Detroit rigging collapse

[boatman]

Just out of interest, could tensioned steel ropes be used for that type of bracing? Or, does it need to be solid to resist compression as well?

[Joe Betts]

Just out of interest, could tensioned steel ropes be used for that type of bracing? Or, does it need to be solid to resist compression as well?

 

 

Steel wire rope is commonly used for this purpose, providing that bracing is applied in 2 directions to form either an X brace or a K Brace, Solid braces are used when you can only brace in one direction ie. behind the outside tower in a PA / Video wing.

 

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Interestingly, if you look closely at the pics, you will see that the main trusses appear to have been disconnected

from the angled sections on the sleeve blocks.

 

It looks asif the point of failiure may have been the angled section, if the trusses were heavily loaded, they would deflect,

and without horizontal wires to stop the towers moving apart at the top, these sections would be twisted as the trusses tried to

bend downwards.

 

The towers have fallen in ontop of the grid, which seems to have almos landed flat, which supports this THEORY.

 

I emphasize THEORY as I was not there, im just passing comment from observing the pics.

[Jamtastic3]

Who calls the shots with the overall design and safety? It's clear the lighting designer went a bit mad with the moving heads but then did he know how much weight could be added to the structure? Did the structural engineer not advise him to take out moving lights or design the stage trussing around the designer?

How does it work in this part of the industry?

[Jivemaster]

Will any US regulatory body actually publish a report detailing the things to be learned from this incident?

[cedd]

Look carefully in the collapsed photos, there's also quite a fair sized pair of line arrays in the wreckage. These aren't visible on the initial erected photo.

 

Do we think theywere hung when the first pic was taken, but outside of the shot, or do we think they gor hung inbetween the first pic and the collapse?

Even more weight to consider.

[Joe Betts]

Look carefully in the collapsed photos, there's also quite a fair sized pair of line arrays in the wreckage. These aren't visible on the initial erected photo.

 

Do we think theywere hung when the first pic was taken, but outside of the shot, or do we think they gor hung inbetween the first pic and the collapse?

Even more weight to consider.

 

If you look at what the speakers are attached to, there appear to be PA / video wings that were out of shot.

There are also projector screens in the same area.

[RogerB]

Hi all

 

Looking at the pictures, this ground support was unusual in that the main cross trusses formed what was in in effect a shallow arch. It is a fundamental principle of arches (known about for thousands of years) that there is an outwards horizontal force at the ends of the arch. The flatter the arch, the higher the horizontal forces from loads on the arch. (It is the same principle that causes loads in rigging bridles to go up as the angle between them gets wider). This incident might well have been avoided if the tops of the towers on SL had been joined to those on SR by tensioned horizontal steel wire ropes to prevent that outwards movement. As that clearly didn't happen, the outward movement of the towers, coupled with the attempt by the main spans to go into tension in the bottom chords as the loads were applied would simply shear off the main spans from the sleeve blocks on the towers. In effect, the outwards forces from the arch principle were working in the opposite direction to the tension forces in the bottom chords and the aluminium simply wasn't strong enough. Speaking as someone who wasn't there, it is pretty clear from the pics that that is what happened.

 

Had the main spans been horizontal to start with, they would have tended to pull the towers inwards slightly as they deflected and may well have stayed up (just!).

 

This is my specialist area and I would say it was 100% predictable that this installation would fail exactly as it did.

 

Roger B