typically correct!  One of these days I'm going to have to get myself to a proper rigging course!  Gut feelings don't always hold up in court!
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Karl P(eterson)

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Re: Securing Truss to Genie Lifts
« Reply #11 on: October 02, 2009, 03:40:39 pm »

In addition to the great advice by Phill, there are some good solutions from The Light Source for rigging truss and genies.

The Genie Fork Adapter which locks onto the fork and gives you a place for an eyebolt & shackle is a good piece and the truss pick-plate and truss-picks themselves are all useful products in their own ways.

Just making those aware who may not be already....

Karl P

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Collin Donohue

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Re: Securing Truss to Genie Lifts
« Reply #12 on: October 02, 2009, 05:40:29 pm »

Karl P(eterson) wrote on Fri, 02 October 2009 14:40

In addition to the great advice by Phill, there are some good solutions from The Light Source for rigging truss and genies.

The Genie Fork Adapter which locks onto the fork and gives you a place for an eyebolt & shackle is a good piece and the truss pick-plate and truss-picks themselves are all useful products in their own ways.

Just making those aware who may not be already....

Karl P




Sometimes when I'm bored, I browse their website just to see all the cool gadgets they come up with... nifty!
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Philip Roberts

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Re: Securing Truss to Genie Lifts
« Reply #13 on: October 03, 2009, 11:18:26 am »

Phill, (and others)

Thanks for the information. You'd think my physics background would have prompted me to hang the truss differently. I guess some of it is that some times Engineers think in slightly more practical terms.

To clarify a couple of things.
Is there any reason to want to use one of the truss pick plates in an application like this, or is using a spanset on each of the lower "pipes" of the truss and then going up the the genie forks sufficient.

Also Colin, how exactly do are you using the spanset in the picture you posted. It appears like its a larks head know around each edge of the truss and around each genie fork. Is this correct?

Last is there any reason to use the genie fork adapters and an eye/shackle combination or is what Colin doing with just a spanset sufficient.

Thanks

Philip
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Philip Roberts
Director of Media Engineering
Pioneer Memorial Seventh Day Adventist Church
Berrien Springs MI

E. Lee Dickinson

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Re: Securing Truss to Genie Lifts
« Reply #14 on: October 03, 2009, 12:12:06 pm »

Colin, I've done it that way and ultimately didn't like it. I don't like that there's nothing other than friction holding the spanset on the fork.

What I've started doing when I span off the Sumners is to choke the corner of the fork, instead of choking the fork itself.

Here's one for you statics guys:

This weekend we put up a square grid for an event that had to move inside. 40x40 box on six Sumners.

The tour rigger came back behind us and added steel cable "Cross bracing" because "all lift structures have to be guy wired down, no matter what."

The red is the cable he added:

index.php/fa/25328/0/

To me, this accomplished exactly two things, neither of which was desirable, without adding any safety:

1: Horizontal Vector: side load on the top of the lifts, which is horrible.

2: vertical vector: additional load, reducing weight capacity of lifts.


----

He also claimed that the lifts had to be derated 75% because the load center was 20' out from the forks, and the lifts specifically say "650lbs at 16" load center."  What is the load center of a truss or square supported evenly at each end/corner?

The final bit of enjoyment was the point in the day where he decided to call Sumner to explain what we were doing and voice his concerns. He told the client if Sumner deosn't sign off on it, he wouldn't sign off on it and the show would be cancelled.

Sumner, of course, wouldn't say a word either way on the phone. I asked the guy what his take on the first page of the CM manual (Not for use for overhead lifting) is, and he had nothing.

So. Is my 40x40 rig unsafe, or was this guy wrong, or both?
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E. Lee Dickinson
Advanced Visual Production Inc.
sound - lighting - video - design
www.avpric.com

Collin Donohue

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Re: Securing Truss to Genie Lifts
« Reply #15 on: October 03, 2009, 12:46:19 pm »

Sorry, Philip, not sure on the exact terms, but in that photo, we did a choke on the bottom rail of the truss, then a wrap on the top rail.  From what I've just googled, yes, that's a larks head (and I call myself an Eagle Scout!?!?!?!)  We then wrapped the fork of the Genie enough times to basically get the truss as close to the fork as possible... no official method, just the best method to keep it close and avoid any unnecessary movement.

In this particular instance, I wouldn't use a truss pick (such as this... http://www.thelightsource.com/products/34/view) because it takes away from the stability of the projectors, making it a real PITA to focus.  Having all four points (two on each end) gives us the most stable structure for focusing.  That and the bridal needed to rig it from the two forks would put unnecessary inward force on the forks... not my cup of tea!

E...  This picture was taken a while back.  Since then, yes, we've been adding some sort of lateral safety.  Be it wrapping the spanset to the back of the fork, adding some sort of clamp or bolt on the fork, etc.

---

On a side note, does anyone else have issues with the adjustable forks on Supertowers having just a bit too much play in them?  Causing the fork to be at a pretty scary downward angle?  Disclaimer... they really were like this before the speakers were hung on them!

index.php/fa/25330/0/
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E. Lee Dickinson

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Re: Securing Truss to Genie Lifts
« Reply #16 on: October 03, 2009, 07:12:01 pm »

Collin Donohue wrote on Sat, 03 October 2009 12:46


E...
Lee. The E. is just because sometimes people get confused that it's not my first name. Razz It's especially funny on my drivers' license: My signature is "Lee Dickinson" - but the DMV insisted that I have to sign "Edward." So on my license, "Edward" is neatly printed followed by my signature. Smile

Quote:


On a side note, does anyone else have issues with the adjustable forks on Supertowers having just a bit too much play in them?  Causing the fork to be at a pretty scary downward angle?  Disclaimer... they really were like this before the speakers were hung on them!



Yeah.. that's exactly what prompted us to choke off to the back instead of just out on the forks. It's especially bad because it LOOKS so unsafe!
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E. Lee Dickinson
Advanced Visual Production Inc.
sound - lighting - video - design
www.avpric.com

Collin Donohue

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Re: Securing Truss to Genie Lifts
« Reply #17 on: October 03, 2009, 08:04:27 pm »

E. Lee Dickinson wrote on Sat, 03 October 2009 18:12

It's especially bad because it LOOKS so unsafe!



I hear that.

When possible and when it fits the need, I use the forks from our Superlifts instead.
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Mike Slay

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Re: Securing Truss to Genie Lifts
« Reply #18 on: October 05, 2009, 02:29:36 am »

That style of truss is called 12 x 18 plated ballroom truss.  It is wide enough to accomadate a double hang yet low profile for low ceilings. Thomas, Applied Electronics, Tomcat and others make that style.  I never stab those from the ends.  I always lay them on the forks and choke with a spanset.  Outdoors The spanset is then connectected to one or two guy wires or load rated ratchet straps that is secured to a large tent peg or 55 gallon drum.  (one if part of a roof system two if free standing truss).  I'll try to get some pictures for you this weekend.

A couple of rules of thumb for lifting truss in general.

1.  Hire an experienced rigger.

2.  Lift from the bottom chords (Chords are the big pipes that run the length of the truss).  This puts the truss into compression.  In other words the load is not pulling the top chords away from the bottom chords.

3.  Know the half point load, quarter point load, and uniform distributed load tolerances for your truss's intended span.  (If you don't know what I'm talking about see rule 1.)

4.  ISPECT the RIG PRIOR TO FLYING TO TRIM!!!  Check the bolts for tightness once the truss is floating.  (One big indicator is a gap in the plates at the end of the truss sections on the bottom.)  Less experienced or lazy stage hands are notorious for under tightening bolts.  Since I climb a lot of truss I take that a little personal.  Walk the truss and make sure all lights are tight and have safeties.  Check for loose items like crescent wrenches, leathermans, and rolls of tape (they seem to be magnetically attracted to the tops of moving lights.)  Secure cable and strain relief the cable drop.

5.  Always have a dedicated oberver watching the rig as it is going up, That person should signal a stop if their are any problems.  Also clear the area of unnecessary people and signal loud and clear any time the truss is moving.
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Mike Slay

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Re: Securing Truss to Genie Lifts
« Reply #19 on: October 05, 2009, 02:50:20 am »

Phillip Graham wrote on Fri, 02 October 2009 15:14

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?


Don't forget that both forks have to be at an identical height also to insure uniform distibution of weight.  
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