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Scenario: lowering off awkward set-up
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By Robert Jordan
Nov 29, 2009

You arrive at the top of a pitch and wish to lower off. There is a two bolt anchor with two quick links attached, BUT they are far apart(Perhaps 2 feet). Threading the rope through them both you see an american triangle situation happening.

Do you

A. Thread through both and rap off thinking two bolts? cant go wrong

B. Thread through one and lower off

C. look for another anchor/ walk off

I was setting a top rope when I saw someone elect for B. which went against my initial climber safety mentality ....

thanks


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By Buff Johnson
Nov 29, 2009
smiley face

As long as the bolt hardware is in good condition, I'd use both; either rap or lower, this anchor situation is fine with me.


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By Jesse Davidson
From san diego, ca
Nov 29, 2009
n cascades <br />

I'll agree with buff. 2 would be ok if the bolts looked good, if the walkoff was easy I'd consider that, but just threading one bolt? Never


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By Evan1984
Nov 29, 2009

Thread them both and rap. No question in my mind.


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By kBobby
From Spokane, WA
Nov 29, 2009

The following analysis has a hidden mistake in it. See the post below for the correct numbers.

Robert's point is an interesting one: does the safety factor from the redundancy outweigh the multiplication of forces?

For the situation he describes, yes it does. In fact, the multiplication of forces really isn't that much.

Robert, with the anchors 2 feet apart, if you set up your rappel two feet from the bolts, then when you first weight the anchor, each bolt we see about 85% of your body weight. And this value will decrease the farther along the rappel you are, with a lower limit of around 71% of your body weight.

On the other hand, if you thread just one, then that bolt sees all of your body weight.

In case you are wondering, if the bolts are close together and on the same horizontal level, they will each see about 71% of your body weight. If they are aligned vertically, with the higher one extended with chain, then each will see 1/2 of the weight.


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By ChrisG
From Westminster
Nov 29, 2009
Sedona Headshot

Europeans Rap off of one. THat's all they need.

Americans rap of of two. Because 1) They like redundancy. 2) they get to toprope more rock.

In a pinch I'd rap off one. but prefer the two and don't see a problem with the distance between.

Generally Speaking:
If you look at the strength ratings on most fixed hardware. it's well above the limit most people are cabaple of putting on it, static or dynamic.

Though,you gotta check the placement, that's subjective.


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By Robert Jordan
Nov 29, 2009

Thanks,

I didn't think of it that way before. When you initially lean back on rappel, there is multiplication of forces, but as you get lower you your weight is distributed better over the two pieces. I think I am understanding your post?

But then, why do folks spend more effort and money to build equalized rappel station like quicklinks equalized at the end of chain link or two rap rings that line up a single masterpoint? This is done to avoid multiplied force is it not?

Thanks again


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By ChrisG
From Westminster
Nov 29, 2009
Sedona Headshot


1) When you initially lean back on rappel, there is multiplication of forces, but as you get lower you your weight is distributed better over the two pieces.

2) But then, why do folks spend more effort and money to build equalized rappel station like quicklinks equalized at the end of chain link or two rap rings that line up a single masterpoint? This is done to avoid multiplied force is it not? Thanks again

Agree on 1)

On 2) It's primarily a point of redundancy.

Have you heard of S.I.R.E.N.? It's good when building anchors.
Solid, Independent, Redundant, Equalized, Non-extending.


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By Evan1984
Nov 29, 2009

ChrisG wrote:
Europeans Rap off of one. THat's all they need.


Us Americans tend to be fatter, too. :)

The reason your loads gets better distributed as you lower is because the angle to the masterpoint(or person on rap in this case) gets more acute as the rays of rope to the anchor gets longer. This is a handy something to keep in mind when building anchors. If I think the angles are going to be too obtuse, I use a longer cordalette or extend a couple pieces with runners to make the angles more acute.

As to why people spend the time to extend with chains, I think there are several reasons. First, it ensures that the angle will be acute for the whole time(this is, as the post has discussed somewhat of a non-issue, but good form). Secondly, people love to TR through the anchors and multiple rap rings allow you to replace worn ones and the chain gives more clipping options. Third, Tr'ing from largely spaced horizontal bolts kinks your rope something aweful(I don't do this, don't yell at me).

My personal favorite setup is what I've heard reffered to as a "Euro rap anchor" which is two bolts placed vertically above eachother and connected with a piece of chain. The bottom bolt hanger has a rap ring on it. You only clip into the ring, but the load is backed up and equalized if the bolts were placed properly.

Evan


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By Crag Dweller
From New York, NY
Nov 29, 2009
My navigator keeps me from getting lost

is the added force created by the extended distance between the bolts even substantial when rapp'ing off solid bolts? i would hope not.

my thinking is that the bolts, which are (hopefully) just as solid as most bolts you clip into when leading a sport climb, are strong enough to catch a lead fall. a lead fall would generate a far greater force than static body weight. so, if one bolt is strong enough to catch a lead fall, wouldn't two bolts be sufficient to hold body weight regardless of the distance between them?


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By Buff Johnson
Nov 29, 2009
smiley face

I'm working on something, SEA ALE:

Anchors by the General -- 1 if by Land, 2 if by:

Solid
Effective Material(s)
Adequate


Load distributing -- use beer distilled with the sea:

Angle
Limiting Extensions
Efficient


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By mcarizona
From Flag
Nov 29, 2009

Good topic.

So why are we so afraid of the

'american triangle' (putting pressure on two angles of the bolt, risking wallowing (is that a word) over time and eventually failure)

YET at many sport areas we are left to lower or rap off these two links of chain that clearly do not organize the weight vertically?

An ongoing discussion with my circle.

Steve


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By Buff Johnson
Nov 29, 2009
smiley face

It's just a simple matter of having redundancy; if one bolt is inadequate for whatever reason, there is another one.

Since you have one modern bolt that far exceeds in rating compared to the intended load; as far as a safety factor, let alone two bolts, this argument of the ADT angles is just discussion for the sake of discussion.

Unless you are looking at "experienced" 1/4" type bolts; but why would anyone consider that an adequate anchor in this day and age when better materials are available???


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By kBobby
From Spokane, WA
Nov 29, 2009

Ack. I made a mistake in my above analysis.

The situation that Robert describes actually loads each of the two bolts with 100% of the climber's weight, when first starting the rappel.

An equilateral ADT (60-degree internal angles) places 100% of the test load on each of the two bolts.

By comparison, equalized slings at the same angle places only 58% of the test load on each of the two bolts.

I completely agree that for rappels it probably doesn't matter. In this case, I think that the redundancy is way more important than the multiplication of forces.


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By Gunkiemike
Nov 29, 2009

"Multiplication of forces" is a misnomer. There is no multiplier involved.

The angles in an American Triangle are wider than they need to be, but as Bobby H. points out, when you do the geometry it's not as bad as most folks would have you believe.


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By John Maguire
From Boulder, CO
Nov 29, 2009
Bastille Crack Final Pitch

Bobby, could you explain the math behind these calculations. Just curious. Thanks


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By kBobby
From Spokane, WA
Nov 29, 2009

John Maguire wrote:
Bobby, could you explain the math behind these calculations. Just curious. Thanks


I will try. Here is a picture that will help:

--- Invalid image id: 106614971 ---

In the picture, [;W;] is the weight of the climber, and [;T;] is the tension in the webbing.

Look at the second figure. We want these forces to be balanced. Doing the trigonometry reveals
[;T = \frac{W}{\sqrt{3}}\approx 0.58 W.;]


Now look at the third figure. Those forces add up to a resultant force whose magnitude (again using trigonometry) is [;\sqrt{3}T.;]

Therefore, in the ADT, each bolt sees a total force equal to [;W;], the climber's weight.

If we had two equalized slings, there would be no horizontal tension element, and the load would simply be
[;T = \frac{W}{\sqrt{3}}\approx 0.58 W.;]


I hope this helps.


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By kBobby
From Spokane, WA
Nov 29, 2009

EdAsh wrote:
I can't believe you guys actually worry about such stuff... But then again I've rapped off some weird stuff: questionable chock stones, small bushes and very small trees, and some imagined anchors (I don't want to boor you with details)... but I am still here on the planet (well mostly?). If you're really that into safety maybe you should just carry a bolt kit when you sport climb (just kidding ;-) but I wouldn't care if you improved some rap stations.) Please forgive me for this next statement: Some folks can climb, and others are gear heads... cheers, ...

Ed, I actually never worry about such things. Someone asked a question that I can answer with a very very easy calculation. Why not answer it?


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By Greg D
From Here
Nov 29, 2009
Out of the blue.  Photo by Mike W. <br />

In the op's scenario: Simply put, load "multiplication" takes place when the angle formed at your belay device is beyond 120 degrees when rapping. With a 2 foot spread in you anchor points you would have to put yourself onto rappel with only 1.15 feet of rope coming from each anchor, an unlikely and difficult task. As your distance from the anchor increases the angle at your belay device must decrease.

If the anchors are not worth worth your body weight or two or three times that, they are not worthy in the first place.


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By JPVallone
Nov 29, 2009

Angles
As angles in the anchor increase so does the forces applied to each placement.
Angles like this are not only created in the construction of complex anchor systems,
multiplication of force can be created when tying around boulders or large trees and
tying them off too tight. Remember to check for these angles in all anchor setups.
As a rule, it is advisable to keep angles under 90. As angles increase, loads will
also increase on each component, so these components would need to be built to
withstand such extreme forces.
Bottom angle Load per anchor (V arrangement) Load per anchor (triangle arrangement)

en.wikipedia.org/wiki/American_death_triangle

As a rule I try never to trust my life to one anchor point, there are exceptions with good judgement and experience

Bolts are fixed gear, to the invincibles in this thread, Bolts do fail, They should be treated and assesed just like any piece of fixed gear. I know people that have been pretty messed up due to bolt failure. Who put that bolt there? was it there first bolt or there 500th bolt, What kind of bolt is it, Was it over tightened, is it undertightened, is it the right depth and diamater for the rock?

Unless extreme emeregency or proper knowledge of the placement and history of the bolt, I would be shaking to Rap off of one bolt I didnt know about and would avoid it and first find the alternatives


here is another good link for some good info on angles and forces

www.uoregon.edu/~opp/climbing/topics/anchors.html


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By kBobby
From Spokane, WA
Nov 29, 2009

EdAsh wrote:
Bobby, I just looked at some of your links.... You're a cool dude imo (but maybe that's the kiss of death coming from me). Number theory...seems to be missing ... It can be cool and draws from all sects of mathematics (for give me for being the prof, and maybe you're one too?) cheers, eduardo

Thanks, Ed. I was briefly interested in number theory ten years ago. Not really anymore. I have a strong affection for geometry. Where are you a prof? (I'm not, btw).
[;\sum_{n=1}^\infty\frac{1}{n^3};]


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By Mike Anderson
From Dayton, OH
Nov 30, 2009

This should be in the Trad forum. Sport climbers don't worry about stuff like this.


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By half-pad-mini-jug
From crauschville
Nov 30, 2009

Agreed Mike, depending on the type of bolts, each one holds anywhere from 3000 to 7000 lbs... I don't think you need to worry too much...


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By Forestvonsinkafinger
From Iowa
Nov 30, 2009

The American triangle sucks because it is American. The European death triangle is far superior because it has been around for thousands of years, and has killed more than its American counterpart. It is also le plus joli than the practical american triangle de laid.

Celtic Anchor <br />
Celtic Anchor


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