Rope rescue: Understanding resultant factors
Here are the key principals to making secure high-directional anchoring systems for rope rescues
We covered the importance of high-directional anchors in the rope rescue environment last month. The primary focus was the reliability of the anchor from a design load or strength perspective.
Here we’ll take a deeper look at this issue from a more technical perspective. Many times, we do not have a high-directional anchor that the rope gods have simply placed in position for us. These scenarios forced rescuers to use manmade options and deploy portable anchor systems like tripods, bipods and monopods.
Understanding both the capabilities and limitations of these systems is one of the biggest challenges for rescuers. We can break it down into two essential concepts.
This term refers to the surface area that exists between the feet of the portable anchor. For a tripod, this is identified by drawing lines between each of the feet that results in a triangle. The interior surface of the triangle is the footprint.
If using a bipod, the footprint becomes a line between the two feet. When using a monopod, the footprint is simply a point directly under the one foot.
The footprint is essential because the forces that are eventually applied to the portable anchors must be translated to resultant forces within the footprints. The more centralized the resultant forces are within the footprint, the more stable and equalized the anchor.
In field terms, this is easiest to understand with a tripod. Setting up a tripod with all legs equal lengths on flat ground, gives a simple pyramid having a triangular foot print with the center top point of the tripod is dead center above the footprint.
If we then hang a simple system (4:1/5:1) from the tripod and haul or lower by pulling straight down, the resultant force will be directed straight down to the center of the footprint. However, pulling laterally on the system instead of straight down, may create instability in the tripod and topple it in the direction of the pull. This all depends upon the next essential theory.
This mathematical term describes the magnitude, force and vectors. Without going into all of the equations, formulas and coefficients, the lines coming in and out of the pulley at the top of the portable anchor create an interior angle. The resultant that we are looking for is the imaginary linear force that bisects this angle.
To better see this, envision a rope that is attached to a lower/haul system some distance behind the tripod. The rope travels up to a pulley at the top of the tripod and redirects down into a manhole at the center of the tripod footprint.
The interior angle created by the section of the rope coming in and the section of the rope going out of the pulley is 60 degrees. The resultant would then be a line of force that goes from the pulley to the ground at a 30-degree angle.
The point where this imaginary line hits the ground must be within the footprint or additional rigging is needed. As soon as the point is outside the footprint, establish elements that resist the forces being applied. There are two basic ways to resist these forces.
The first option is to redirect the pull so that the resultant is redirected within the footprint. Using the same scenario, instead of the lower/haul line going directly to the head of the tripod pulley, attach a change of direction pulley near the ground at the center of the footprint.
With a manhole entry as the point of access, attach an anchor strap to a suitable anchor within the hole, such as bombproof anchor rungs, and then attach it to a pulley right above the surface of the hole.
The lower/haul line would the redirect up through this pulley the head of the tripod pulley. This places the resultant point right back in the center of the footprint.
The second option is to create adjustable back ties. A self-minding simple system works great for this application.
Using the same initial scenario, the resultant force would be behind the footprint and the pull would culminate in the tripod falling over backward. To resist this backwards motion, apply back ties in the opposite direction.
Attach the tensioning system to the head of the tripod and connect it to an appropriate anchor toward the front of the tripod and tension it. This tensioning process requires a load to be established so that the tension applied is commensurate to the load.
Unequal forces can quickly destabilize the tripod. This is why it is important to use adjustable back ties. More simple rigging solutions can be used with simple rope and Prussiks, but they often lack efficiency when it comes to adjusting the tension.
These fundamental theories can be applied to more complex situations. Often times, we need to beam out or articulate the portable anchors.
In the tripod application, this involves extending one leg out, leaving an upright A frame or bipod with a long kick stand behind it. This will move the high point closer to an edge line.
Using the previous theories, we can surmise that the load will now be in front of the footprint. This does not mean that back ties or COD pulleys are required. It means that we must analyze the resultant.
If the anchor is established behind one extended tripod leg and the rope of the lower/haul system is run up through a pulley at the head of the tripod and then down to the load, look at the interior angle created by the rope and bisect it with an imaginary line from the head pulley to the ground. Look at the footprint of the tripod, if that point is in front of footprint and outside of its boundaries, either change the angle of the rope or apply back ties.
A bipod requires tension in two directions. This can be accomplished in a variety of ways, but the essentials don’t change. When the bipod is beamed forward, it immediately requires back ties toward the rear.
Once the head of the bipod is past the linear footprint, the feet of the bipod will be unstable and will want to move back toward the anchor. This can be managed by back tying the feet toward the front of the bipod or by driving pickets or earth nails through the feet to create shear resistance and prevent this motion.
Some portable anchors also offer variable feet that include claws or spikes for this scenario. In a more advanced depiction, a portable anchor may be used in a high line application to elevate the track lines.
Here, adjustable back tie systems are almost imperative. As the load moves across the track lines, the angle of the track lines coming in and out of the head of the anchor may change. For example, as the load gets closer to the portable anchor, the interior will reduce and as the load moves away from the anchor the interior angles will increase.
We now know this directly impacts the resultant, which is the driving force behind the back ties and securing systems. As the resultant changes, we may need to adjust either the lines or the position of the anchor to maintain the anchor’s stability.
There are many other things to consider when it comes to advanced rigging and portable anchors, but these fundamentals provide a safe and efficient foundation. Understanding the engineering and physics behind why we do what we do always leads to effective problem solving and efficient rigging.
Stay safe and train hard.