Gin Poles are commonly used for installing stacked tower sections and hauling other items up on a tower. Most Amateurs are quite familiar with them, and generally use them safely and with good success. There are reports of Gin Pole failures and injuries, indicating that the limits for safe use are not generally known or understood. There have been some discussions on the TowerTalk reflector about using a Block & Tackle lifting system with a gin pole to 1) make it easier to lift the load, and 2) reduce the load on the gin pole, allowing it to lift heavier loads.
This page is not meant to cover everything there is to know about Gin Poles. It is a limited look at a few examples to try and see what is going on, and hopefully will help readers find a few new things to think about when using them.
There is more than one kind of Gin Pole. For the sake of this discussion, I only looked at the Rohn EF2545, Erection Fixture, which many amateurs are familiar with. This pole is intended to be used to raise Rohn 25G and 45G tower sections for installation on a stacked tower. The fixture I have uses a 2.0" OD x .125 wall x 12 Ft long aluminum tube for the pole. Larger fixtures are available for 55G & 65G, and SSV tower sections.
Before getting into the discussion, a little Gin Pole and Block & Tackle information might be helpful for readers who are not already familiar with these things.
The Gin Pole (Rohn EF2545)
The Erection Fixture is attached to the leg of the tower, a line is routed through the inside of the tube and over a sheave (pulley) at the top of the pole, and down to the tower section (the load). The section is lifted far enough for the bottom ends of its legs to clear the top of the section it is to be connected to, and then the section is lowered onto the tower and the leg joints are engaged and fastened. The end of the line exiting the bottom of the tube is pulled by the ground crew to hoist the section along the side of the tower to the top, and eased off to lower the section into place. The crew on the tower directs the section into place and fastens it.
REF: Page HA-19 (Form No. 88-2200-20R3, Titled "Rohn Erection Tools") in the Rohn Commercial Catalogue, Stating "EF2545 12' for towers with 1-1/4" Tubular Side Rails." Drawing C630645 R10 cites the weight of a 10'-5" overall length section of 45G as 70 Lbs.
The Block & Tackle Mechanical Advantage & The Gin Pole
A Block & Tackle can be rigged between the load and the top of the pole to lower the force required to lift the load. This is the preferred way to use the load lifting advantage of the Block & Tackle. A more detailed discussion of Block & Tackle systems can be found on the Block & Tackle Systems Page.
Below is a diagram of a pole rigged with a 2:1 advantage:
The end of the line is connected to the pole. It runs down around a sheave that is attached to the pick point on the section, back up over the sheave on the pole, and down to the ground through the center of the pole.
Ginpole Loads and Stress in the Standard Configuration
Let's assume that we are lifting a 70 Lb section of Rohn 45G and that the load is directly below the pole, such that the angle between the pole and the line from the load to the sheave is 0 degrees.
The loads applied to the top of the pole look like this:
The sheave is offset from the pole centerline by 1 inch, which creates a bending moment in the pole, plus the axial compression load. If we want to just take a look at stress, the place to do it is at the base, where it is held by the clamp. There we have the weight of the pole (9.5 Lbs) plus the top load to get 149.5 Lbs of compression, plus 140 In-Lbs of bending moment. Running these thru the linear formulas for stress we get P/A = 203 psi stress due to compression and MC/I = 431 psi stress due to bending.
So, the peak combined pole stress is 634 Psi, that's not much! If the yield strength of the aluminum is 35 ksi, the Safety Factor for stress would be 55. So, that would be a clue that the pole, which is apparently rated for a 70 Lb load, is probably not limited by stress.
Gin Pole Loads and Stress with the Block & Tackle
Let's assume again that the load is below the pole, such that the angle between the pole and the line from the load to the sheave is 0 degrees. And, that the block and tackle system is rigged like the diagrams on the Block & Tackle Notes Page , to allow the pole to react the load with a purely vertical force running through the center of the sheave. The stress values in the following table come from the linear formulas above.
The table below shows the effect of 4 different Block & Tackle ratios on the pole stress:
For these load cases we get the expected reductions in the applied load and lifting line load from the use of the mechanical advantage. The stress reduction is slightly different from the load reduction because the component of stress that comes from the pole weight is not being reduced by the mechanical advantage.
Let's put a small angle between the load line and the pole, by attaching the line to the top of the section, and say that we have the section plumb and directly over the tower. The configuration would look like this:
Nothing magic about this angle, just what came out of the configuration. With the pole extended 11 Ft above the clamp, the loads and linear stress calculations look like this:
The applied load and lift line reductions from the block & tackle are nearly the same, but the reductions in stress are insignificant, and the safety factors for stress have been reduced 93%. Notice how the angle of the resultant load changes to maintain a constant horizontal load on the pole. That is because the block and tackle has done nothing to change the lateral load on the pole. The lines on the load side of the pole are still carrying the same 70 Lb load at the same angle. The thing that is changing is the lift line load, which is only reducing the vertical load on the pole. The lateral load on the pole is responsible for almost all of the stress. The pole is not close to failing due to stress, but the mechanical advantage has little impact on reducing stress when the load is off at an angle.
This example might not be practical as we probably couldn't fit a 4:1 system in the available space. It is merely to illustrate the point. I'm sure someone has enough imagination to figure out how to develop an angle like this another way.
What About Buckling?
Since, this Erection Fixture is only supposed to be rated for lifting a 70 Lb 45G tower section, and the safety factors for stress seem to be huge, stress is probably not what is limiting the pole. The pole is a long slender column subjected to compression and bending loads. I ran some non-linear FEA models to look at the pole buckling behavior. Purely analytical buckling predictions are not entirely reliable unless accompanied by real world testing. So, this stuff here should only be viewed in relative, not absolute, terms.
The stress values, listed below, are the values reported by the non-linear analysis for the vertical and horizontal forces shown for each case, including the moment caused by the sheave offset. The Ultimate Load values are the dead weights, to the nearest pound, that were just below the onset of buckling instability.
Example: The maximum dead load for the 1:1 ratio load case (Table 3) = 128 Lb. At this point the pole was slightly below the point where it became unstable. Using a 2:1 system, instability occured at ~171 Lbs, raising the dead load limit to ~170 Lb.
The non-linear analysis for the first load case (same as Table 1) where the load is pulling straight down produced these values:
All of the ultimate loads were defined by buckling instability and the lowest safety factors exist for them, confirming that buckling is the limiting factor. That shouldn't be surprising, considering the pole. The block & tackle is effective in increasing the pole capability.
Looking at the load case with a 17.1 Deg angle between the load line(s) and the pole:
The Ultimate loads were still buckling driven. The safety factors for stress are much lower, indicating that the bending stress has become a more important factor. The non-linear analysis shows higher stress values than the linear calculations, which is not unusual. It is reporting the pole stress in its deformed state, which is a more accurate representation of what is happening. The block & tackle definitely helps the pole carry more load here, but care should be given when estimating how much more load the B&T system will facilitate. It is starting to diverge from what we would expect for a pole with a purely vertical load.
Looking at a load case with a 25 Deg angle between the load line(s) and the pole:
The ultimate load for the 1:1 load case was defined by buckling stability. The other load cases were limited by stress. We are no longer getting the expected increase in pole capability from the mechanical advantage of the block & tackle because bending stress has become the primary failure mode and the B&T isn't changing that.
What Does All of This Mean?
Well, it means that I am nuts for even looking at this business! I got into this because a Towertalk discussion got me worried that some reader would think that by just putting a block & tackle on a Gin Pole would always mean they could lift the amount that was indicated by the block & tackle mechanics. It was way more work than I had planned on, but was at least interesting.
Many of us intuitively know that it is not real safe practice to do things that side load these Gin Poles. I have heard many reports of amateurs side loading these things.
There is abundant evidence that block & tackle systems do what they are supposed to do. When we take a block & tackle system and put it on a Gin Pole lifting system, there seems to be enough evidence to suggest that we ought to be careful about our assumptions, regarding how effective the block & tackle system will be in allowing us to increase the loads we will lift, if we are not going to keep the load close to the pole at all times.
For this gin pole, when the load line gets to about 20 degrees from the pole, the danger alarms should start going off.
I hope this prevents more headaches than it has created.
As, is customary with everything on this website, I only offer comments to stimulate thought, and hopefully help fellow Amateurs. None of the information provided is authoritative in any manner or guaranteed to be correct. The reader is encouraged to research these subjects and make his own determinations about these things, before trying to apply them in the real world.