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.
Frame
- (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.
Overhead Anchor Point
- (1) rope, 12.5mm static kernmantle utility line, 50-foot length
Rig a multi-loop anchor sling on the "X" formed by the joining of the legs, leaving at least 18-inches of "dangle" between the legs. This anchor sling will be the connection for the load and the 18-inch dangle will keep the loops from creating a critical angle.
Photo By Capt. Michael "Mick" Mayers
This anchor sling is a triple-wrapped multi-loop anchor constructed from 12.5 mm static kernmantle. Webbing or a chain sling will work as well, so long as it is rated to hold the weight of the load. We will attach the lifting rig to the anchor sling you have just created to attach the load to the apex of the gantry. The load should be attached to the lifting rig either by bolting or slinging; we are assuming that you are familiar with how to secure a load for lifting- a whole series of articles could be dedicated to that subject. I recommend that the rig you choose to attach to the anchor sling be something that you can adjust, like a chain hoist or come-along. This way, if when you are lifting the gantry to the 45-degree position to prepare for the lift you find that your rig is too short (thus loading the gantry BEFORE you get to 45 degrees) you can easily adjust the rig until the position is correct.
Fore and Aft Line Attachments
- (2) rope, 12.5mm static kernmantle utility line, 50-foot length
- (2) carabiners
A-Frame Gantries use two lines attached to the apex of the gantry to control the fore and aft movement of the gantry; a hauling line to bring the load from its resting place to vertical and a belay line to control the line from vertical to a new resting place. These lines must be attached to the apex to control the top of the gantry; as the haul line pulls the apex up toward centerline, the gantry begins to lift the load. The belay line simply remains ready to capture the load if the gantry passes too quickly over the centerline and the load starts to fall too rapidly toward ground. Ideally, the load will be lifted until the gantry is completely vertical; then the belay line will be tensioned and the load will be gently lowered across the centerline and toward the finishing position, allowing the weight of the load to do the work. The belay needs to be carefully managed to keep the load from gaining momentum and running out of control.
Photo By Capt. Michael "Mick" Mayers
We have found the fore and aft line attachments are best done in heavy loads by creating two independent slings for attachment. The placement on the gantry is relatively easy; create a three-loop sling, twist it to form a figure-eight, then put one loop over one "ear" of the apex and the other loop over the other "ear" of the apex. The crossed part of the figure eight should be sitting in the "V" created by both of these "ears". This will create a self-equalizing load distributing anchor arrangement. Make another of these and do the same thing; one of the slings should pass to the front with a carabiner gigged through the "X" and one should pass between the ears to the rear in the same fashion.
Now the gantry should look like this: it should be an equilateral triangle, with a line from one leg to the other; each foot should be lined up to go into a hole when the gantry is lifted; there should be a three-loop anchor sling secured across the intersection of the two "ears" at the apex and dangling down approximately 18 inches underneath; there should be two figure-eight anchors, one going forward and the other to the rear where the haul line and the belay line will be attached.
Haul Line
- (6) pickets, windlass, rebar
- (1) anchor webbing
- (2) carabiners
- (4) prussiks, two short, two long
- (1) 4:1 rig with 12.5 s/k utility line, 200-foot length
- (1) carabiner
- (1) 5:1 rig with 12.5 s/k utility line, 150-foot length
Belay Line
- (6) pickets, windlass, rebar
- (1) anchor webbing
- (1) 2:1 rig with 12.5 s/k utility line, 150-foot length
- (2) carabiners
- (1) brake bar rack
When ready, the haul line and the belay line will each be attached to the gantry at the apex using carabiners gigged through one each of the figure-eight self-equalizing, load distributing slings. Insure the fore and aft anchors are placed at least 250% of the length from the foot to the apex lashing to decrease the effect of the load on the anchors. In this case, it is the effect of the angle between the rope mechanical advantage systems and the gantry itself that can create increases in force.
For example, with this 16-foot 4x4 timber gantry, the apex lashing will be placed at 13 feet. Thus, the foot-to-apex length we will be using is 13 feet. Placing the guy anchors around 32 feet (250% of 13 feet) from the anticipated centerline will provide around a 23 degree angle between the rope MA and the gantry. Using a 23 degree angle keeps the force at less than 110% of total load.
Rig the haul line by securing an anchor suitable for the load; in this case we are using a 2:1 picket system. Reeve a 4:1 mechanical advantage between the apex and the anchor. The first anchor should be placed 250% of the length of the apex-to-foot length, as discussed. Something to consider: in this example, realize that you are using a 4:1 system which depending on how you rig it, may require at least 200 feet of line; to hook the running pulley directly to the load will require at least 45 feet- 13 feet from the apex to the intended centerline and the 32 feet from the centerline to the anchor- and with the 4:1 you obviously have four returns for 180 feet of working line. Rigging it this way, you should not have to reset this first system at all, but a suitable progress capture device should be placed on the running end of the line so that the secondary (piggy-back) can be reset.
The second anchor should be established for piggy-backing a 5:1 haul system to the 4:1, creating a 20:1 mechanical advantage. This line should be attached to the running end of the 4:1. Finally, attach the running pulley of the 4:1 to the gantry.
Next, rig the belay line by securing a substantial anchor. Tie off to the anchor and attach a running pulley to the gantry to create a 2:1 mechanical advantage. Reeve a brake bar rack on the running end and attach it to an anchor. This is the belay line; when the load passes over the centerline, the gantry will momentarily be distributing 100% of the load weight to ground by way of the legs. The moment the load passes over to the other side, the belay must be ready to immediately arrest the load's motion and guide it gently to the ground. Realize that the closer the gantry gets to 45 degrees on the other side, several forces begin to act upon the system to make the load dangerous if not tended to properly. For one, the load's momentum will act on the line and the belayer should anticipate a moderate amount of stretch in the belay line, depending on the weight, even though you are using static line. This needs to be considered. Further, the closer to 45 degrees, the more mechanical advantage will be exerted on the line by the gantry. Finally, as the gantry settles toward a more horizontal position, there is less axial force being transferred to gantry legs and more lateral force; therefore, the feet may kick out if efforts aren't made to resist that force. This usually doesn't require anything more than using the pinch bar to keep the feet in the holes, but it's something your team must pay attention to.
In the next article, we will move the a-frame into position, load it, and move the object.
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