A Tripod, a Rope, and a Harness: The Safe Load of a Lift Built from Three Poles

For a tripod of timber poles with a stated Young's modulus of 10 GPa, the Euler buckling load of the weakest pole calculated, with a diameter of 60 mm and a length of 3.5 m, is 5125.5 N. With a safety factor of 4 on buckling, an uneven load share in which the most-loaded leg carries 0.5 of the total, and legs inclined at 20 degrees from vertical, that weakest combination gives a safe total vertical load from the legs of 2408.2 N. The rope, with a stated breaking strength of 20000 N, a knot retention factor of 0.65, and a safety factor of 5, allows 2600.0 N. The governing limit is therefore the legs for the combination of 60 mm and 3.5 m, at 2408.2 N, and the rope for every other combination calculated, at 2600.0 N. Every combination covers a suspended mass of 95 kg, whose weight is 932.0 N, including when a dynamic allowance factor of 1.5 is applied, giving a dynamic load of 1397.9 N. This device, as calculated, is not a substitute for a rated hoist.