Rigging Safety and Load Calculations
Core concepts
Every lift starts with two numbers: the load's weight and the rigging's capacity. Candidates must be fluent in the relationship between working load limit (WLL), the angle between sling legs, and the tension each leg actually carries. A vertical single-leg sling carries the full load; as legs spread, tension rises sharply. Also understand center of gravity: an off-center pick shifts load onto one leg and can rotate the load unpredictably. Know the difference between static weight and dynamic forces from acceleration, shock loading, and wind, and remember that the safety factor is built into the WLL, so never apply your own multiplier to published ratings.
Traps to watch
Questions often give a total load and a sling angle and expect you to recognize that leg tension exceeds the share of weight. Another common trap is a load with an asymmetric center of gravity where the "obvious" even split is wrong. Watch for unit mix-ups (pounds versus kilograms) and for capacity questions that quietly change the number of legs in use. Distractors frequently quote the vertical capacity for an angled configuration.
Worked example
A 2,000 lb load hangs from a two-leg bridle at a 60° angle from horizontal. Each leg's vertical share is 1,000 lb, but tension equals share ÷ sin(angle): 1,000 ÷ sin 60° ≈ 1,000 ÷ 0.866 ≈ 1,155 lb per leg. Rule of thumb: at 60° from horizontal each leg carries about 1.15 times its vertical share; at 45° about 1.41 times; at 30° double. Keep included angles at or below 90° whenever possible.