Low-Angle Two-Leg Lift of a Steel Base Plate
A 6,000 lb steel base plate, 8 ft wide by 12 ft long, must be lifted from a truck bed using two lifting eyes located 10 ft apart on the long centerline. The rigger plans two single-leg 1/2 in. wire rope slings, each 6 ft long from hook to eye, with an eye-and-eye configuration. With the hook centered, each sling makes roughly a 34° angle to the horizontal. The proposed slings have a vertical working load limit of 4,000 lb each. The plate has no spreader bar, and the lift is a straight vertical pick with no side pull. A nearby worker asks whether the slings are strong enough because “the load is only 6,000 lb.” The rigger says the sling angle “doesn’t matter much” as long as the hook is centered. The plate’s lifting eyes are rated for 8,000 lb each in a vertical lift.
1. Calculate the tension in each sling leg and compare it with the proposed sling’s capacity at the actual angle. Show the geometry.
Read the worked answer
The eyes are 10 ft apart, so each half-span is 5 ft. With a 6 ft sling length, the vertical height is √(6² − 5²) = √11 ≈ 3.32 ft. The angle from horizontal is arctan(3.32/5) ≈ 33.6°, which matches the stated 34°. For a symmetric two-leg lift, each leg carries half the vertical load: 6,000/2 = 3,000 lb vertical. Tension per leg is 3,000/sin 33.6° ≈ 5,420 lb. The sling’s rated vertical capacity is 4,000 lb; at 33.6° from horizontal, its reduced capacity is approximately 4,000 × sin 33.6° ≈ 2,210 lb per leg. The proposed slings are overloaded by more than a factor of two. The conclusion is not based on the 6,000 lb total alone; it comes from the sling angle and the resulting leg tension.
2. What horizontal force does the sling configuration impose on the plate, and why does that matter?
Read the worked answer
Each sling leg has a horizontal component of T cos θ ≈ 5,420 × cos 33.6° ≈ 4,510 lb. Those inward components compress the plate between the lifting eyes with roughly 4,500 lb. That matters because the plate may not be designed for that compression, especially near the eyes, and because the lifting eyes are rated for vertical lifts. A low sling angle also increases the load on the eyes and hardware beyond the vertical rating. The rigger’s statement that angle “doesn’t matter much” is incorrect; it changes both sling tension and the forces introduced into the load.
3. Propose a safe rigging change and verify it.
Read the worked answer
Use a spreader bar so the slings hang vertically, or use longer slings to increase the angle. With a spreader bar, each leg tension is 3,000 lb, which is within the 4,000 lb vertical WLL, and the horizontal compression on the plate is eliminated. If longer slings are used instead, check the new angle: 10 ft slings give a height of √(10² − 5²) ≈ 8.66 ft and an angle of about 60°. Tension becomes 3,000/sin 60° ≈ 3,464 lb. At 60°, the reduced capacity is 4,000 × sin 60° ≈ 3,464 lb, which is exactly at the limit, so that is not an adequate margin. I would require a spreader bar or slings with a higher WLL, and verify the lifting eyes for the actual angular loading.