Technical Articles | Anchors Part 1
This will be the first of a two-part series on anchors. This section, purposely, will not include situations that necessitate vehicles, pickets or other such portable anchors. Nor is the series meant to be an exhaustive source of information… there are numerous books on this subject. If you have any questions regarding the below information, please feel free to contact RRG.(Excerpts taken from “Rigging in the Vertical Realm, Thorne and Rhodes; The North American Working at Height Handbook 2004, North Sea Lifting Ltd.)
Considerations in building an anchor
Anchors are truly the most important aspect or component of any system when used to lift, perform work or rescue someone. Without a solid base or foundation, the entire project is in jeopardy. Whether you’re dealing with natural or artificial components for the anchor system, take special care and make sure the integrity and strength of the component is not in question. In times of question, backups, opposition or back-ties must be employed (this will be discussed later). In building an anchor system, it is best to keep the line of pull directly “above” (or fall line) the load. When this is not doable, using advanced placement skills to focus the main direction of the anchor to a viable position will be needed (such as re-directs).
Marginal anchors are those that are generally questionable and believed not to be able to hold the entire force or load (including a margin of safety). These include (but are not limited to) corroded or weather materials, small-diameter trees, or rock features.
Bombproof anchors have been determined to be sufficient for all component requirements and will hold any load or fall (while maintaining a margin of safety at all times).
The anchor systems themselves will generally fall into three categories; self-distributing (or self-equalizing), multi-point or fixed multi-point (load sharing). Employing a critical thinking test is mandatory in examining the type of system to be built. Will it pass the critical point (in terms of failure)? Don’t always assume that the line of fall is always down; a belay anchor to a climber is vastly different to that of top-down lower, as is that of a traverse (will be put lateral forces on the anchor). Force will be applied to all aspects of the anchor system (compression, tension, and friction). Hidden factors that need to be looked at are:
- The focus of the anchor (direction of pull)
- How many points of contact
- Friction (on or within the system)
- Cutting or breaking
Critical thinking and using an Anchor Tension Formula
Though physics is applied to all aspects of anchor systems a general understanding of forces applied is more than just a good idea. As a general rule for building any multi-point anchor system (which will be discussed later), the angle should be kept at 90 degrees or less. At this angle, each of the two-point components will see 71% of the load; at 60 degrees, each two-point component will see only 58% of the load. At the extreme (such as highlines), and angle of 180 degrees (though this is not actually achieved), the two-point components will see 56 times the force or load!
There can be considered three anchor types:
- Point Anchors
- Linear (tensioned) Anchors
- Linear (non-tensioned) Anchors
Point anchors are specific to one location (a crack, pole, tree etc…). Several point anchors can be serviced to build an anchor system but must be interwoven to join all points together to prevent the total loss of the system.
Components that are tensioned between two point anchors are called linear anchors (tensioned); such as a back tied anchor. Normally consists of multiple bundled ropes, which serve to reduce rope stretch (in conjunction with a pulley system).
Those anchors, which remain without tension between two point anchors and provide only “back up” to the main anchor, is a linear anchor (non-tensioned).
Peace on your days…