Lifting Heavy Objects: Timber A-Frame Gantry - Part II

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.


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.

22_gantry3.jpg
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.

22_gantry4.jpg
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