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.