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.


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.

22_gantry5.jpg
Photo By Capt. Michael "Mick" Mayers

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.