Steep Highline for Roadside Rescue

CAUTION!! LARGE FORCES EXIST. RIG CAREFULLY.

Because the AHD on a steep highline usually needs to be right at the edge in order for the track lines to be effective, a tighter-than-desired catenary angle through the head of the AHD is virtually unavoidable. This creates amplification of forces on the AHD. Often times you'll get a 90 degree catenary with a 141% net force increase.

Let's do a quick whiteboard analysis on the forces at the head of the AHD. We were at 930 lbf on the load cell and could have easily gone to 1,000. Let's assume we did hit 1,000 lbf. The safety factor on each track line in this case is 9:1. This is fine for the lines, however this is not what we should be looking at. The critical part here is the AHD. Since we basically have two lines running through the AHD that are each at 1,000 lbf in tension, and the catenary angle is 90 degrees, that puts a 2,820 lbf downward force vector through the double sheave pulley and the VORTEX head. The pulley and the VORTEX both have listed MBS of 8,000 lbf. The safety factor on these components is much lower, coming in at about a 2.9:1 safety factor, and less than the listed WLL of these components. We could perhaps improve this safety factor to about a 6:1 if we only rigged one line through the AHD instead of both.

As for the Far Side: Assuming the chosen far side anchor is unquestionably strong, the weak link here is the swivel with a MBS of 9,000 lbf. If a single track line is under 1,000 lbf of tension, then it doubles through the pulley and becomes 2,000 lbf net resultant force. The safety factor on the far side anchoring system is thus 4.5:1.