Unequal-Leg Bridle on an Offset Pick Point
A 9,600 lb skid must be lifted from two existing lugs that are not symmetric about its center of gravity. Lug A is 4 ft from the CG line; Lug B is 8 ft from the CG line, both on the same side of the load's vertical axis, so the hook sits between them. The rigger plans a two-leg bridle from a single hook directly above the midpoint between the lugs, with each leg at 60 degrees from horizontal. The available slings are rated 6,000 lb vertical each with a 2-leg, 60-degree capacity of 10,400 lb per assembly. The crew wants to add a third leg to the shorter side to 'share the load.'
1. Determine the static tension in each bridle leg and assess whether the proposed two-leg arrangement is acceptable.
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Because the hook is not above the CG, the legs do not share load equally; tension is inversely proportional to horizontal distance from the hook's vertical line to the CG. With the hook at the midpoint (6 ft from each lug along the beam), the CG sits 4 ft from A and 2 ft from B on the hook side, so the load reacts 2/6 at A and 4/6 at B.
- Vertical share at A = 9,600 x (2/6) = 3,200 lb
- Vertical share at B = 9,600 x (4/6) = 6,400 lb
- Leg tension = vertical share / sin(60 deg) = share / 0.866
- T_A = 3,200 / 0.866 = 3,695 lb
- T_B = 6,400 / 0.866 = 7,390 lb
Leg B exceeds the 6,000 lb single-leg vertical rating and is far above a safe working margin. The assembly rating of 10,400 lb applies only to a symmetric two-leg pick; it does not authorize an overloaded leg. The pick is not acceptable as planned. Corrective action: relocate the hook over the CG (4 ft from A, 8 ft from B, i.e., 12 ft span split 1:2), which equalizes the vertical shares at 4,800 lb and gives 5,540 lb per leg at 60 degrees, still tight against the 6,000 lb leg rating. Better: use a spreader beam above the CG so legs run vertically, or select higher-rated slings and verify lug capacity for the angled pull.
2. Evaluate the proposal to add a third leg on the short side.
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Adding a leg does not reliably reduce tension in the long leg unless the geometry and stiffness make it share load. In a statically indeterminate multi-leg system, load distribution depends on leg stiffness, length, and precise hook position; a slack or longer third leg carries little or nothing while the crew believes the load is shared. The third leg can also introduce a horizontal force component at the lug that was not in the lug's design case, and it changes the included angle at the hook.
Judgement: reject the 'add a leg' fix as a load-sharing solution. The defensible options are (1) reposition the hook over the CG and re-verify leg tensions, (2) use a spreader or equalizer so each leg is vertical and the load path is determinate, or (3) upsize slings and verify lugs and shackles for the calculated tensions. Any of these requires recomputing tensions and documenting the rigging plan before the lift.