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.


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.

27_lifting1.jpg
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.

This content continues onto the next page...