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 and put it wherever they want on a rubble pile. Anyone who has been in the business for a while knows that this isn't always the case. In fact, during major disasters where more than one structure is involved, rescuers may not even have ready access to pneumatic lift bags or hydraulics, depending upon the immediate urgency of the situation or the availability of resources.
A-Frame Gantries have a lot of ancient history; Archaeologists speculate that A-Frame Gantries were utilized as far back as the Pyramids to raise structures and materials. Even in modern times, when the Standing Stones Monument was constructed in Glen Innes, NSW commemorating Celtic influence on Australia, A-Frame Gantries were used to keep the project true to their heritage. According to one of the leaders of this project, John "Trigger" Tregurtha, theory has it that ancient Celts used A-Frames to raise huge stone monoliths throughout Ireland, Scotland and Wales. In Europe during World War II, rescues from bombed out structures utilized A-Frame Gantries to lift away fallen structural members.
Photo By M.S.Mayers
A-Frame Gantries can be constructed by the use of pneumatic shoring struts and an accessory kit, but for those departments out there that don't have those tools yet, timber can be used to create this system. As is eloquently described in the FEMA Structural Collapse Technician text, an A-Frame Gantry is "a fairly complex application of leverage that involves floating an object in air between two horizontal points". 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.
A-Frame Gantries have a number of advantages. Use of a gantry can change the direction of pull from straight on to overhead, which is especially useful in a short haul if the object is "digging in" and rescuers don't have the ability or the time to run a timber track or put down rollers.
The primary use for an A-Frame Gantry, however, is during situations where no suitable overhead anchor points exist and/or crane access isn't practical. They can be moved to any position on a rubble pile, which if vibration is an issue, or the crane simply can't reach, is more advantageous. The gantry can even be positioned inside the building if necessary.
Unlike pneumatic lift bags, gantries provide enough lift to move the object clear of the area. Lift bags are wonderful tools and are usually the first weapon in the arsenal to create lift. But lift bag strength is in the first few inches from inflation; after that, the air column diagonal decreases and the lift is not as effective. Furthermore, a gantry is able to lift the item and then move it to one side- lift bags can only move the item straight up. A-Frame Gantries are wide enough to accommodate large items and spread out enough to move weight concentration away from the item being lifted.
A-Frame Gantries are not without their disadvantages, however. Whereas a gantry is a "simple" machine and they have been around for centuries, in today's age they violate the principal rule of rescue- "Keep It Simple". With the prevalence and relative ease of use of lift bags, hydraulics, and cranes, I would not recommend the A-Frame as the weapon of choice. Depending upon the size of the load to be lifted, constructing an A-Frame may be time and labor intensive and the mechanical advantage required to use one can be cumbersome.
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.
Insure the fore and aft guy anchors are placed at least 150% 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 a gantry using 20-foot 6x6 timbers, the apex lashing will start at 17 feet. So the foot-to-apex length we will be using is 17 feet. Placing the guy anchors around 43 feet (greater than 150% of 17 feet) from the anticipated centerline will provide close to a 23 degree angle between the rope MA and the gantry. Using a 23 degree angle keeps the forces at less than 110% of total load.
This series of articles on the A-Frame Gantry is meant to give the reader a working familiarization with the dynamics of the gantry system. Since your system size may vary based on available materials or the type of load being lifted, we want you to understand the system itself. In the next few articles, we will go through the actual building of some A-Frame Gantries and putting them to work for you.