Deflecting angles and high directional anchors
“Tripods”, “bi-pods”, “mon-pods”, “gin poles”, and “A-frames” are common names for a category of anchorages known as artificial high directionals (AHD's). These anchor types are often essential for safely raising or lowering in a rescue operation, especially while working in areas that have limited options for overhead anchor points. The uppermost platforms on industrial towers, or cliff sides lacking available anchorages present rigging difficulties to rescue teams. AHD’s, which can be engineered for industrial rescue or constructed using natural materials in the wilderness, can greatly enhance the safety of a technical rescue by adding an element of control to the operation.
Negotiating whats called “the edge transition” remains THE most delicate step in a technical lowering operation. This is commonly the moment where things have the greatest potential of going awry. For edge transitions high directional anchorages offer security, maneuverability, and give rescuers options as they attempt to move from a level staging area to vertical terrain.
The word “tension” probably best describes the value of high directionals. By utilizing a higher anchorage we are able to pre-tension the rescue system (actually lean into and fully load on flat ground) effectively testing our entire technical operation before we commit to being over the edge. The amount of safety and reassurance this adds to our rescue team is substantial. Pre-tensioning provides us the advantage of seeing just how every component loads and settles into position as the weight of the rescue load is applied to it.
In the absence of high anchor points we would have to approach the edge with some slack in our system, commonly resulting in a "semi-dynamic" event- as the edge is being negotiated there is typically a 2-3 foot settlement distance as the system becomes fully tensioned.
If you’ve ever rappelled while using an anchor at the ground level then you'll quickly remember how the rope rises roughly 3 feet from the anchor to your rappel device at your waist. As you move toward the edge and start your rappel you must delicately walk yourself over the lip of the cliff until that 3 feet is taken out of the system; trying to avoid an abrupt drop in the process. This is difficult to do as an individual on rappel, let alone as a rescuer trying to control an occupied litter. Evidence of how awkward and dangerous these transitions can be are easily found on the internet.
High directional anchorages, whether natural or engineered, alleviate these difficulties provided that they are constructed correctly and with due consideration to the resultant forces that occur when we begin to deflect our rescue load.
High Directionals are not usually “anchors”
Typically, an artificial high directional acts as a “deflection” ( or “deviation”) and not necessarily as an “anchor” of the rescue load. In other words, unless we are building our primary anchors directly off of the AHD (which is seldom the case-with the exception of winching tripods used in confined space rescue) then the high directional assists in deflecting the patient package above or over an edge. The advantages of this are mentioned above and also includes the benefit of reducing the level of contact the tensioned rope has with a potentially abrasive edge. Putting a 90 degree bend in a tensioned rope as it runs over a sharp surface is a recipe for disaster. Lets examine some of these details further starting with angles.
First off, lets establish a baseline for understanding angles and the forces that they either distribute or amplify. Most of us are familiar with the optimal angle for establishing our anchors: an interior angle of 90 degrees or less. Lets do a quick review of that now. . .
Lets assume we have a two piece (multi-point) anchor selected to evenly distribute the weight of our intended load. To keep the math easy lets also assume that the load is 200 pounds. At a 45 degree interior angle the load being supported by this two-part anchor is evenly distributed; each of the two anchors is holding 100 pounds of the load. This is optimal.
If we increase this interior angle to 90 degrees (perhaps our two anchors are spaced further apart than in the first example) then each of the two anchors now sees roughly 150 pounds of that 200 pound load. While this is completely fine in many circumstances (after all we've still reduced the amount of weight each anchor sees below the 200 pounds of the actual load) we are moving further away from an even load distribution.
If our interior angle increases beyond 120 degrees then we actually begin to amplify the forces each of our two anchors see. The two anchors are experiencing compounding forces. Forces pulling perpendicular from the direction of the initial load and additional forces from the anchors wanting to collapse inward toward each other.
With deflections (also known as deviations) its important to recall that anchors are just that: fixed (non-mobile) points of attachment for our ascending, descending, lowering or raising operation. I'd like to stress the word “fixed” because its essential in understanding how angles in deflections differ from interior angles formed when constructing a multi-point anchor.
Lets examine a couple of details using the image below as a point of reference.
1. The rescue load at the ends of our ropes are not a fixed anchorage. To be clear, the ropes are indeed fixed to the patient package however, they are constantly moving around a pulley at the master point (the head of the tripod) thus we lose the compounding effect we would otherwise experience with fixed anchors. The load at the primary anchor (out of view) will remain constant when lowering.
2. The deflection angle is measured at the intersection of the rope entering the pulley to the outside of the bight. In other words, imagine the rope leaving the lowering device at the anchor. It moves up toward the high directional pulley (head of the tripod), around the pulley and down to the litter being lowered. Reset. Now imagine the continuation of that rope traveling from the anchor on a straight trajectory and not bending around the pulley- just continuing on into space. The intersection of this imaginary trajectory and the rope moving down to the litter is the angle of the deflection.
Some points of reference using a theoretical 500 pound load:
• A deflection angle of 20 degrees will place 34% of the load onto the AHD.
• A deflection angle of 45 degrees will place 76% of the load onto the AHD.
• A deflection angle of 60 degrees will place 100% of the load onto the AHD.
• A deflection angle of 90 degrees will place 141% of the load onto the AHD.
The load at the primary anchor will remain a constant 500 lbs. during the lowering operation regardless of the deflection angle.
*Less efficiency at the high directional anchor ( i.e carabiners instead of pulleys) yields a slight reduction in load. 141% load at a 90 degree deflection angle becomes closer to 120%.
*If you want to DECREASE the load on the AHD then you would work to INCREASE the angle highlighted in blue by adjusting a) your tripods height and b) the proximity of your primary anchor.
While the load on the lowering system anchor remains constant in spite of the angles referenced above, an increase in load does occur on the high directional itself. This will occur most commonly because of 2 primary factors.
The higher we extend the high directional the larger the deflecting angle we create causing an increase in the forces.
A straightforward example of this happens when we preform a vector pull on a tensioned rope. This is a common rock climbing tactic to assist a following climber trying to make a difficult move below you. You can simply pull the rope outward (perpendicular) a short distance away from the rock face while it is under tension. This "vectoring" of the rope requires only a little force but can help "tug" a climber up a few inches which may be enough to see him through that difficult move.
Now, if we were to try vectoring the rope in this same way but instead of a short distance we attempted to vector outward 6 feet it would become progressively harder the further away we tried to pull. We, in the act of trying to help our climbing partner up through the difficult move, have become the deflection for the rope.
2. The second common way we can inadvertently increase the load on our AHD’s is by operating our lowering function (with an MPD or SCARAB for example) closer to the AHD.
The closer we move our lowering/raising station to the high directional the greater the forces on the high directional. If we were to set up our lowering system directly underneath the tripod we could effectively create a 2:1 mechanical advantage on the head of the tripod, turning our 500 pound rescue load into a load reaching twice that amount. The further away we position this lowering/raising system away from the tripod the shallower the angle becomes decreasing the load on the high directional anchor.
Tripod stability is essential
Often our choice of AHD's are manufactured solutions such as CMC’s Arizona Vortex or SMC’s TerrAdaptor- devices which are more than capable of safely withstanding the forces encountered in a technical operation. However, improvised methods are also available in more remote settings where the luxury of a manufactured AHD’s are absent. Its critical to observe where these forces will develop in your system if you elect to construct your AHD from, for example, a pile of logs in the wilderness - a method taught in some technical rescue courses. Here’s why:
Lets assume a rescuer weighing 180 pounds is preforming a rescue of a 180 pound victim using an improvised AHD that deflects the rope 6 feet off of the ground.. If this AHD fails the rescuer and patient will fall 6 feet or until a backup system catches them. If there is not a secondary line in place, or if both main and belay lines have been run through the AHD then the arresting forces on the anchor could reach 15 kilonewtons or 3,300 pounds (with low-stretch rope). This force may be enough to result in a catastrophic failure of the primary anchor.
Additionally, if the rescuer is directly attached to the ropes using static materials or the low-stretch rope itself (common is some pick-off maneuvers) then the rescuer could see a potentially fatal arresting force resulting from sudden stop trauma (the force would be 7.5 kn to the rescuer in this scenario). For this very reason 8 kn is the maximum allowable arresting force a worker/rescuer can experience in the eyes OSHA. In Europe and Canada the maximum allowable arresting force is set more conservatively to 6 kn.
In the above scenario not running both lines through the AHD when its stability is in question is an important safety consideration. Moreover, using dynamic materials or lanyards (such as a Petzl Jane) to connect the rescuer to the patient package is also wise.