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

It seems to me that A-Frame Gantries catch a lot of slack because there are plenty of rescuers out there who think they can just summon up a crane without any problem.

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. When completely vertical, that is; the gantry is perpendicular to the ground; weight is distributed along the long axis of the columns. In a timber system these columns, or legs, can be constructed from 4x4 or 6x6 lumber, or in an engineered system using commercially-made pneumatic shoring struts. In the timber system, the compressive strength of the dimensional lumber is substantial enough to resist fairly substantial loads, provided the rescuers consider the rated capacities of the species they are using.

A-Frames are made from two timbers lashed together at the top. The legs should be connected at the bottom by a "ledger line" using either webbing, rope or a chain. This line will help resist the tendency of the legs to force outward, laterally from the load. When I first learned how to make an A-Frame, we were taught to use a board or other piece of timber lashed crosswise between the two legs. The major problem with this, however, is that if control of the load is lost and the load hits the board, there could be a potentially catastrophic side impact. A rope, chain, or webbing, however, provides a little "give" if necessary. The legs should be spaced apart as far as the distance from the foot to the apex, so that an equilateral triangle is formed.

The load is attached to the apex of the gantry using a short rigging strap. We have used a triple-wrapped multi-loop anchor constructed from 12.5 mm static kernmantle. Webbing or a chain sling rated to hold the weight of the load can be used. As the gantry is tensioned by the haul line and the A-Frame is elevated, the load starts to rise. A hoist or come-along may also be used to initially suspend the load.

As lifting begins, the force in each of the gantry legs will be about equal to the load. Spotters must use pinch bars to help guide the feet into the bearing points and to help resist the lateral forces while the gantry moves into position. Once the load moves off the 45 degree point, the horizontal force on the "foot" of the gantry legs will be about 66% of the load.

There will also be forces acting into the ground as the weight force shifts to the long axis of the legs. These forces can be resisted by digging a hole. In soft ground, gusset plates (12-inch x 12-inch plywood) may be used to spread the load at each foot. I will say, however, that at Rescue School 2003 in Columbia, SC, we used an A-Frame in the pouring rain to lift 4800 pounds and at no time did the poles dig any deeper into the ground than the original depth of the holes. On concrete or other paved surfaces, depending on the size of the load, resisting these outward forces might get a little hairy. We have tried several solutions, from using pinch bars from multiple angles to chipping out the concrete to create a hole. I have also seen pickets driven shallowly into the ground then moved as the frame came to vertical. Whatever solution works best, my suggestion is that you get out there and experiment.

Photo By M.S.Mayers

As the gantry moves becomes perpendicular to the ground, the load continues to elevate until the gantry is straight up and the object being lifted is dangling beneath the apex. As the load continues past perpendicular, the haul system becomes useless and the belay system takes over the lowering of the load. When the load is settled on the ground, we usually we have the haul team move up and assist walking the gantry into a safe position.

An A-Frame Gantry can be used in a smaller scale than is being taught at many USAR schools to move "smaller" objects or for practice; two 14-foot long 4x4 timbers can support the move of an object 8 feet from center-point, which makes a relatively easy drill. Practice with smaller A-Frames to begin with to increase comfort in their use and to decrease the time it takes to set them up.

Safety considerations include never allowing the gantry to bear the load more than 45 degrees from vertical in either direction. At 45 degrees, once the system is placed in tension, the forces on the haul line exceed 100% of the load. As the load begins to move closer to vertical, the forces will decrease, but if the gantry is tensioned too early, like for example at 50 degrees from centerline, the forces can exceed 130% of the load.