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Technical - Anchors, Pt. 2
This is the second of a two part series on anchors -- the first part is available here. This article will intentionally omit situations which 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.; "The Tree Climbers Companion, 2nd Edition Revised and Expanded" - Jeff Jepson)
Fixed Multi-point (Load Sharing) Versus Distributing (Self-Equalizing) Anchors
The debate over multi-point/load sharing (MPLS) and that of distributing (self-equalizing) DSE anchors is one of focus and elevation (high directional). The debate may have quieted down a bit over the years, mainly due to many "anchor" policies within the guide service, rope access and rescue teams... never the less, the problem -- and thus confusion -- is still there.
In general, with a DSE anchor, if the load shifts, the legs of the system will "equalize" and continue to share the load. The potential problem is… what if the legs don’t alter with the shift. A failure at a compromised anchor could prove monumental. Many times, this issue is found with "ground dwelling" rigging; no high directional (natural or artificial). A small drop of 1' could bring as much as 5x the load onto any given anchor. This could prove disastrous. One way this happens is when the DSE anchor system actually locks up under extreme forces. Last month, we went over how much of this could be mitigated. Fixed multi-point anchors are almost always the best way to go. For more on this, see Part One if this article in last month's Newsletter (05/2005).
So now that we have the proper anchor foundations in mind... lets move on to a few sub anchor set ups. These could be considered, for some, more advanced anchor considerations. They are back ties, oppositions, and floating anchors.
Back Tie Anchors
Many times a questionable anchor can be reinforced into what would be considered a bombproof anchor... one way this can be done is through a back-tie. Typically done by using a Wrap3 Pull 2 technique on both the primary and secondary anchors and connect them by using a 3:1 MA (non working and very inefficient MA using carabiners). A jigger or AZTEK kit can also be used very effectively (but normally in combination with a non working system).
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A pre-tensioned Back-Tie in an industrial setting using one AZTEK jigger and one non- working MA system. |
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A pre-tensioned Back-Tie in a backcountry setting using one AZTEK jigger and one non-working MA system. |
Diagram of a Non-working 3:1 Mechanical Advantage Back-tie1 |
Opposition Anchors
Opposition anchors do what the name implies... They pull opposite of the direction of activity. Opposition anchors (OA) are often used to secure multi directional pulls (whether they are rescue systems, climbing systems or working systems), serving to keep or hold positions with no slack in the line or anchor legs.
Floating Anchors
Floating anchors are constructed in harmony with the combination of multi-point, back-ties and opposition anchors. In general, these anchors are focused in a needed position or direction where no natural anchors are found. Also, floating anchors provide suspension where horizontal movement must be controlled.
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A detailed diagram of an opposition anchor, as used in localizing a floating multi-point anchor for use with an artificial high directional.2 |
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A floating multi-point anchor, using two AZTEK jiggers and one non-working 3:A MA. |
Redirects
There are two general techniques for redirects: artificial and natural. Mechanical redirects are rigged by choking a sling to a life rated anchor (wood or steel) with a girth hitch or basket hitch. A natural redirect is used many times with arborists by using the natural features (a natural crotch in a tree for example) to accomplish their goal. The techniques are important and applicable to all rope technicians, as they provide a safer and more comfortable working environment.
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Example of planning a natural redirect to facilitate a safe and comfortable work location.3 |
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Example of using an artificial redirect, preventing the need for either dangerous over-extension from the anchor, or recrotching the climbing line.4 |
In conclusion, I think it can be said that anchors are the most critical components of any rope rescue system. They are the true foundation upon which all other components depend. Choosing an anchor (natural or artificial) and rigging it properly for each scenario -- these are elements of prime importance. So be critical in your thinking, know your limitations as well as the resources available to you; then build it as if your life depends on it… because it does.
Bibliography of Resources
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