In Part 1, we discussed theory involved in building and using an A-Frame Gantry. In this article, we will build one to practice with. At the risk of sounding like we're concocting a recipe, we are going to have to secure some "ingredients" that may or may not be at your immediate disposal, so I have listed the materials here.
A few safety points, however; as should be noted for any kind of technical rescue training, these techniques should be performed under supervision of an experienced rescue instructor. Furthermore, someone should be watching the load at all times while it is suspended and at no time should anyone be permitted to work under the load unless it is properly cribbed.
Photo By Capt. Michael "Mick" Mayers
While comparing different a-frame construction techniques, I noticed the information was very vague or non-existent. A-Frame Gantries can be constructed by the use of pneumatic shoring strut kits, but for departments that don't have those tools, timber can be used to create one. For training purposes, we will diagram an a-frame gantry capable of lifting a small load so that you can get the feel for how this system works.
A frequent question regarding the a-frame gantry involves capacity. First, consider that the functionality of the frame is limited by the interior dimensions of the frame; you are obviously not going to be able to pass an item that has a width of 10 feet through a space that is 9 feet wide. That being said, the other capacity limitation of the frame would be the weight of the object. Several components come into play here. We realize that the compression strength of the frame's weight bearing members (the legs) is limited by the dimension and the length of those members, and the species of wood. The capacity of the hauling and belay systems will likely be the weak point of the system, where the hardware, the rope and webbing, or the anchors may influence the entire operation. I am mentioning all of this so that you consider that this is truly a system, and each component must be evaluated as to suitability in moving the load.
The system that we will be constructing then, will be a very simple (read: small) gantry using 16-foot 4x4s to lift a load of up to 3000 pounds.
- (2) timbers, 4x4, 16-foot length
- (1) rope, 12.5mm static kernmantle utility line, 50-foot length
- (2) wedges, wood, 4x4x18 inch
- (1) rope or webbing, 50-foot length
A-Frame Gantries utilize two legs, connected or "lashed" together near the top to form a triangle, then positioned to transmit load forces from an overhead point to ground. Using dimensional lumber is recommended, basing the size of the lumber on the size of the load to be lifted.
Photo By Capt. Michael "Mick" Mayers
Begin by placing the two timbers side by side, ends even. Place a spacer block (2x4 scabs) between the "legs" in several places. Tie a clove hitch around one of the legs approximately 36 inches from the top, and then go tightly around BOTH legs performing a round turn lashing 10 times. After the 10th turn, go between the legs and create five counter-wraps, finishing with a clove hitch and a double overhand safety on the opposite leg from where your first clove hitch was initiated.
Drive two wedges from the bottom up under the round turn lashing to tighten the lashes and repel the tendency for the rope to slide on the lumber. We have found this step especially important with big timbers or treated lumber. Turn the lashed legs onto one side and spread the legs to create an equilateral triangle.
Attach a 50-foot section of rope with a clove and three round turns approximately 6 inches from the foot of one leg, and cross to the other side, finishing with three round turns, a clove hitch, and a double overhand. This is your ledger line, which will aid in resisting the lateral forces of the gantry. Position the a-frame with the legs spread TOWARD the desired landing area with the apex over the load.