Lighting truss collapse April, 2019

[EdSavoie]

I don't remember this one making the rounds on here yet:

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No idea on locale / when this actually happened.

Speculating with some others, it looks like 2/3rds of the way between motor 1 and 2, the truss hit some part of the grid, turning the truss into a lever. Some napkin math says that motor one would be applying roughly twice it's lifting power as downward force on motor two. I'm only looking on my phone though, I might be missing something.

 

[Van]

Looks to me like someone forgot to run the House Left motor up and the HL motor failed under the strain.

 

[RonHebbard]

Looks to me like someone forgot to run the House Left motor up and the HL motor failed under the strain.

Forgive my ignorance; I'm failing to understand how not running the House Left motor up caused the House Left motor to fail under the strain.
I could understand how not running the House Left motor up would cause the House Right motor to fail under the strain but not the motor that's not being operated.
Please explain to this retired geezer. @What Rigger? @egilson1 @Van @anybody???
Toodleoo!
Ron Hebbard

 

[Van]

Forgive my ignorance; I'm failing to understand how not running the House Left motor up caused the House Left motor to fail under the strain.
I could understand how not running the House Left motor up would cause the House Right motor to fail under the strain but not the motor that's not being operated.
Please explain to this retired geezer. @What Rigger? @egilson1 @Van @anybody???
Toodleoo!
Ron Hebbard

Yeah, that's what I meant. The HL motor wasn't run so the HR was hauling the load and it failed. At least that's what it looks like to me...

 

[MNicolai]

Saw that on FB. There's one guy on the left side of the frame who looks like he got very lucky.

Seems like in this failure method, the inside pick on the left side would first absorb the load from the right end of the truss that's not getting lifted. Then the inside/left motor would reach its overload threshold and slip, by design. The outside/left motor may keep lifting and quickly relieve the inside/left motor from holding any of the load, absorbing the full load of the truss until the outside/left motor fails and brings the full rig down.

I don't play with chain hoists much so take that theory with a grain of salt.

 

[What Rigger?]

Forgive my ignorance; I'm failing to understand how not running the House Left motor up caused the House Left motor to fail under the strain.
I could understand how not running the House Left motor up would cause the House Right motor to fail under the strain but not the motor that's not being operated.
Please explain to this retired geezer. @What Rigger? @egilson1 @Van @anybody???
Toodleoo!
Ron Hebbard

Hang on.....I gotta watch this thing a few time more. I can't keep up with time lapse like I used ta.

 

[Milt Hathaway]

My ignorant speculation: Those two short trusses are permanently installed in the venue and contain internal cable lifts (two each) which also explains the pantograph power cable feeds. My wild guess is that those weren't designed/installed to carry much more than a basic fixture load and not the full added truss with movers seen here. Compound that with someone not insuring that both lift trussed were activated together and here we are.

BTW, YouTube has a speed control in that settings menu (the gear on the bottom right of the video window) that allows you to slow down the video to 25% speed. Speeding it up to 150% is my favorite way to survive online video tutorials.

 

[MNicolai]

From AV Rigging Disasters:

"I know exactly where this is...and I know EXACTLY why it fell."
"The house had it's own lighting trusses that could be raised and lowered by cable winches... a outside lighting company was told (by the house guy) to rig to their trusses..."
"The problem was... the house trusses were rigged to eye bolts that unraveled from the weight..."
"It was "ace hardware" grade eye bolts"

 

[BillConnerFASTC]

Please explain

contain internal cable lifts (two each)

 

[Milt Hathaway]

Please explain

A variation of this: http://stage-directions.com/2312-tse-unveils-new-self-climbing-truss-hoist.html

It's possible it's very much like this with a single center motor driving two cable drums. Hard to tell from that video.

 

[BillConnerFASTC]

Line shaft in a truss. Rigging Innovators, Texas Scenic, ETC, and I'm sure others.

I'd agree - probably some overloading and two hoists that were never meant to be "synchronized" . Watched a video of four hoists "synchronized" by hand and eye. The load cells told the story of what a terrible idea that is.