Anchoring: Death Triangle

[PhysWiz]

I was up the canyon over the weekend doing some rappelling and noticed an anchor that a group near by was using. They were using two bolts with hangers maybe 12" apart, each hanger had a biner attached to it with the rope running through both biners and they were rapping double strand. I've seen this setup used a few times and I think it's somewhat commonly used by climbers. Isn't that a death triangle? I'm sure the anchor isn't going to fail but that can't be best practice. What do you think?

It looked like this:

or like this:


You can see in the first picture the angle is less severe and with a little more extension it wouldn't be an issue at all.

[ratagonia]

I was up the canyon over the weekend doing some rappelling and noticed an anchor that a group near by was using. They were using two bolts with hangers maybe 12" apart, each hanger had a biner attached to it with the rope running through both biners and they were rapping double strand. I've seen this setup used a few times and I think it's somewhat commonly used by climbers. Isn't that a death triangle? I'm sure the anchor isn't going to fail but that can't be best practice. What do you think?

It looked like this:

or like this:


You can see in the first picture the angle is less severe and with a little more extension it wouldn't be an issue at all.

Yes it is an American Death Triangle, of sorts.

Disturbing in the second picture is that the rope is threaded the wrong way through the far ring. Was this an actual anchor in actual use?

Perhaps as a Phys Wiz you would throw up a free-body diagram and explain, for the class, what is inappropriate about this setup.

Then we can discuss further from there.

Tom

[Sombeech]

So what makes this so dangerous? Is it the angle at which the rope glides through the rings?

[PhysWiz]

Yes it is an American Death Triangle, of sorts.

Disturbing in the second picture is that the rope is threaded the wrong way through the far ring. Was this an actual anchor in actual use?

Perhaps as a Phys Wiz you would throw up a free-body diagram and explain, for the class, what is inappropriate about this setup.

Then we can discuss further from there.

Tom

That picture wasn't of the actual anchor but it was very similar. I will do my best with some pictures I found online. I'm not the greatest at explaining things in writing.

The diagram shows the weight of the object being transmitted through the rope that is running through two anchor points. If this was a proper V shaped anchor the weight would be split evenly between the two anchors. However the force on the anchors isn't always going to be 1/2 the weight of the object. The two anchors are exerting equal and opposite forces against each other, each is pulling the other toward it's self.
In a death triangle setup you now have a horizontal line running between the two anchors, this increases the horizontal force on the anchor. If you take a look at the right anchor on the diagram. The blue arrows show the forces exerted on the anchor from the rope and the red arrow shows the total force on that anchor. The total force on the anchors is almost 175% of the weight of the object. If it were a V anchor it would be close to 55%. Its all in the horizontal forces. The forces that you don't think about because you don't see them.
Hopefully that made a little sense . Clear as mud right.

[kiwi_outdoors]

"V" is good, "triangle" is bad.

That's all you need to know.

Keep it simple.

Like riding a motorcycle: "push left - go left". (words to live by)

P.S. I am also a structural engineer, and I can do the math.

[ratagonia]

Can't say I am the best at either the math or the explaining, but...

I get a resultant, using the same angles you do, of 174lbs, give a 200 lb rappeller.

What is the liklihood of these bolts failing at 174 lbs? At the most perfect, the bolts would only see 100 lbs each.

A normal sport-climbing lowering would put pretty much twice the load on the anchors, or 282 lbs each. (=2*cos(45 degs)*200 lbs).

Modern, well placed bolts in good rock are generally worth about 5000 lbs. Even using not-best technique and making an elevated Resultant, you are not getting to loads on the anchors where you have to worry about the bolt failing due to excess load.

I have seen this scenario go wrong, and I believe this configuration has fallen out of favor, because pulling your rope through one ring, across and through another ring seems to result in the rope twisting up badly, and at times has gotten the rope stuck because the twisties form a tangle before the end gets pulled through the first ring.

Tom

[Bo_Beck]

Can't say I am the best at either the math or the explaining, but...

I get a resultant, using the same angles you do, of 174lbs, give a 200 lb rappeller.

What is the liklihood of these bolts failing at 174 lbs? At the most perfect, the bolts would only see 100 lbs each.

A normal sport-climbing lowering would put pretty much twice the load on the anchors, or 282 lbs each. (=2*cos(45 degs)*200 lbs).

Modern, well placed bolts in good rock are generally worth about 5000 lbs. Even using not-best technique and making an elevated Resultant, you are not getting to loads on the anchors where you have to worry about the bolt failing due to excess load.

I have seen this scenario go wrong, and I believe this configuration has fallen out of favor, because pulling your rope through one ring, across and through another ring seems to result in the rope twisting up badly, and at times has gotten the rope stuck because the twisties form a tangle before the end gets pulled through the first ring.

Tom

I believe I'd be more concerned about the alignment of the "Hanger" as shown on the top anchor. If the carabiner were to slide on top of the right side hanger it would create "Leverage" as more of a "Pullout" opportunity as compared to a "shear force" opportunity. Granted, the pullout of a well placed quality steel bolt is substantial, I fear that often "resultant forces" are overlooked in multiple anchor points brought to a focal anchor point?

[Scott P]

Here's a question for physics/climbing experts. What would be the advantages/disadvantages of placing bolts one above the other instead of side by side? I can see several advantages of placing them one above the other, but I seldom see this out on climbing routes or in bolted canyons.

[PhysWiz]

One above the other would defiantly reduce the load on each anchor. That seems like a logical configuration but I'm sure there is a reason, that I can't think of, why it's not done that way. Maybe the load on the rock stacking up?

[Brian in SLC]

Yes it is an American Death Triangle, of sorts.

Naw...not really. Here's one (stole from your site):



What you got is a rope going through both anchor and extending to the ground. Not a fixed triangle at all. Angle on the rope goes to fairly nil not far below the anchors. Very common climbing anchor. Most folks who'd use it to climb on would extend each side down with a sling or long quickdraw, and, top rope or climb from that.

Yeah, vertical better:



Folks hate the single ring, even if its rated stronger than their harness, rope, belay device...(!).

I prefer a vertically oriented anchor for most things. Rock tends to be layered horizontally and spacing anchors vertically could gain better rock in a different vertical position, methinks. Also, no angle to worry about.

[ratagonia]

One above the other would defiantly reduce the load on each anchor. That seems like a logical configuration but I'm sure there is a reason, that I can't think of, why it's not done that way. Maybe the load on the rock stacking up?

I think it comes from tradition.

On regular trad climbs, where both people will be at the anchor, spreading the bolts horizontally makes the anchor easier to work with (as there is a left side and a right side; two people hanging from the anchor are not hanging atop each other, but one from one bolt and one from the other). And that is the way it was done for a long time.

Can also be easier to place two bolts at the same height, when drilling by hand (less so by powertools). When choosing where to place bolts, the biomechanics of the stance and the location are important.

But yes, the Vertical orientation of the anchors often works better. Especially on Sports Climbs. And especially if people have an understanding of how the vectors work.

Tom

[ratagonia]

Naw...not really (an American Death Triangle). Here's one (stole from your site):

What you got is a rope going through both anchor and extending to the ground. Not a fixed triangle at all. Angle on the rope goes to fairly nil not far below the anchors. Very common climbing anchor. Most folks who'd use it to climb on would extend each side down with a sling or long quickdraw, and, top rope or climb from that.

Well, it is a triangle, with the angle changing as the rappeller descends.

I think the "key" point of the ADT is the line of tension from one bolt to the other. Which is present in this geometry. I think my trig shows that it is not a concern in this case.

Thus, "of a sort".

Tom