Two-Leg Bridle Sling With Unequal Leg Angles
You need to lift a 12,000 lb steel tank using a two-leg bridle of 1-inch 6x19 IWRC wire rope slings with mechanical splices, choked at the tank's two lifting lugs. The lugs are 8 ft apart and the hook is 5 ft above the line between them, so each leg runs at an angle from vertical. The rigger proposes a single-shackle connection at the hook and says 'two legs, so each takes 6,000 lb, and 1-inch rope is good for way more than that.' The slings have been in the yard for two years and show some broken outer wires near one splice.
1. Calculate the actual tension in each sling leg and evaluate the rigger's claim.
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The rigger's 50/50 split is only true if the legs are vertical. Here each leg is inclined. Half the lug spacing is 4 ft, hook height is 5 ft, so leg length = sqrt(4^2 + 5^2) = sqrt(41) = 6.40 ft. The angle from vertical has cos(theta) = 5/6.40 = 0.781.
Vertical share per leg = 6,000 lb. Tension per leg = 6,000 / 0.781 = 7,680 lb. So each leg carries about 7,680 lb, not 6,000 lb - a 28% increase. The horizontal component (6,000 x 4/6.40 = 3,750 lb per leg, 7,500 lb total) squeezes the lugs inward; confirm the lugs and tank shell can take that.
Also account for the choke: a choker hitch derates wire rope capacity (commonly to about 75-80% of vertical capacity), and the sling angle factor compounds it. So the required vertical-rated capacity per sling is roughly 7,680 / 0.75 ≈ 10,240 lb equivalent. Check the manufacturer's vertical capacity for 1-inch 6x19 IWRC with the mechanical splice; spliced slings are rated per the splice, not the bare rope.
2. What do you do about the damaged slings and the connection at the hook?
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Broken outer wires near a splice are grounds for removal from service - rejection criteria depend on the number of broken wires in a lay and on the splice condition. I would tag the slings out and have them inspected by a qualified person; do not use them for a 12,000 lb lift.
On the hook connection: two sling eyes on one shackle at the hook creates a loading condition where the shackle pin and the hook see a resultant that depends on the included angle. With this geometry the included angle between legs is 2 x (90 - angle from vertical). Since cos from vertical is 0.781, angle from vertical is about 38.6 degrees, included angle about 77 degrees. That's acceptable for a shackle if rated for the resultant, but confirm the shackle's WLL exceeds the total leg tension vector and that the hook throat is not side-loaded. Use a properly rated master link or shackle sized for the resultant, and verify the hook latch is functional.