Post by: Philip Roberts on October 01, 2009, 01:03:06 AM
Wondering if there is a better/safer/more standard way of doing this?
Thanks
Philip
Post by: Rob Timmerman on October 01, 2009, 08:35:47 AM
http://www.jthomaseng.com/instructions/Truss%20User%20Instru ctions01.pdf
Post by: Philip Roberts on October 01, 2009, 01:11:45 PM
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
Post by: John Birchman, CTS on October 01, 2009, 04:14:14 PM
John
Post by: James Feenstra on October 01, 2009, 07:01:25 PM
it'd be better if the truss was sitting on top of the forks
Post by: Phillip_Graham on October 02, 2009, 01:04:34 AM
| 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.
Post by: Collin Donohue on October 02, 2009, 09:06:20 AM
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??
Post by: Phillip_Graham on October 02, 2009, 09:48:47 AM
| 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
Post by: Collin Donohue on October 02, 2009, 09:56:20 AM
| 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!
Post by: Phillip_Graham on October 02, 2009, 10:14:35 AM
| Collin Donohue wrote on Fri, 02 October 2009 09:56 | ||
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?
Post by: Collin Donohue 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!
Post by: Karl P(eterson) on October 02, 2009, 03:40:39 PM
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
Post by: Collin Donohue 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!
Post by: Philip Roberts on October 03, 2009, 11:18:26 AM
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
Post by: E. Lee Dickinson on October 03, 2009, 12:12:06 PM
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:
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?
Post by: Collin Donohue on October 03, 2009, 12:46:19 PM
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!
Post by: E. Lee Dickinson on October 03, 2009, 07:12:01 PM
| Collin Donohue wrote on Sat, 03 October 2009 12:46 |
E... |
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. 
| 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!
Post by: Collin Donohue 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.
Post by: Mike Slay on October 05, 2009, 02:29:36 AM
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.
Post by: Mike Slay on October 05, 2009, 02:50:20 AM
| Phillip Graham wrote on Fri, 02 October 2009 15:14 | ||||
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.
Post by: Mike Slay on October 05, 2009, 09:55:49 AM
The way those guy-wires are installed is inefficient in my opinion. They only protect the towers in one plane. Plus it does nothing to help protect the mast from deflecting towards the forks.
Outdoor or if there were to be a live load on the grid I would require a guy-wire on each corner at about 135 degrees from sides going to the ground. I would require 2 guy wires on the center towers going to the ground.
Post by: Phillip_Graham on October 05, 2009, 02:36:48 PM
| Philip Roberts wrote on Sat, 03 October 2009 11:18 |
Phill, (and others) 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. |
Perhaps spansets give you more flexibility for the height of the placement in some situations? A pick plate with proper clamp to the lower chord should be just as good.
| Quote: |
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. |
If you had a beam that attached to both fork tubes, and then dead hung the truss using a lower chord clamp from that point by chain/shackle/wire rope, this would remove the inward tension on the lift forks that exists in colin's picture. That would be a "simply supported" bearing member both at the truss and the lift forks.
In Colin's picture the spansets pull slightly inward, and downward, on the forks.
Which genie fork adapter were you considering?
Post by: Phillip_Graham on October 05, 2009, 02:56:47 PM
| Mike Slay wrote on Mon, 05 October 2009 02:29 |
= 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. |
Strictly speaking, a pipe section suspended from the bottom chord is partially in tension, and partially in compression. Points below the centroid are in tension, points above are in compression.
One real-world example of this is that rebar goes in the lower half of a concrete span, to improve the concrete's behavior in tension.
Post by: Rob Timmerman on October 05, 2009, 06:08:44 PM
| Phillip Graham wrote on Mon, 05 October 2009 14:36 |
Perhaps spansets give you more flexibility for the height of the placement in some situations? A pick plate with proper clamp to the lower chord should be just as good. |
The big advantage to a spanset is that it places the center of mass of the truss well below the pivot point. A pick plate places the pivot point in the same plane as the bottom chords of the truss, which can lead to a tippy truss in some situations. Think pendulum instead of see-saw.
Post by: Phillip_Graham on October 05, 2009, 08:22:24 PM
| Rob Timmerman wrote on Mon, 05 October 2009 18:08 | ||
The big advantage to a spanset is that it places the center of mass of the truss well below the pivot point. A pick plate places the pivot point in the same plane as the bottom chords of the truss, which can lead to a tippy truss in some situations. Think pendulum instead of see-saw. |
Rob,
This makes tremendous sense, and shows I was picturing the wrong type of plate for the genie/sumner.
I was imaging a locking plate that went over both forks, and then had a central shackle, aligned with the center point of the lift. Hung from that shackle would be a short chain or wire rope that connects to a truss pick like this one:
http://www.thelightsource.com/products/80/view
In this case the CG would be lower, too, as the spanset example. The type of pick plate you are describing, though, would not have that advantage.
Post by: Mike Slay on October 05, 2009, 09:10:25 PM
Post by: Philip Roberts on October 06, 2009, 12:53:20 AM
| Collin Donohue wrote on Sat, 03 October 2009 12:46 |
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. ... 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. |
Collin,
Don't feel to bad about not knowing the Lark's Head by name. I didn't either (knew how to tie it but not what it was called).
I'm not fully following you as to what you are doing with the span set, especially regarding wrapping to the back of the fork. A more recent and closer in picture would be helpful if you get a chance some time.
A couple more general questions:
Are spanset lengths quoted as the length of material in the loop or the lengths of the loop it's self? (Diameter or circumference?)
Last (for now) are there any good rigging training classes or books? I've seen recommendations of Donovan's book but it's title makes me think it may be overkill for my situation where I'm mostly using ground support and may at some point move to a few (2-6) motors, at weights well under 2T.
Thanks
Philip
Post by: Rob Timmerman on October 06, 2009, 06:01:41 PM
Donovan's book is an excellent reference, and includes quite a bit of "how to". A Statics textbook will give you the fundamentals, but none of the "how to".
Also, FWIW, it may be a Lark's head when tied with rope, but it's a choker when used with a sling.
Post by: Philip Roberts on October 13, 2009, 09:29:41 PM
I've attached a photo where I mocked up what I think Collin is doing with short XLR cables instead of spansets. Assuming I get proper span sets is something like my mock up a good and safe way to hang the truss? Any recommendations of a better way to handle the upper end?
I'd still be interested in any other recommendations books or training classes for practical / how to for basic rigging like I'm doing. I'm planing to by Donovan's book when I get the money, is there any thing else good?
Thanks
Philip
Post by: Collin Donohue on October 13, 2009, 10:04:32 PM
Also, you have the "spanset" secured by looping it around the back end of the forks. There is nothing really holding this secure. Wrap the spanset around the fork and then use a shackle to secure it back to itself.
I'll try to get a picture sometime soon. We're getting busy again so it might be a while!
Post by: Karl P(eterson) on October 15, 2009, 09:23:45 PM
| Philip Roberts wrote on Tue, 13 October 2009 21:29 |
I guess I'll reply to my self with a follow up question. I've attached a photo where I mocked up what I think Collin is doing with short XLR cables instead of spansets. Assuming I get proper span sets is something like my mock up a good and safe way to hang the truss? Any recommendations of a better way to handle the upper end? I'd still be interested in any other recommendations books or training classes for practical / how to for basic rigging like I'm doing. I'm planing to by Donovan's book when I get the money, is there any thing else good? Thanks Philip |
I have totally seen truss rigged like that.
As in, with 20AWG
Karl P