American Death Triangle
How bad is it?
This is typically a two point anchor system, usually involving bolts. You would use a single piece of webbing or rope and create a continuous triangle like the illustration below. The ‘load attachment point’ would be a carabiner (typically) at the bottom of the illustration.
(The blue lines show the direction of force placed on the anchors when a load is applied to the ADT.)
With all two point (or more) anchor systems the goal is to equally distribute the forces of the load so that each anchor supports its share of the load. The American Death Triangle accomplishes this. The problem with ADT, as we know, is that it is a force multiplier. Instead of having each of the two anchors support ½ of the load, each anchor has to support nearly twice the load (based on a 120 degree angle) because of the physics involved.
The following table shows the percentage of the load's force that is transferred to the two anchor points with the ADT compared with the standard ‘V’ type anchors that we typically use.
Load per anchor – based on 100 pounds of force
Bottom Angle |
Load – V Anchor |
Load - ADT |
0 |
50 |
50 |
30 |
52 |
82 |
60 |
60 |
100 |
90 |
70 |
130 |
120 |
100 |
190 |
150 |
190 |
380 |
So, aside from being a force multiplier, the ADT will work. With a piece of 11 mm rope and a knot you still have 20 kN of strength left, which easily meets a 10:1 safety ratio. (Based on a 200 pound load or less and 120 degree angles within the ADT.
Aside from the magnification of forces, the ADT violates several anchor construction principles. These include:
- Redundancy: if the webbing fails on one leg of the anchor, the entire anchor will fail.
- Extension: if one of the anchors fails, the webbing will extend and will shock load the remaining components of the system. Depending on the size of the ADT the shock load could be huge.
- Third, and perhaps most important, the ADT is a force multiplier. If all things are ‘static’ and there is no dynamic motion in the system, things may work out ok. However, when things are dynamic it gets ugly quick.
According to Schafer’s study, a 1-foot drop of 400 pounds will generate nearly five times of force at the anchor. Assuming the ADT has three angles of 120 degrees each a drop of this nature would create a force on each ADT anchor of approximately 4000 lbs, or 18 kN. (400 pounds x 2 (resulting form the ADT physics) and this force would be multiplied by five due to the 1-foot drop) This comes very close to destroying the 11 mm rope as well as the two bolts.
There are some situations where an ADT may actually be desirable, mainly in the rock climbing world when a climber needs to have a strong opposing force to keep chocks or camming devices in place. However, outside of that it does not have much practical use.
It’s obvious that if things remain static the ADT isn’t that bad. But with a very simple change in forming a ‘V’, the anchor becomes much better and offers far greater safety margins than does the ADT.