Close Call: Rigging Point Failure

[Mick Dempsey]

I think you're right Matty, now I think about it.

[MattyF]

I'm waiting for an expert to tell me what I'm wrong still...

this was all I could find in forces and angles as the misses has nicked my rigging books !

The extreme angles are what looked like in the vid...

Like I said last few dodge trees I've done I've tried two rigging points one with a tip tie so by the time the butt rope has ran a bit the forces are spread directly below the anchor points ... also brought a safe block as the safe block does not double the forces at the rigging point as your not using a base rigging point for when the occasion arises that is not an option or just for such occasion! I forget the formula for negative rigging but I think it tripples the weight of the bit being lowered?

Also the OP is totally right that big bit to get the job done is not worth the risk unless you can be sure.. even then I've seen rigging points fail or whole trees split in half from unseen defects.

[billpierce]

Not really understanding the doubling force thing your talking about. Can you explain I different words?

 

Might be wrong but seems that certainly in the first mewp vid the stem that broke was totallyaide loaded as the rope between the other block and the brake tried to get as straight as possible which is why it broke. Obv timber is stronger when the force is directly vertical down a stem rather than bending it.

 

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[Arrozdoce]

http://vtio.org.au/wp-content/uploads/2010/07/Working-the-Angles-i.pdf

 

Probably a lot of people here already know this document but I will leave it here again.

[TreeMuggs]

We have access to a 60ft spiderlift through a friend, so that's the size of lift that we were using. No, it was not ideal for this particular tree by any means, but it was still safer than climbing. This tree was sketchbag. This is one of the biggest cemeteries in the city, and they let stuff go way, way too long. But at the same time, they are our biggest client, so I couldn't just pass on the work. Yes, at the point that I had to make that final cut, I was way too invested to just pack it in and tell the guys that we would have to figure something else out.

 

As far as the rigging is concerned, as a concept, I have always split the load between at least two spars. This concept however makes far more sense when the spars in question are at roughly 45 degree angles, so that they load in compression. The leader that broke in this case was basically vertical, which meant that it was being side-loaded. It snapped at a big squirrel's nest 25 feet below where I was cutting. There were 3 little gaffers all curled up inside. I should have been much more critical of that spar and done a much more thorough inspection before rigging off of it, but it had been supporting a live crown, so I figured it would be ok.

 

For my first 3 years, I used natural crotch rigging almost exclusively, and I agree, it makes more sense in a ton of situations. In this case where I was using pulleys, NOT splitting the load would actually have been much safer.

 

Like most accidents, it was not just down to one single error, there were a whole bunch of things that I did wrong that compounded to produce that result. What I didn't explain very well at all in the video is that, in the moment, the thing that I was actually the most concerned about was that big hanger, which completely blinded me to the actual danger staring me in the face...

[Daniël Bos]

Not really understanding the doubling force thing your talking about. Can you explain I different words?

 

Might be wrong but seems that certainly in the first mewp vid the stem that broke was totallyaide loaded as the rope between the other block and the brake tried to get as straight as possible which is why it broke. Obv timber is stronger when the force is directly vertical down a stem rather than bending it.

 

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If you hang 100kg on a rope, pass it up through a pulley and down, it needs another 100 kg force on the other side of that pulleyto equal the force.

So if you're hanging 100kg from that pulley, the force pulling down on the pulley is 200kg.

[ben_y]

So at what point/angle does using two pulleys not share the load and actually increase the load on the rigging point? I'm a little confused.

[Clicky]

This is how I saw it in the video! The spar that snapped was taking some of the doubling effect of the spar that didn't snap by decreasing the angle and not making it so harsh. By doing this it resulted in side loading of the other spar causing it to snap?

[MattyF]

So at what point/angle does using two pulleys not share the load and actually increase the load on the rigging point? I'm a little confused.

 

 

Ben after doing a bit more reading especially of arrozdoce link I don't think it does I'm just talking bollocks ! Like I said I'm no expert on physics but it looks like it just side loads the stem so on a defective stem that's the last thing you want..

would be nice to have some more opinions?

[Mark Bolam]

Aye, side loading played a big part in that.

I've done it, you set a rigging point for a specific section of the tree and due to being idle end up taking big lumps from the other side of the tree without moving the rigging point or redirecting, creating massive lateral stresses.

 

Simple equation:

 

Rigging point snaps out = You've cocked up somehow = No money