A-Frame Loading
Discussion and Assumptions
Below are the three basic
loading configurations for an A-frame in different phases of the rescue
evolution. For the purposes of making the calculations as real
world as possible, the following assumptions were considered.
These we based on conversations with a leader in the rescue community.
First, the dimensions of environment: The height of the A-frame,
when placed vertically, is set at 20 feet. The trees, which
represent the necessary anchors for the rigging lines (R1 and R2), are
set at 40 feet from the base of the A-frame and the rigging lines are
secured at the base of these trees. The rescuer is positioned thus
that the pull line is basically inline with the rigging line, even
through a slight angle is present for clarity. Second, the loading
factors: The load being lifted is set at 1000 pounds for this
example. Using this number makes its relation to the other numbers
easier to understand. (If an actual load is then 2000, one can
merely double the loads on the rest of the system.) There is a 5:1
haul ratio between the load and the top of the A-frame, making the
resulting pull required by the personnel (P) to be 200 pounds.
Now, between the top of the A-frame and the person, one can place a 2:1
or 3:1 haul system here, and it will reduce the load on the person, but
that system will not affect the loading on the A-frame. Finally,
the angle of the A-frame: In these examples we are using the
absolute worse case scenario of the frame being 45 degrees from the
ground. Angles that place the frame any lower to the ground would
result in forces that increase very rapidly with a resulting failure of
many aspects of the system including the rigging lines, the horizontal
thrust of the A-frame with the ground, and the inline forces on the
A-frame itself. So while in actual rescues it should always be
remembered to keep the frame as vertical as possible and that the
further one tilts the frame the higher the loads on many elements of the
system.
For each of the phases of the rescue below, the
loading is stated for each element. "R1" represents the tension on
the rigging line (or system) towards the side where the animal is to be
delivered. "R2" is the tension in the back rigging line which
holds the frame in the last phase of the rescue. "L" is the load,
in this case predetermined to be 1000 pounds. "P" is the haul load
for the rescuers either directly or through a haul system, here
determined to be 200 pounds. And finally, "A" represents the
inline loading on the A-frame. For this case we are only
considering the total load. To take the calculations further and
determine the loading on each of the frame legs and the horizontal
thrust with the ground would require more information on the angle
between the legs and other elements that are beyond the intent of this
discussion. Below each diagram, each loading element, other than L
and P which are already determined, is shown.
R1 = 1200 pounds
A-frame = 1914 pounds
R2 = Slack
Not only does the A-frame take the load, but also the downward forces of the
rigging and haul lines.
R1 = Slack
A-frame = 1178 pounds
R2 = 200 pounds
Almost the easiest position on the whole system. Other than the load, we
have the added down pull components of P and R2.
R1 = Slack A-frame = 2181 pounds R2 = 1774 pounds
Clearly, this phase of the rescue places the highest loads on the rigging as well as the frame. Fortunately for the patient being rescued, during this time they should be over solid ground should a failure occur. With system failure not an option, however, even for this average sized large animal, the A-frame needs to be strong as well as solid anchor points for both the base of the frame and the rigging!
Further analysis of the system above, especially in this last phase, shows that the 200 pound pull of "P" from the top of the frame is actually adding a significant load to the system. Because of this finding, a different rigging solution was considered. In these examples, shown below, the haul line "P", instead of the direct pull as shown above, is now going through a "change of direction" or "redirect" pulley at the base of the A-frame. With the same load "L" of 1000 pounds, and "P" being 200 pounds, this different set up was analyzed below. As before, when the loads are in line with the A-frame, the slight angle shown in the diagram is present only for clarity.
R1 = 1400 pounds A-frame = 2114 pounds R2 = Slack
R1 = Essentially zero A-frame = 1200 pounds R2 = Essentially zero
R1 = Slack A-frame = 2114 pounds R2 = 1400 pounds
From this configuration, it becomes apparent that the biggest change in the system loading is with R2, or the back rigging line. The load on this portion has been reduced by 374 pounds, or about a 26% reduction. In addition, because R2 has been lowered, the overall system effect is that the A-frame has now a slightly reduced load of about 67 pounds.