Two-Leg Pipe Bundle with an Offset Center of Gravity
A crew is lifting a bundle of six steel pipes from a truck to a rack. Each pipe weighs 620 lb, so the bundle weighs 3,720 lb. The bundle is 24 ft long. The rigger plans two single-leg nylon slings, each rated 3,200 lb in a vertical hitch, attached at pick points 10 ft apart and 7 ft from each end. The hook is 4 ft above the pick points, so each leg makes about a 39 degree angle with the horizontal. The center of gravity is 10 in toward one end because one pipe is heavier. The rigger says each sling only sees half the load, about 1,860 lb, so the 3,200 lb rating is fine. The load will be landed on a rack with sharp edges, and the crew plans to use the slings directly over the edges. A tag line is available but not attached.
1. Calculate the actual maximum sling tension, accounting for the center-of-gravity offset and the sling angle. Is the rigger correct?
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No. The two pick points are 10 ft apart, so the midpoint is 5 ft from each point. The center of gravity is 10 in, or 0.833 ft, toward one end. The vertical reaction at the heavier pick point is 3,720 lb times (5 + 0.833) divided by 10, which equals about 2,170 lb; the other reaction is about 1,550 lb. The sling tension is the vertical reaction divided by the sine of the angle with the horizontal. At 39 degrees, sine is about 0.629, so the heavier sling tension is 2,170 divided by 0.629, or about 3,450 lb. That exceeds the 3,200 lb vertical rating by about 250 lb. The rigger's half-load assumption is wrong because the center of gravity is offset and because sling tension increases as the angle flattens.
2. What rigging changes would you require, and what edge protection is needed?
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Reject the planned lift until the rigging is changed. Move the pick points so they straddle the actual center of gravity and produce roughly equal reactions, or use a spreader beam to keep the legs more vertical. If the geometry cannot be balanced, select slings with a rated capacity above the calculated tension plus the required design factor, and account for the hitch type and any hardware. Use corner protectors or softeners at the rack edges because nylon can be cut or abraded on sharp steel, and inspect the slings after landing. Attach the tag line before lifting to control rotation. Verify the rack's capacity and the landing area before the load is moved.
3. If the bundle must be landed with one end 2 ft lower to fit the rack, how does that affect the rigging?
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Tilting the bundle changes the load distribution and can make one sling go slack while the other carries much more. It also changes the angle of each leg and may allow the pipes to slide. The crew should not rely on a tag line to hold the tilt. Use a lifting beam, an adjustable leg, or a chain fall to control the angle, and recalculate the tension in each leg for the tilted position. Confirm that both pick points remain loaded and that the slings cannot slip off the bundle. If the tilt cannot be controlled with the existing rigging, change the lift plan or land the bundle flat and reposition it on the rack.