Lowering wood with bull rope

[TreeandLand]

I have only used 16 strand climbing line for my rigging up to this point.
This line has been used a lot though, and is looking worn, so I decided to retire it from any heavy pieces.
I bought 600' of 9/16" bull rope from Bailey's and plan to cut a 200' piece to use for rigging. I'm a little nervous about the first time we try it though, because the climbing line always had that stretch in it to soften the impact of a heavy peice. If the groundman doesn't let the line run, am I in danger of breaking the hardware and jarring my teeth out when it shock-loads?

 

[sgreanbeans]

As long as you stay under the WLL of each piece in the system, you should be OK. Read up on Dynamic loading and cycle to life on the rope, each time it takes a hit, one more nail in the coffin. Let it run as much as possible, even if a little bit, it will soften the blow. Its not a bad idea to practice "letting it run" with your ground guy. On easy tree's that you can bomb, rig it instead, good time to try the risky stuff, don't wait until yur over a hot tub and have to. Buy "The Art and Science of Practical Rigging" by ArborMaster, good read and will help you greatly.

 

[squad143]

Rig smaller pieces until your groundie gets the feel for the new rope and then work your way up to bigger chunks.

 

[the Aerialist]

What are you gonna tie it to?

I just bought a stainless steel Porta-Wrap and it recently arrived, what a slick piece of gear! I clicked through the Sherrill Tree link from this site to help support it, and you should too. It's on sale right now for $120, the same price as the regular steel was. A great bargain when you consider the difference.

I also have an aluminium one which dissipates heat better and is less than half the weight, but in some gear the weight is worth it.

 

[TreeandLand]

No need to be nervous if you've got much experience using 1/2 inch rope. You're not going to notice much difference between the two.

My new rope is "stable braid", doesn't that stretch much less than climbing line? I'm glad to hear you say that I won't notice much difference. After all, I want less stretch in the line so I can control tops when working close to a house.
I will be sure to practice with this rope on less risky trees.

 

[TreeandLand]

I just bought a stainless steel Porta-Wrap and it recently arrived, what a slick piece of gear! I clicked through the Sherrill Tree link from this site to help support it, and you should too. It's on sale right now for $120, the same price as the regular steel was. A great bargain when you consider the difference.

I also have an aluminium one which dissipates heat better and is less than half the weight, but in some gear the weight is worth it.

AA, we use a steel porta-wrap that was given to me by another arborist. It was made for him by a welder....it seems pretty rugged and is the same size as the medium porty that Sherrill Tree sells. (they claim you can use 9/16" rope with it, so I hope they're right). We attach the porta-wrap to the base of the trunk with a tenex whoopie sling, and use a smaller whoopie sling for the block up in the tree.
What size rigging line do you use? This new bull rope is by Yale Cordage and has an average breaking strength of 15,000 lbs. Way better than my old climbing line.

 

[the Aerialist]

Lowering Line ...

I've been using 1/2 " bluestreak with my aluminum Porty, but I wanted to get something heavier for heavier wood. I do have a 3 strand 3/4' bull rope, but it is not recommended (3 strand) for Porta-Wraps.

I'm thinking of 9/16 or 5/8 12 strand but I'm open to suggestions.

 

[tree MDS]

AA, we use a steel porta-wrap that was given to me by another arborist. It was made for him by a welder....it seems pretty rugged and is the same size as the medium porty that Sherrill Tree sells. (they claim you can use 9/16" rope with it, so I hope they're right). We attach the porta-wrap to the base of the trunk with a tenex whoopie sling, and use a smaller whoopie sling for the block up in the tree.
What size rigging line do you use? This new bull rope is by Yale Cordage and has an average breaking strength of 15,000 lbs. Way better than my old climbing line.

I have the large and mid size steel portys. I dont use the mid size much, but it seems to me the last time I did, my 9/16 didnt seem run too smoothly through it (you have to remember that thats max rope size for the mid), a wrap seemed to be more than with the big one.. Probably something to do with bend radius or something. I could be mistaken though..

 

[TreeandLand]

I've been using 1/2 " bluestreak with my aluminum Porty, but I wanted to get something heavier for heavier wood. I do have a 3 strand 3/4' bull rope, but it is not recommended (3 strand) for Porta-Wraps.

I'm thinking of 9/16 or 5/8 12 strand but I'm open to suggestions.

AA, I've heard good things about 12 strand but haven't used it. I'll try and let you know how my 9/16" bull rope works out. It's double braided....I don't know how many strands.

 

[Capt. Blye]

Never took calculus

What is the formula used when determining shock load? For the sake of discussion, a 500# log falling 3' vs 6' or does no such rule of thumb for the industry exist? I know wwl and let lines run when applicable, however, knowing the fomula and calculating prior to the cut would reduce some of the sweat in the tree I expieriance over property.

 

[Blakesmaster]

What is the formula used when determining shock load? For the sake of discussion, a 500# log falling 3' vs 6' or does no such rule of thumb for the industry exist? I know wwl and let lines run when applicable, however, knowing the fomula and calculating prior to the cut would reduce some of the sweat in the tree I expieriance over property.

That's a damn good question. There's a coupla guys with good math skills on this site that might be able to speculate. I'm not one of them.

 

[ropensaddle]

Vector,force,weight,friction,energy,acceleration,anchor forces are all used in the calculation.

 

[Mike Cantolina]

What is the formula used when determining shock load? For the sake of discussion, a 500# log falling 3' vs 6' or does no such rule of thumb for the industry exist? I know wwl and let lines run when applicable, however, knowing the fomula and calculating prior to the cut would reduce some of the sweat in the tree I expieriance over property.

This software can be very helpful:

http://www.sherrilltree.com/Professional-Gear/Videos_3/Rigging-Software-v-10-1176

I have it and used to use it a lot. I don't need to as much anymore.

 

[ropensaddle]

I chose to keep it a little on the small side in weight and limit drop all I can,too many times we are nearing potential failures of our gear through kinetic energy. Math is not my strongest virtue yet lol so I err on the cautious side. I used to do wild stuff but I have broke ropes and got lucky a time or two and decided smaller or even chucking a better result.

 

[TreeClimber57]

What is the formula used when determining shock load? For the sake of discussion, a 500# log falling 3' vs 6' or does no such rule of thumb for the industry exist? I know wwl and let lines run when applicable, however, knowing the fomula and calculating prior to the cut would reduce some of the sweat in the tree I expieriance over property.

Making one think.. and not sure if I can get this 100% correct or not, digging out my old notes so will try. But am missing some info.. to get accurate need specs on the rope (amount of stretch, etc)..

But lets plug in some #s and see how it goes.. feel free to point out where I make errors

Now my formulas all are in metric, not sure if what the other formulas are .. so will play it by ear.

500 lbs is around 227 kg.. and we will call 3 ft as 1 m.

227 kg dropped 1 m is traveling at 4.4 m/s at place of stop. This equates to around 2225 J of energy (rounding here).

Now if the rope stretches .1 m or 10 cm (not sure how much it would realistically be).. then this means that there is an impact of around 22,000 N or 4,400 lbs of force.

That same 227 kg dropped 6 ft or 2 m would be traveling at around 6 m/s and have around 4450 J of energy.

If we have the same .1 m stretch (again not sure how realistic this is), then we have 44,500 J of energy or 8,900 lbs of force. With more J of energy, I expect the stretch would really be slightly more but not sure how much.

BUT if I recall correctly the rope stretch changes based upon mass and energy.. so the amount changes here.

Also dynamic rope, again if I remember correctly, looses it stretch rather quickly when used in heavy dynamic loading situations and each drop will be different loading due to the change in stretch of rope.

Now don't use the #s above for anything serious.. as too much guess in the #s ..

Now as it falls farther, it will be dropping faster.. 4 m would be dropping almost 9 m/s and would be around 17,000 lbs of force... and 10 m would be up to 14 m/s and 45,000 lbs of force.

Keep in mind the block at top, if the rope is going straight up and back down (at 180 degrees) is handling twice the stated load .. so at 1 m the block would have around 8,800 lbs of force.. As that angle decreases so does the impact on the block. I think 120 degrees would half that load as I recall..

Think this is right but been a long time since I did this stuff.

This is also taking some very simple concepts, and ruling out any friction, etc.. if using porty there is also some slippage as rope tightens (no matter how hard you try there will be some with this amount of impact).. and depending on wraps on porty and how much you letting it run.. will dramatically impact that force actually experienced.

Bottom line, you get into big #s very fast.. and we were taught to apply the following:
10:1 safety factor on all ropes
7:1 on slings
5:1 on steel hardware

Some apply the 10:1 for everything.. so this could be a point of discussion and may be controlled by WSIB or similar rules in your area (so be aware).

 

[Mike Cantolina]

The stretch in the rope will vary with the amount of rope in the system. (above and below the rigging)

3 foot drop

6 foot drop

From the software linked to earlier

 

[Capt. Blye]

Insightful with practcality

Thanks for response on the Q. I will research this a bit more. Sometimes the ground crew isn't expierianced enough to take the proper amount of wraps to let run or the owner wants the BF Cherry over his pool brought down in 8 footers. I use 5/8 stable braid for big stuff but I occasionally wonder if were using strong enough line. I should be able to figure it out with the info provided here. Thanks again

 

[ropensaddle]

Thanks for response on the Q. I will research this a bit more. Sometimes the ground crew isn't expierianced enough to take the proper amount of wraps to let run or the owner wants the BF Cherry over his pool brought down in 8 footers. I use 5/8 stable braid for big stuff but I occasionally wonder if were using strong enough line. I should be able to figure it out with the info provided here. Thanks again

I use 3/4 stable braid, there is times I believe it is not enough. 8 foot stick of cherry if 20 inch in dia is near 800 lbs. Its easy to see how 800 lbs changed from potential energy and then kinetic and not allowed to run can easily get past the design factor if not actual breaking strength. I wont attempt it no more too many chances for damage and injury. I have seen ropes bind in the bollard and normally groundies don't get it. They get scared they are not going to be able to stop it and end up either completely stopping or coming to a quick stop with little run. The best groundie is an older climber imho.

 

[TreeClimber57]

The stretch in the rope will vary with the amount of rope in the system. (above and below the rigging)

3 foot drop

6 foot drop

From the software linked to earlier

Correct!!

Which is why the first piece, and last piece typically show the greatest loads. Top being because it is usually bigger and falls farther, the bottom because it can be large, but also because rope is shortest. At least that is what the formulas always show. Realistically the formulas we were given years ago, were designed to get the concept of shock load into your head, they were not designed to use manually in field. With computers and software today, that is actually possible I guess.

As you mentioned, the brand of rope, type of rope, diameter of rope, age of rope, if wet or dry.. just about anything can impact the ability of rope to stretch and absorb some of the energy.

I will take a look at software Rope.. thanks.. I never really spend much time doing calculations, when you have done it for a while you know what works or what has worked in past.

 

[TreeClimber57]

The stretch in the rope will vary with the amount of rope in the system. (above and below the rigging)

3 foot drop

6 foot drop

From the software linked to earlier

This is actually more accurate for day to day work, as it is taking in to consideration factors like how far up the wood the rigging is attached, length of rope under rigging point - from block to porty, length of rope from block to wood, type of knot used, and type/size of rope. All factors that impact the stretch of rope and ability to absorb shock.

Would be interesting to see how that 12 ft of rope after rigging point will impact the numbers.