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[Mike Slay]

In my opinion I don't believe properly installed guy wires add that much load.  I also don't believe guy-wires are necessary indoors if there is not a live load on the grid.  

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.

[Phillip_Graham]

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?

[Phillip_Graham]

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.

[Rob Timmerman]

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.

[Phillip_Graham]

Rob Timmerman wrote on Mon, 05 October 2009 18:08

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.

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.

[Mike Slay]

I should clarify.  I mean particullarly the welds at the panel points.  

[Philip Roberts]

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

[Rob Timmerman]

Spanset lengths are the length when used as a vertical sling, so roughly half the circumference.

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.

[Philip Roberts]

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

[Collin Donohue]

What you have on the bottom chord is correct.  On the top chord, instead of wrapping the spanset around NEXT to itself, wrap it THROUGH itself.  That way the point is still centered on itself.

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!