Rigging, load charts, and lifting calculations
Rigging, load charts, and lifting calculations tests the math behind a safe pick: load weight, center of gravity, sling tension, hardware capacity, and chart deductions. Expect calculation questions, not just definitions.
Rigging geometry
For a symmetric two-leg bridle, each leg carries half the load divided by the sine of the angle measured from horizontal: tension per leg = (load/2) / sin θ. As the sling approaches horizontal, tension rises sharply. A basket hitch does not double capacity unless the sling is vertical and the load is balanced; choker hitches reduce capacity. Always include the weight of slings, shackles, and spreader beams in the load.
Worked example
A 10,000 lb load is lifted with a two-leg bridle at 60° from horizontal. Tension per leg = (10,000/2) / sin 60° = 5,000 / 0.866 = 5,774 lb. If the same load is rigged at 45°, tension = 5,000 / 0.707 = 7,071 lb. At 30°, tension = 10,000 lb per leg. The load has not changed; the geometry has. Select slings and shackles with a WLL above the calculated tension, and check the crane chart with the total suspended weight.
Traps
Measuring the angle from vertical instead of horizontal; assuming all legs share load when the COG is off-center; ignoring sling and hardware weight; using the same capacity for choker, basket, and vertical hitches; forgetting that a spreader beam adds weight but reduces sling angle.