Forces in a Guided Rappel

[mtngoat59102]

Most of us are aware of the force multipliers that occur when loading an anchor where the degrees between attachment points is too high. (american triangle)

What are your thoughts on this principle as it applies to force generated on the opposing attachment points of a guided rappel? Would the body hanging in the center of the taught line create weight multipliers at both ends of the rope? I'm sure this force is reduced due to movement, angle of decent, stretch in rope, etc.

These where just some random thoughts spinning around in my head and thought I would post them for comment, or not. :rolleyes:

[ratagonia]

Most of us are aware of the force multipliers that occur when loading an anchor where the degrees between attachment points is too high. (american triangle)

What are your thoughts on this principle as it applies to force generated on the opposing attachment points of a guided rappel? Would the body hanging in the center of the taught line create weight multipliers at both ends of the rope? I'm sure this force is reduced due to movement, angle of decent, stretch in rope, etc.

These where just some random thoughts spinning around in my head and thought I would post them for comment, or not. :rolleyes:

Well, it depends. It depends on geometry, and one can figure it out by doing the math. As a rule of thumb, the top anchor for a guided rappel would expect to have a load of 5 times rappel weight. Given a rappel weight of 2X body weight, and a body weight of 200 lbs, that would be about 2000 lbs. Thus, be sure the anchors for a guided rappel are "truck".

Perhaps it should be noted here that forces generated in a rappel can be higher than many people would expect. It is fairly easy to generate 1000 lbs from bouncing on a rappel, harder to generate much more than that (unless one tries, such as jumping, as happenned recently at a climbing area in SLC).

Tom

[mtngoat59102]

Well, it depends. It depends on geometry, and one can figure it out by doing the math. As a rule of thumb, the top anchor for a guided rappel would expect to have a load of 5 times rappel weight. Given a rappel weight of 2X body weight, and a body weight of 200 lbs, that would be about 2000 lbs. Thus, be sure the anchors for a guided rappel are "truck".

Exactly my thought. The additional force of a tensioned line prior to body weight would drive the number even higher than the above math I would think. The question of weight distribution over the longer length, etc are the unknown. Bottom line, solid anchors are important for guided raps.

[ratagonia]

Exactly my thought. The additional force of a tensioned line prior to body weight would drive the number even higher than the above math I would think. The question of weight distribution over the longer length, etc are the unknown. Bottom line, solid anchors are important for guided raps.

Also a good reason to not repeatedly tighten it up. Generally, a guided rappel should be done with as little tension as will get the job done, kinda sorta maybe. I think Rich teaches to tighten it up once after the first person, but not to keep cranking on it after that.

Tom

[rcwild]

Yep. Tighten. Send the first person down. Tighten again. No more. Tighten it too much, taking every bit of stretch out of the rope, and all force will be directed to the anchors.

Noticing more focus on guided rappels lately. Wondering if people understand the forces involved. Lots of manky old bolts on the Colorado Plateau that may not hold up.

[mtngoat59102]

Rich,

Manky old bolts? Dude, we pile rocks up. If it looks questionable, we just sit on it. :eek:

[sonnylawrence]

We set up a guided rappel much like is taught by the ACA. The rope was 11 mm low stretch nylon. The angle was about 30 degrees (yeh, I forgot to measure it). A dynamometer was inserted at the anchor point at the top of the tower. The guideline was initially tensioned with one person at the truck anchor point. That resultant baseline guideline tension was 0.9 kN. A person weighing 0.9 kN rappelled. The peak force the guideline anchor (combination of rappel rope plus guide line rope) saw was 2.85 kN. In essence, in this example only, the rappeller tripled the forces on the anchor by using the guideline.

No one was injured when the tower was pulled over.

[skianddive]

No one was injured when the tower was pulled over.OK, I'll be the first to ask Stooopid Question #1: When the tower was pulled over, was the multi-pound helo sitting on top?

[sonnylawrence]

We did not pull the tower over. Uhh...I exaggerated that part.

[sonnylawrence]

Guided rappels may result in huge forces being applied to the anchors. As the angle of the guideline with respect to the horizontal lessens, the forces greatly increase on the anchors. In a sense, the worse case scenario is when the guideline is horizontal. This of course would not work for a rappel. It would now be called a highline. If the rope could be strung taut horizontally between two anchors, the forces would approach an infinite amount. But the rope stretches and hence sags. The angle then changes. The resulting load on the anchors decrease. Since the angle of the rope is important, so is the stretch in the rope. Consequently different canyoneering ropes behave differently. Even low stretch nylon has significant stretch. However polyester and Spectra ropes may be designed with very little stretch. A person accustomed to rigging a guided rappel (or highline) with nylon rope potentially could get into trouble when setting up the same system in the same way with one of these other ropes. I went to the lab to get a sense of the forces involved. I used Canyon Pro rope which has very little stretch. I set up a highline, rigging it via one method taught by the ACA with a 5 to 1 theoretical MA. With great difficulty (eight feet above the ground), two of us pre-tensioned the line (as per ACA system) to 0.9 kN. We hung a person as a 0.7 kN load on the line. Initially the anchor experienced 3 kN in tension. As the rope and knots relaxed, the tension dropped to 2.8 kN. The load? then starting bouncing around on the line to simulate a rappel. That was spooky. The peak force experienced by the anchor was 3.5 kN. In other words, simply by hanging a person off of a horizontal line rather than a vertical line as would occur in a standard rappel (with some bouncing), the force on the anchor was multiplied 5 fold.

[dccampen]

That was spooky. The peak force experienced by the anchor was 3.5 kN. In other words, simply by hanging a person off of a horizontal line rather than a vertical line as would occur in a standard rappel (with some bouncing), the force on the anchor was multiplied 5 fold.
It would be interesting to compare using "static" nylon rope vs spectra rope.

[ratagonia]

Many of us who use guided rappels fairly often have always wondered what kind of forces are generated using this technique. I had the opportunity to measure a guided rappel last week, and here's what we found.

I went training with the other ZAC guides in Water Canyon last
Friday, March 7, 2008. We set up a guided rappel for the last big
rappel in the canyon. The rap is 90 feet high, and the rock that
forms the bottom of the rap is about 80 feet away. My 155 foot rope just barely makes it.

For both guide and rappel lines, we used Bluewater Canyon Pro 8.0 mm ropes. The Dynamometer was set up between the bottom anchor and the bottom of the rope. The Dynamometer was a 4000 lb model, and had ticks at every 25 lbs, and a max-load hand. The rope was tensioned (only moderately tight) at the bottom using a Voodoo system.

We sent one 150 lb person down the guide line. Normally, I would
tighten the system a foot or two at the top, at this point, but in
this case I decided against it, so we could see where different
weights of people ended up. Next we sent a 120 lb person down, then another 150 lb person. I then tightened it up reasonably snug from the top, using a 1:1 counterweight and tying a block in with a couple inches of slack.

Results

My 190 lbs on the guided rappel generated a maximum load of 625
lbs. Maximum force was reached at the very bottom of the rap, just before stepping onto the sand. Most people tend to bounce more at the very bottom, also, which probably contributed to the max load.

Discussion

This was much lower than I had previously suspected, as being about 5X bodyweight. 5X is probably still a good, conservative upper limit. This 45 degree overall angle (roughly) would produce higher loads than most guided rappels I have done (which seem to be for the most part, steeper).

Application of Results

The geometry of each guided rappel will be somewhat different. This guided rappel with a 45 degree overall run only generated 3.5X bodyweight. Steeper angles are likely to produce lower forces.

The BW Canyon Pro rope is considered to be highly static, thus we would expect that using almost any other rope would result in more rope stretch, and therefore lower loads. (A high level of staticity is usually considered desirable when setting a guided rappel).

Tom

[indieandie]

Tom, can I assume that a more dynamic rope over 9.0 is a good choice for a 200+ person doing 45's or shallower with little experience? I am not a math whiz but I understand the concept but not the math. Thanks for the infor.

[rcwild]

There is another reason for the higher loads at the bottom of the rappel. Unless the guide rope is steel cable that retains the same angle during the entire length of the rappel ...

The angle of the rope between your tether carabiner and the bottom anchor will be changing constantly. The closer you get to the bottom, the closer to horizontal the angle will be.

[rcwild]

Tom, can I assume that a more dynamic rope over 9.0 is a good choice for a 200+ person doing 45's or shallower with little experience? I am not a math whiz but I understand the concept but not the math. Thanks for the infor.

The issue is not with the strength of the rope, but with the strength of the anchors used. The benefit gained using a dynamic rope for an occasional guided rappel is not enough to justify it. Lesson should be: (1) use bomber anchors, (2) set your guide rope with a steep angle.

[rcwild]

Shallower angles are likely to produce lower forces.


:question:

Shallower angles meaning steeper? or flatter?

[ratagonia]

:question:

Shallower angles meaning steeper? or flatter?


Good catch. Steeper angles mean lower forces. I will edit the original.

Tom

[ratagonia]

Tom, can I assume that a more dynamic rope over 9.0 is a good choice for a 200+ person doing 45's or shallower with little experience? I am not a math whiz but I understand the concept but not the math. Thanks for the infor.

A dynamic rope will stretch a lot with the forces on the guide line. I like using the most-static rope available for the guide line, as it makes the path of the guide line (and thus the canyoneer) the most predictable.

To minimize forces on the anchor, I suggest carefully setting the tightness of the guide line to the point that it gets the job done, and no tighter. Sequence people down carefully, and ignore the desire to tighten it up more than once.

Tom

[rcwild]

To minimize forces on the anchor, I suggest carefully setting the tightness of the guide line to the point that it gets the job done, and no tighter. Sequence people down carefully, and ignore the desire to tighten it up more than once.

Sounds familiar. You were paying attention in class.

[ratagonia]

Sounds familiar. You were paying attention in class.

Si, Senor.

T

[sonnylawrence]

Discussion

This was much lower than I had previously suspected, as being about 5X bodyweight. 5X is probably still a good, conservative upper limit. This 45 degree overall angle (roughly) would produce higher loads than most guided rappels I have done (which seem to be for the most part, steeper).

Your results are similar to mine.

EDIT FROM RICH: Because the threads were related I merged them into one. Sonny's results are in post #10 of this thread.

[barleywino]

For those who might be interested in the nerdy details, here are some results I got from doing the math (disclaimer: this has not been verified or validated by anybody so caveat emptor). I can send the spreadsheet to anybody interested if you IM me with your email address.

t1 = Angle between ground (horizontal) and lower segment of rope (degrees)
t2 = Angle between ground (horizontal) and upper segment of rope (degrees)
F1 = Tension on lower segment of rope (as multiple of person's wt)
F2 = Tension on upper segment of rope (as multiple of person's wt)

t1 t2 F1 F2


30 60 100% 173%
30 45 273% 335%
30 40 441% 499%
30 35 940% 994%

45 60 193% 273%
45 50 738% 811%

60 75 100% 193%
60 70 197% 288%
60 65 485% 574%

The tighter the rope tension, the closer the angles t1 and t2 are to each other and the greater the force multiplier or tension on each segment of the rope.

(for those who want to reproduce these results on their own, I just set the sum of the horizontal and vertical forces (respectively) on the rappeller from each of the rope segments and from gravity to zero and solved the 2 equations in 2 unknowns using Cramer's rule)

[ratagonia]

For those who might be interested in the nerdy details, here are some results I got from doing the math (disclaimer: this has not been verified or validated by anybody so caveat emptor). I can send the spreadsheet to anybody interested if you IM me with your email address.

t1 = Angle between ground (horizontal) and lower segment of rope (degrees)
t2 = Angle between ground (horizontal) and upper segment of rope (degrees)
F1 = Tension on lower segment of rope (as multiple of person's wt)
F2 = Tension on upper segment of rope (as multiple of person's wt)

t1 t2 F1 F2

30 60 100% 173%
30 45 273% 335%
30 40 441% 499%
30 35 940% 994%

45 60 193% 273%
45 50 738% 811%

60 75 100% 193%
60 70 197% 288%
60 65 485% 574%

The tighter the rope tension, the closer the angles t1 and t2 are to each other and the greater the force multiplier or tension on each segment of the rope.

(for those who want to reproduce these results on their own, I just set the sum of the horizontal and vertical forces (respectively) on the rappeller from each of the rope segments and from gravity to zero and solved the 2 equations in 2 unknowns using Cramer's rule)

Are you putting the suspension point at the half-way of the rope?

On this one, we are saying that the overall angle of the guided rap is 45 degrees. Which means, with the weight at center, the average of the upper and lower segments is 45 degs, I think. Not like trig is my forte, any more...:oldman:

Tom

[rcwild]

t1 = Angle between ground (horizontal) and lower segment of rope (degrees)
t2 = Angle between ground (horizontal) and upper segment of rope (degrees)
F1 = Tension on lower segment of rope (as multiple of person's wt)
F2 = Tension on upper segment of rope (as multiple of person's wt)

t1 t2 F1 F2

30 60 100% 173%
30 45 273% 335%
30 40 441% 499%
30 35 940% 994%

45 60 193% 273%
45 50 738% 811%

60 75 100% 193%
60 70 197% 288%
60 65 485% 574%

The tighter the rope tension, the closer the angles t1 and t2 are to each other and the greater the force multiplier or tension on each segment of the rope.


Very interesting. Thank you for crunching the numbers, Hubert. It's easy to see two variables that effect the load – (1) tension, by comparing t1 and t2; and (2) angle, by comparing the group values for 30, 45 and 60.

One takeaway for me that I will emphasize more with students in the future – when it is not possible to set a steep guide rope, it's better to leave it a bit looser than normal. Looking at the numbers in the group values for 30 degree angle – multiplier gets quite dramatic as difference in angles gets smaller. 30-35 @ 940-994 :worried: Let that sucker sag.

[barleywino]

Are you putting the suspension point at the half-way of the rope?

On this one, we are saying that the overall angle of the guided rap is 45 degrees. Which means, with the weight at center, the average of the upper and lower segments is 45 degs, I think. Not like trig is my forte, any more...:oldman:

Tom

OK, here are some of the numbers that come out when the average angle is 45 degrees:

35 55 168% 240%
40 50 370% 441%
41 49 471% 542%
42 48 640% 711%
43 47 978% 1048%
44 46 1990% 2061%
(Danger, Will Robinson!!)

[ratagonia]

OK, here are some of the numbers that come out when the average angle is 45 degrees:

35 55 168% 240%
40 50 370% 441%
41 49 471% 542%
42 48 640% 711%
43 47 978% 1048%
44 46 1990% 2061%
(Danger, Will Robinson!!)

And the load on the rappel strand is zero in these cases, correct?

On steep rappels, some of the load is taken by the rappel strand for much of the rappel. On shallow rappels less so.

Tom

[barleywino]

And the load on the rappel strand is zero in these cases, correct?

On steep rappels, some of the load is taken by the rappel strand for much of the rappel. On shallow rappels less so.

Tom

Right, the numbers given for the tension on the upper rope should strictly speaking be interpreted as total force on the upper anchor (sum of tension on the rappel strand and tension on the upper segment of the guiding rope).

[rcwild]

I moved some off-topic posts about rope to a more appropriate thread titled "Rope ... General Considerations (http://www.canyoneering.net/forums/showthread.php?t=51)"

[barleywino]

Right, the numbers given for the tension on the upper rope should strictly speaking be interpreted as total force on the upper anchor (sum of tension on the rappel strand and tension on the upper segment of the guiding rope).


Ok here's an example of how the angles and the tensions below and above the rappeller vary as the rappeller descends:

Cliff height (ft) 100
Rope length (ft) 142

(column headings:)
Distance traveled
Lower length (amt of rope below rappeller)
Lower angle
Upper angle
Lower tension
Upper tension

10.0 132.0 44 64 127% 209%
28.4 113.6 42 55 254% 330%
42.6 99.4 42 53 308% 382%
56.8 85.2 41 51 341% 413%
71.0 71.0 40 50 357% 428%
85.2 56.8 39 49 356% 426%
99.4 42.6 37 48 339% 407%
113.6 28.4 35 48 301% 367%
127.8 14.2 29 47 231% 293%

We're assuming the rope is very static, rigged at 45 degrees with about 6" of slack. Tensions above and below the rappeller increase rapidly until past the midpoint then decrease more slowly.

[barleywino]

Very interesting. Thank you for crunching the numbers, Hubert. It's easy to see two variables that effect the load – (1) tension, by comparing t1 and t2; and (2) angle, by comparing the group values for 30, 45 and 60.

One takeaway for me that I will emphasize more with students in the future – when it is not possible to set a steep guide rope, it's better to leave it a bit looser than normal. Looking at the numbers in the group values for 30 degree angle – multiplier gets quite dramatic as difference in angles gets smaller. 30-35 @ 940-994 :worried: Let that sucker sag.

one popular tyrolean where this might be especially relevant is the Lost Arrow Spire tyrolean (Yosemite), generally traversed in the uphill direction and pulled very tight, with an average angle somewhere around 30-40 degrees iirc

[brucefrombryce]

It seems to me that someone told me, and I believe I did it in Spry, that the last person rappels from the side of the rope with the biner on the rope and uses the other side for the guided portion of the rap (I believe that the team already on the ground grabbed the rope (all six of them) and held it taught while I rappeled and used the guided portion to keep me out of the water.

Would someone please explain this to me if it is correct! And if not, please let me know the correct way for the last person to rappel with a guided rappel in place.

Thanks,

bruce from bryce

[smorga]

Bruce,

I believe you are correct. The pulley attachment to your harness should be on the side of the rope that is being anchored by the team below. The side of the rope with the block (side with biner) should be rigged through your rappel device.

--Steve

[rcwild]

Yes. True for everyone in the group, not just the last person.

Let's be careful with techniques like guided rappels. There are people on the internet suggesting that guided rappels should be done "just because they're fun". This is inappropriate and reckless advice. Accidents have resulted from the misuse of much simpler systems like blocks. Unless you really know what you are doing and understand the forces and the risks involved, why take the risk?

[brucefrombryce]

You're right Rich. I'm just practicing and want to be sure of the proper method. No canyons so this is what I do to keep the skills sharp.

bruce

[wiese]

Unless you really know what you are doing and understand the forces and the risks involved, why take the risk?ummm.... pretty much the entire "sport" is risk and we do it "just because it's fun".

[rcwild]

ummm.... pretty much the entire "sport" is risk and we do it "just because it's fun".

ummm ... yes, the entire sport involves risk, but only a fool would go into it with no understanding of the risks and make no effort to do everything they can to mitigate the risks. It stops being fun as soon as someone gets hurt.

[barleywino]

When i do the calculation for a highline situation (where the rope can sag below the lower anchor due to slack or stretch), say, 100' long highline with 10'-25' of elevation loss and 2% rope stretch, I get maximum tensions of about 2.5x body weight if there is no pretensioning of the highline. If there is pretensioning, that takes some of the stretch out of the system and increases the tensions, for example, if the rope is pretensioned so that 3/4 of the stretch is taken out of the rope, then tensions go up to about 5x body weight (maximum tension occurs when the person is about halfway down the highline). I can run the numbers for other scenarios if anybody is interested.

[oldno7]

Barley
I'm guessing your measuring tension on the top anchor?
Using 2.5X doesn't seem to be adequate considering this anchor already has 2X just by nature of being on a 2:1 MA system.
Now throw in a flattened angle of descent and those numbers would rise significantly. I don't have a large enough scale or I would test this theory real time.

[barleywino]

oldno7, actually i'm looking at rope tensions both above and below the person (they are pretty comparable at these low angles, with the tension above the person being slightly higher). If a 2:1 is used to pre-tension the rope, then the rope tensions will be higher. For example, let's assume for simplicity that the rope is rated at 2% stretch. So under this *approximation*, a 100' rope will stretch by 2 feet under load. Suppose now that 1' of rope is pulled through the anchor using a 2:1, ie. after tensioning the rope, there is now 1' of slack hanging out the backside of the 2:1. As an *approximation*, we assume this means that the 100' rope can now only stretch by 1 more foot under load. This tightens up the angles that the rope makes under loading (so that the rope will be more straight and less "bent") so the tensions will be higher. So if you can tell me how much slack gets pulled through the 2:1 under pre-tensioning, and what the total available stretch of the rope is (2% in this example), I can tell you the tension on the rope under load etc. Let me know if this makes any sense...

[oldno7]

We are talking 2 different things. I'm talking about the "load" applied to the top anchor only. Your talking about "tension" on the guide line. So dis-regard my input, it was not directed towards line tension.:worried:

[barleywino]

We are talking 2 different things. I'm talking about the "load" applied to the top anchor only. Your talking about "tension" on the guide line. So dis-regard my input, it was not directed towards line tension.:worried:

Sorry for the confusion (my bad)

[twobadfish]

I plan to setup a guided rappel in a dry canyon for two reasons:

1) Fun
2) Practice

I nearly lost my life when I entered a wet canyon before me or my partner had any experience. The ability to confidently setup a guided rappel would have made the difference between a near-death experience and a pleasurable one.

Whether it's just for fun or not, it is an advanced technique that could save a beginner's (or poor swimmer's) life when or if the party gets in over their head.

Besides, a large component of canyoneering is fun. If someone wants to master a technique and practice with it for fun, more power to them. You can't however stop foolish people from playing with techniques whom take no time to master and practice them before trying the real thing. That goes for just about anything.