Rigging forces

[stewmo]

Two times loading only applies to a static load. When pieces are freefalling (before the slack is taken up) the forces multiply significantly. Up to 12 times the weight of the piece, although up to about 6 times is more the norm. With a good groundie letting stuff run and slowly decelerate that greatly lessens shock loading the system.

If you have a 100kg piece, rigged from below, with 1 to 2m slack, stopped dead you could create a force equivalent to 600kg on each side of the pulley, meaning 1200kg at the rigging point.[emoji15]

Something to think about.

J.

I thought it was 11 x for safety factor on negative?

[jfc]

I thought it was 11 x for safety factor on negative?

I cant remember the exact figure from my rigging ticket. There is some good research by the hse on rigging forces. They were dropping some big sections and I think it was 4 to 6 times the weight of piece. Its a few years since I read it so best check for yourself.

The pulley doubling effect is why things like the safe block and x rings are popular, and natural crotch, as they add friction at the anchor point and don't double the loading.

J.

[jfc]

RR668: Evaluation of current rigging and dismantling practices used in arboriculture

WWW.HSE.GOV.UK

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This report presents the results of a comprehensive study into a number of topics related to riggingoperations used in the dismantling of trees in the UK. The information it contains should enable thearboricultural industry to determine good practice

[stewmo]

Ressurrection of my thread here.....

Been going through rigging calculations on my rigging course and wondering am I correct!

 

So I looked up biggest rigging rope I could see quickly - 20mm English Braid has a MBS of 11,000kg

 

Safety factor for ropes is 10:1 (5:1 for hardware) so that leaves me with 1100kg as a safe working load.

 

So for positive rigging I can lower up to 550kg (dividing by 2 as using a pulley)

 

And for negative rigging I can lower up to 77kg (dividing by 14.3) 

 

This seems like a small lump for a big rope? Looking on the log mass charts that's a 40cm diameter green oak log with 60cm length. I see plenty of much larger lumps coming off on people's pics/vids!

[htb]

Think your calcs are a bit out.

[Haironyourchest]

[stewmo]

Think your calcs are a bit out.

Where am I going wrong then? I can’t figure it out. Thanks

[Blah]

Values might have changed but I'm pretty sure when I did my rigging course WLL is MBS/5, and the rule of thumb for negative rigging is divide by 11. So that leaves 200kg of your 11,000kg line.

 

You can up that number by having more rope in the system, having a good groundie who can let it run etc.

[stewmo]

Values might have changed but I'm pretty sure when I did my rigging course WLL is MBS/5, and the rule of thumb for negative rigging is divide by 11. So that leaves 200kg of your 11,000kg line.
 
You can up that number by having more rope in the system, having a good groundie who can let it run etc.

Thanks Blah

Safety factor I thought it 5:1 but apparently it 10:1 for the ropes.

The 11 is the anchor force but also factor of safety as well is 1.3 so 14.3. See photo.

[Blah]

I see. They changed how they want it explained.

 

When I did the course there was a separate question on estimating the weight of the log, where a species correction factor and a safety factor (the 1.3) came into play.

 

Then for the load at the rigging point it was 2x and 11x log weight. 

 

14.3 is a much easier number to use. I'm really good at my 14.3 multiplication table. 

 

Edited March 2, 2021 by Blah