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Week 8 - Anchor Principles
 

Jay Raps off the Columns, Eugene, OR










 

PEOL 252
Wed. 2:00-4:00: Indoor Climbing Facility

Instructor: Ryan Ojerio
Phone: 341-1485
Email: ryan_ojerio@hotmail.com

Anchor Principles

Building anchors is a complex topic with many variables that are beyond the scope of this website. However, understanding a few basic principles will give you a good foundation to build on and help you avoid some of the most heinous mistakes. With the information provided here you should be able to construct a position equalized and self-equalizing anchor given two fixed bolts using a cordelette or webbing. You should understand the pros and cons of each and be able to evaluate an anchor for rappelling.

All anchors whether they are for belaying, top roping, or rappelling should adhere to the concept of S.R.E.N.E.

 

Strong: Good anchor systems are built off of solid components, such as a good bolt, stout tree or immobile boulder.

Redundant: Anchor systems must be constructed of multiple components so that if any one component fails, the anchor will not fail.

Equalized: Building an anchor system so that the load is shared by all of the components decreases the chance that any one component will fail.

No Extension:If a component does fail, the anchor system should be constructed so that the remaining components are not shock loaded.

Using cord or webbing you can connect anchors together to equalize them and create redundancy, but also they are useful to adjust the length and position of the system so that the rope is not rubbing across the rock.

 

Intuitively you might think that two anchor points will split the load 50/50 in an anchor system. In fact, the geometry of the system determines how the load is distributed in an equalized system.

Consider the diagram at right. Of the four examples, which angle is will create the safest anchor?

When building an anchor system, how can you adjust the angle to ensure that you don't create an unsafe system?

 

 

Top Rope Anchors With Webbing

American Triangle

Rather than distributing the load between the two bolts, this setup actually puts greater force on each of the bolts because of the mechanics of the triangle. Doubt it? Get out your calculator, physics and trigonometry book, and look of the chapter on vectors.

What's wrong with this anchor?

What will happen if one of the bolts or carabiners clipped to the bolts fail? What will happen if the webbing fails?

Twist and Clip

By twisting and clipping, the rope's carabiners are clipped into the loop of the webbing so that if one of the bolts fails, at least the loop will catch the rope.

Will this anchor extend if one of the bolts or carabiners fail? How far?

What happens if the webbing is cut by a sharp rock?

Self-Equalizing Anchor

1. In this setup the anchor will equalize in a variety of positions with out having to be retied

 

2.

Extension!

If one of the bolts or carabiners fail, the loop will catch the rope, but notice the extension that will severely shock load the remaining components of the system.

If you tie an overhand knot in the webbing before clipping it the anchor it will still self- equalize, but won't extend as far as in the preceding example.

Furthermore, if you tie off both pieces of webbing you create redundancy because if the webbing is cut in one spot, the knots keep the anchor intact.

Self-Equalizing anchor tied off to minimize extension.

Mmmm....don't have enough carabiners, what to do? (buy some more!)

Girth hitching the bolt as in this picture reduces the risk that it will slide across the sharp edge of the bolt. Maybe OK for rappelling, but DO NOT belay or top rope off of such a setup! Is this anchor system redundant?

Position Equalized Anchor

A quick way to equalize an anchor in a single position is to tie off the entire bunch of webbing with an overhand or figure eight (easier to untie after loading).

With this configuration the anchor doesn't extend very much at all should one of the bolts or carabiners fail.

Cordelettes are great for building anchors too. You can build position or self-equalizing anchors just as with webbing.

Create a loop by tying the ends together with a double fisherman's or figure 8 follow through, which is easier to untie after loading.

With a long cordelette you can equalize multiple anchor points, such as in this position equalized anchor.

 

Clipping the Rope

Attach the rope to the system with two carabiners for redundancy. Constructed properly, the anchor system will hang the biners over the edge and not be hitting against the rock which may open the gates. Always clip the carabiners so that the gates are opposite and opposed.

A. These biners are neither opposite nor opposed. It is possible that they could hit on the rock and open simultaneously, significantly decreasing their strength or allowing the rope to pop out.

B. These biners are opposite. Biners that are opposite are less likely to open simultaneously if they slap on the rock, but one of the biners could rotate 180 degrees and you would end up with the situation pictured in A. above.

C. These biners are opposed, but not opposite. The rope is unlikely to slide out in this configuration, but like in A. above, if they hit on the rock and both gates are pushed open, they are significantly weaker than when they are closed.
D. These biners are opposite and opposed. This is the best what to set up your biners, way to go!

 

Rappelling Anchors

Its time to retreat on rappel and you don't want to leave any gear behind. You might have to make a choice between your life or your wallet; personally I'd rather be poor than dead.

Bad Idea. This setup creates the dreaded American triangle and the rope will probably be really hard to retrieve because of the friction as it runs through the sharp bolts.

There are at least two problems with this set up.

What happens when one of the bolts or a single chain link fails?

What angle does the chain make between the bolts. Is the load shared or magnified?

Better!

And this rope will be much easier to retrieve.

Okay, you've got a better angle to split the load, but its not redundant because the entire system fails if the webbing breaks or one of the bolts fail.

Better.

Two pieces of webbing, equalized. You might end up with a lot of friction when you pull the rope through that much webbing. It might make it easier to retrieve if you tie in a rappel ring. Costs a little more, but hey, you're worth it baby!


Links

Rock 1 Website

SmithRock.com

PARS Outdoor Pursuits Program
Recreation Center Rockwall
Physical Activities and Recreation Services (PARS)
Outdoor Program

Jay and Kalub at Sisters Boulders
 

Design and Photography: Ryan Ojerio                June 1, 2002