Four-Leg Bridle with an Offset Center of Gravity
An 8,000 lb asymmetric machine base is to be lifted with a four-leg bridle. Each synthetic sling has a vertical working load limit of 5,000 lb, and the master link is rated 20,000 lb. The pick points are 10 ft apart, and the center of gravity is 6 in off-center toward one pair of legs. The legs are planned at 60° from horizontal. The rigger proposes to use all four legs and to rate the assembly at 20,000 lb because four 5,000 lb slings are used. A 3,000 lb capacity shackle is available at each pick point.
1. Calculate the tension in each leg for the symmetric case at 60° from horizontal.
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For a symmetric four-leg bridle, each leg carries one quarter of the vertical load:
8,000 lb / 4 = 2,000 lb vertical per leg.
Leg tension = vertical share / sin(angle from horizontal):
2,000 / sin 60° = 2,000 / 0.866 = 2,309 lb per leg.
This is the starting point only. Because the CG is offset, the symmetric assumption is not valid for the actual lift.
2. Evaluate the rigger's proposed 20,000 lb assembly rating.
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The 20,000 lb rating is not valid. A four-leg bridle is statically indeterminate; the load does not necessarily divide equally among four legs. With an offset CG, two or three legs may carry most of the load, and a slack leg may carry little or none. The assembly capacity must come from the manufacturer's tag or from a competent-person calculation that accounts for geometry, CG location, and leg angles. Do not add the four sling ratings together. Check each shackle against the calculated worst-leg tension rather than the average.
3. What configuration change would you require to reduce the risk?
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Use an equalizing arrangement or a spreader beam so the load share is more predictable, or relocate pick points so the CG is centered between them. A three-leg bridle with the CG centered in the triangle is often easier to analyze than an asymmetric four-leg bridle. Recalculate worst-leg tension and verify sling angle, shackle capacity, and master-link loading after the change. If the offset cannot be corrected, have a competent person determine the actual load distribution before the lift.