Rigging Mathematics, Load Calculations, and Load Distribution
Core concepts
Every lift begins with a free-body diagram. The load weight acts through the center of gravity (CG); the rigging must oppose it with vertical components that sum to the weight and horizontal components that cancel. Candidates must be fluent in: sling tension as a function of angle, the load-angle (sling-to-horizontal) multiplier, share of load among multiple legs, and the effect of an off-CG pick.
Traps in questions
- Confusing sling length angle with horizontal angle. A 60° sling-to-horizontal has a multiplier of 1/sin60° = 1.155, not 2.0.
- Assuming equal leg tension when legs are unequal length or the CG is offset - the steeper leg carries more.
- Forgetting that a two-leg bridle at 90° included angle puts 0.707W on each leg, but at 120° included angle each leg carries 1.0W (the 120° trap).
- Treating a basket hitch as doubling capacity without accounting for the bend efficiency and the included angle between the two parts.
Rule of thumb
For a symmetric two-leg bridle, tension per leg T = (W/2) ÷ sin θ, where θ is the angle from horizontal. At 60°: T = 0.577W; at 45°: T = 0.707W; at 30°: T = 1.0W. Never rig below 30° from horizontal without engineering review.
Worked example
A 4,000 lb load on a two-leg bridle, each leg 60° from horizontal, symmetric. T = (4,000/2)/sin60° = 2,000/0.866 = 2,309 lb per leg. If the CG shifts so one leg goes to 75° and the other to 45°, tensions are no longer equal and must be solved by statics (moments about the CG), not by averaging.