Two-Leg Bridle Sling Spread and Choker Derating
A 12,000 lb steel tank is to be lifted with a two-leg bridle sling. Each sling leg is 10 ft long from hook to shackle, and the pick points on the tank are 12 ft apart. The slings are 5/8 in wire rope with a single-leg vertical capacity of 8,000 lb. The rigger plans to use the slings in a choker hitch because the master link is too large for the tank's lifting lugs. A nearby wall requires the load to be rotated after it is lifted, so the rigger proposes moving the pick points to 16 ft apart using the same 10 ft slings. The tank's center of gravity is centered between the lugs. The ground is level, and the lift is a routine repeat pick.
1. Calculate the tension in each sling leg with the 12 ft spread using vertical capacity assumptions. Show the geometry.
Read the worked answer
Half the pick-point spread is 6 ft. With a 10 ft leg, the vertical height from hook to load is √(10² − 6²) = √64 = 8 ft. Each leg carries half the vertical load: 12,000 ÷ 2 = 6,000 lb. The sling tension is the vertical share divided by the sine of the angle from horizontal, or 6,000 × (10 ÷ 8) = 7,500 lb per leg. The angle from horizontal is about 53°. Compared with the 8,000 lb single-leg vertical capacity, the tension is 94% of capacity. That is within the nominal vertical rating, but there is little margin, and the sling is not being used vertically, so the manufacturer's angle and choker ratings must govern.
2. Evaluate the proposed 16 ft spread using the same 10 ft slings.
Read the worked answer
With pick points 16 ft apart, half-spread is 8 ft. The height becomes √(10² − 8²) = √36 = 6 ft. Tension per leg = 6,000 × (10 ÷ 6) = 10,000 lb. That exceeds the 8,000 lb vertical capacity by 25%, so the arrangement is not acceptable. The wider spread flattens the sling angle and increases tension rapidly. To use a 16 ft spread, the rigger would need longer slings or a spreader beam so the legs stay steeper, and the tension would still have to be recalculated against the correct rating. The load also needs to remain stable and the slings must not contact sharp edges.
3. How does the planned choker hitch change the evaluation?
Read the worked answer
A choker hitch reduces the sling's capacity, commonly to about 80% of the vertical rating, so 8,000 × 0.80 = 6,400 lb. The 12 ft spread already produces 7,500 lb per leg, which exceeds 6,400 lb, so the choker plan fails even before the spread is widened. The 16 ft spread would be far worse at 10,000 lb. The rigger should not use a choker just because the master link does not fit. Use a properly sized shackle, master link, or lifting lugs, and confirm the hitch and angle ratings with the sling manufacturer. Also check that the choker would not crush or damage the tank.