By Mike Donahue
In this month’s article, I will again look into the physics behind the systems you rig and build. As stated in my previous article, understanding the physics involved in what you create is important knowledge a rescuer shouldn’t be without. Why? Loads and forces can be your friend or foe; they will also make or break your operation when not controlled properly.
Fortunately, when harnessing the power and laws of physics, you can counteract those forces and prevent such failures. The methods used to counteract those forces need to be mathematically accurate to provide the ability to do such things; it is essentially a balancing act based on science. When done properly, you can then manipulate and control the forces of gravity and the resultant physics that go along with it! Following is an example.
In my last article, I discussed the physics and math behind load sharing anchor systems and directional pulleys. In this article, I will expand on that topic and look at the tension stresses created when using a mono pole, often referred to as a gin pole. Because equal tension and angles are so important to the stability and success in its use, an understanding of “how” and “why” is key.
As you all already know, mono means “one”; thus, a mono pole is just that—one pole. The stability of the mono pole is based on tension from three or four points.
Photo 1 shows the head of the Arizona Vortex and the various connection points. The multitude of anchor points from which you have to choose makes it ideal for tensioning in multiple directions.
Photo 1. (Photos and Figures by author.)
Figure 2 show a mono pole positioned with a 20° lean. Like everything else in the world, gravity is pulling down on the mono pole. If I released tension on one or both tie backs, the mono pole would of course fall to the ground.
Figure 2
As Newton’s laws states, “For every action there’s an equal and opposite reaction.” In this example, I am going to create that opposite reaction by back tying the mono pole using a mechanical advantage system. Your back ties will be constructed on either side of the pole angled at—ideally—a 45° angle. The greater degree of forward lean you apply to your mono pole, the more chance the legs have of kicking out. You essentially create a class one lever when you load the mono pole. To prevent that, you must manipulate the physics with which you are dealing. Attaching another back tie to the center of the vortex head and applying tension will remove some of the load from the other back ties. When loaded, it will also add more compression to the mono pod, increasing its stability.
When tensioning the mono pole with three lines, you’re essentially making a triangle based on the tension points (Figure 3). When performing this, opposing tension forces are aiding the created compression force, strengthening the stability of the mono pole, which I call “tri tensioning.” I refer to tensioning from four points as “quad tensioning.” By using this method, you can create a triangle of tension on all four sides of the Vortex head. The result is sturdy, stable mono pole now having the ability to receive and support a load on its own!
FIGURE 3.
Figure 4 shows a gin or mono pole, and how gravitational forces will effect a structural member. For explanation purposes, let’s look at it as a vertical structural member. If you have experience with the discipline of structural Shoring, you know that the transferring of captured loads needs to be in a straight line. Why? Because gravity travels straight and down, if a supported load is out of plumb (not straight up and down), gravity will attack that out-of-plumb load and cause it to fail or collapse.
Standing alone this mono pod is very unstable. It essentially has a 2½- x five-inch footprint that, when held, is perfectly plumb. Without tensioned tie backs, much like the bi-pod, it will be useless. We’ll now move forward and create what I refer to as quad tension which would be tension on four sides.
FIGURE 4.
Figure 5 shows four of the connection points used for tensioned tiebacks. By tensioning four points, the mono pod will become self-supporting in a 360° window. When using a quad tension technique, it is imperative to create equal tension between all four points. Think of it as one complete equal window of tension. As the main line is tensioned, fine tune your tension in the other lines. Why? Because as tension is pulled in one direction, the other lines will be affected by the opposite reaction.
FIGURE 5.
A simple way of testing to see if you, in fact, have equal tension, is to pluck each line as if it were a guitar string (Figure 6). When doing this, use only two or three fingers. Your sense of touch will be more sensitive to inconsistent tensions. I also have students perform this test with their eyes closed. I’ve found that by doing this it enhances your ability to detect differences. You’ll perform this test with two tie backs at a time.
FIGURE 6.
When positioning the back ties, pay attention to the distance or span of the back tie in relation to the height of the mono pole. The closer the back tie anchor point is in relation to the pole, the more leverage that is created. Finally, when equal tension has been applied to each point, take a small torpedo level and place it against the mono pole to ensure it is plumb. Gravity and generated force will exploit the percentage of your pole that is out of plumb. Also, have someone hold a level on the mono pole as tension is applied to ensure the pole is plumb on completion of the tensioning operation.
At this point, you’ll apply what I refer to as a “false load,” i.e., a controlled load mimicking the live one. This is done to stretch the system, allowing you to then tension and fine-tune the back ties, also referred to as “guy lines.” The tensioning and resulting loss of tension in the other of system lines is a great example of Newton’s Third Law.
Physics is a big part of every rescue discipline you perform. I encourage you to explore his side of what you do. Understand it, know it, and you can control it.
Mike Donahue has 17 years of fire service experience and has been a career firefighter in the city of Elizabeth, New Jersey, for the last 13 years, working out of Rescue Company 1 for the past 10 years. Mike teaches a Middlesex County College as an adjunct professor and acts as the fire service program coordinator. Mike is the owner of Progressive Rescue and can be reached at progressiverescue@gmail.com.
