shock loading 1" Stable Braid

Jacques LaForge

Jacques LaForge

Location
Portland
Hello tree climbing brothers and friends,

I am seeking help in understanding some math according to a recent removal where I wrecked a 1" Stable Braid.

I was called in to finish(block down wood) on a 4' DBH cherry removal. No drop zone between two houses. Rope could not run, and due to stretch on 1" was pre-tensioned on the GRCS and locked off, creating an environment for shock load. 1" ISC block and 1" Tenex Whoopie. I was taking 6' x 2'ish-3' logs, tied up by half hitch to ISC Big Dan on double fisherman's anchor knot(choked).

After a few pieces were rigged out, half of the sheath appeared to be compromised(burst strands) and core was exposed, at approximately where the half hitch on the log was being tied. The distance between block and half hitch was as short as possible considering circumference, directional notch, and enough room to not melt the block sling with a 660 muffler(probably about 1.5'-2').

***I recognize that there are better rigging solutions(double blocking, rigging rings, etc.) for this type of work, but I did not think that it was beyond the scope of acceptable risk to continue with the limited tools at hand and by the time expectation set by the tree boss.***

Log Weight Pro says the logs were no greater than 2,200 lbs and in a typical negative block rigging scenario should not generate more than 20k force. So, how did I compromise the shealth on a rope that has an ABS of nearly twice the potential force at 39K?IMG_20200310_064644.jpg
 
All I can think of is the rope on rope friction at the half hitch. Also knots (not sure if half hitch would be considered a "knot") significantly reduce strength of rope, so friction and the knot reduction could easily drop the strength of the rope close to the forces generated by basically static blocking. Rope does look pretty used too, which also adds to the reduction.
 
Is the darkness on the rope goop or melting/glazing? Could've just been stress concentration at a bend/contact plus strength reduction from cycles to failure. Doesn't look like a melt plus stretch, just looks like normal temperature fibers snapping under tension. ?
 
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Agreeing with the other comments I'll also add that a marl although may be necessary to hold these pieces more securely does cause more strength loss than a running bowline. I'm not sure strength loss compared to the carabiner setup that you used, but the marl has a history of being the weak link.

I'd cut that section off and continue, then replace it after that job. Ropes after all are a consumable product.
 
The SWL etc. stated for static load
In dynamic hit, no forgiveness from 1" Stable Braid
Especially on hit with shorter length before fractions to support side to spread hit out to yield dampening effect.
Also, if Running Eye was not tightly seated takes run in hard from force.
Especially w/o relief of run.
.
side note: if use same rope in 2:1 on load
>>system is now ~2x as strong
>>dynamic dampening is by loading
>>each leg laded less together get LESS dynamic absorption than single leg
Thus, dampening by how much hit invades SWL less relief
>>similar to parallel resistors in electronics halves resistance response in circuit.
.
But yes, rope on rope friction seems what this is, but no glazing where would expect??
and would try to take hit off of eye anyway, bury the seam form the hit when can.
.
Even in carpets with human weight,
uncrushable sand grains average 18 cutting surfaces per grain
powered by human walking for slow-mo sand blasting away at carpert fibers hidden deep
>>sand cutters power by force
>>clean carpets last longer
Taken to rope model, grains inside of jacket can more quickly work against internals
>>Some mountain/rescue fold go nuts if step on rope, let alone drive over one..
.
 
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Jacques LaForge said:
So, how did I compromise the shealth on a rope that has an ABS of nearly twice the potential force at 39K?
Click to expand...

Because that rope has an ABS of 39K on a static pull test bed. Shock forces are an entirely different matter. Using a safety factor of 10:1 you shouldn't load that rope over about 4K and that's not just the log weight... it's the expected shock forces, too. Even with a 5:1 safety factor, you were over. Add to that everything in the system that lowers the effective breaking strength, like knots, hardware with small bend radius, and the fact that the rope is well used and I don't see how it could have done anything but fail.

If you're going to negative rig, think seriously about using a high-elongation rigging line and taking much smaller pieces. Every time you shock load a rope near it's safe SWL, you weaken it. If you do that 100 times, your 39K rope might only be a 17K rope by the time you try it again. The only way to preserve the rope's strength for a long time is to keep it clean and don't shock load it anywhere near its safe SWL load.

Negative rigging big wood makes great videos, but it's hard on the equipment, the ropes, the crew and on the client's property. Taking smaller pieces might be slower, but it might not. Most crews can fall quickly into a fast routine with smaller pieces and move faster and more efficiently if the pressure isn't on them all the time to not break everything with the slightest miscalculation or mistake.
 
Thank you all for the constructive feedback and suggestions!

@Bart_ Yes, that rope is dirty as hell and I didn't see any glazing. It was also raining all day(not sure if that's relevant).

@evo I have no good excuse, I recognize that a half hitch/marl to running bowline is better than choking with a steel carabiner for negative rigging wood. I should know better, especially when sinking into complacency from an exhausting work week, to be more mindful and triple check everything.

@*useless info* I appreciate your perspective, you've given me some good homework.

@JeffGu I agree with you entirely and I don't go big to make a show. I work on call and often feel the expectations of my employers to meet their budget and do well enough , so that they are inclined to hire me again. I always try to find a strategy in removals to go smooth and safe, rather than fast and hard, but in this case I admit that I was experiencing pressure to go bigger and it effected my decision making process. Your comments are very helpful and your post is good medicine!
 
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CanadianStan said:
50kn isn't a whole lot of MBS for big square rigs ..
Click to expand...
I don't know what a square rig is.
I am not condoning simply cinching a biner around big wood. The wrap plus the half hitch takes so much of the load that the carabinier is in no danger of failing, especially on wood large enough to not cause any substantial side load.
 
SomethingWitty said:
I don't know what a square rig is.
I am not condoning simply cinching a biner around big wood. The wrap plus the half hitch takes so much of the load that the carabinier is in no danger of failing, especially on wood large enough to not cause any substantial side load.
Click to expand...

Square rigging, aka negative rigging.

I also use a steel biner for rigging but when it's Cherry logs, 6' long by 2'-3' in diameter... Estimated to be 2200lbs by weight alone.. I would skip the carabiner personally, especially with the buildings in close proximity
 
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CanadianStan said:
Square rigging, aka negative rigging.

I also use a steel biner for rigging but when it's Cherry logs, 6' long by 2'-3' in diameter... Estimated to be 2200lbs by weight alone.. I would skip the carabiner personally, especially with the buildings in close proximity
Click to expand...

On another note, this is not a bad spot to use double block rigging
 
Yeah, man. Multiple ropes to take less load each, have less stretch because of lower loads, and stabilize sway more because they're not set in the exact same spot.
It's definitely the way when you're negative rigging mill logs.


Square rigging is a new one for me. Is that some Australian slang?
 
SomethingWitty said:
Yeah, man. Multiple ropes to take less load each, have less stretch because of lower loads, and stabilize sway more because they're not set in the exact same spot.
It's definitely the way when you're negative rigging mill logs.


Square rigging is a new one for me. Is that some Australian slang?
Click to expand...

Canadian! What other terms do you know it by ?
 
Jacques LaForge said:
Thank you all for the constructive feedback and suggestions!

@Bart_ Yes, that rope is dirty as hell and I didn't see any glazing. It was also raining all day(not sure if that's relevant).

@evo I have no good excuse, I recognize that a half hitch/marl to running bowline is better than choking with a steel carabiner for negative rigging wood. I should know better, especially when sinking into complacency from an exhausting work week, to be more mindful and triple check everything.

@*useless info* I appreciate your perspective, you've given me some good homework.

@JeffGu I agree with you entirely and I don't go big to make a show. I work on call and often feel the expectations of my employers to meet their budget and do well enough , so that they are inclined to hire me again. I always try to find a strategy in removals to go smooth and safe, rather than fast and hard, but in this case I admit that I was experiencing pressure to go bigger and it effected my decision making process. Your comments are very helpful and your post is good medicine!
Click to expand...

Can we just take a minute to give this guy credit for listening to and responding to criticism and constructive advice without getting all defensive or just leaving? Too many guys/gals show up, ask a question and get an answer they don't like and either blow up or leave.

Refreshing...... Carry on!
 
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