Uneven Four-Leg Lift on a Skewed Load
You are dogging a rectangular steel tank, 4.2 m long by 2.0 m wide, weighing 6.8 t. Its centre of gravity is offset 0.4 m toward one end because of an internal baffle. Four lifting lugs are welded at the corners, but the tank has been slung with a single four-leg chain set of equal leg length (3.5 m each) to a single hook. The hook sits directly above the geometric centre of the tank, not the centre of gravity. During the trial lift the tank tips noticeably toward the heavy end and one chain goes slack. The rigger wants to add a fifth leg to steady it. Wind is light, ground is firm, and the exclusion zone is set.
1. Explain why one leg goes slack and calculate the approximate share of load the two legs at the heavy end will carry during the trial lift (assume the load is effectively carried by three legs once one is slack and that the CG offset is the only asymmetry).
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The hook is above the geometric centre, but the CG is 0.4 m toward one end. The load therefore rotates until the CG lies under the hook, which shortens the distance to the heavy-end lugs and lengthens it to the light-end lugs. Since all four chains are the same length, the geometry can no longer share load evenly; the light-end chains unload and one goes slack.
Once a leg is slack, the tank behaves as a three-point sling. The CG offset of 0.4 m along a 4.2 m length means the heavy end carries the larger share. Treating the two heavy-end legs as sharing the heavy-end reaction and the single light-end leg as taking the rest, the heavy end carries roughly (2.1 + 0.4)/4.2 of the vertical load, about 0.60 × 6.8 t ≈ 4.1 t, split between two legs at about 2.0 t each. The light end carries about 2.7 t on one leg. These are static estimates only; the dynamic factor and any sling angle derating must be added before selecting gear. The point is that equal-length legs do not guarantee equal load.
2. The rigger proposes adding a fifth leg to steady the tank. Assess this proposal and state what you would do instead.
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Adding a fifth leg is the wrong fix. A statically indeterminate sling does not share load predictably; the new leg will either be slack or overloaded depending on tiny length differences, and it gives false confidence. It also does not correct the CG offset, so the tank will still rotate until the CG is under the hook.
The correct approach is to reposition the hook over the CG, or to use a spreader/adjustable sling so the hook line passes through the CG. Options: move the hook by using a longer or shorter leg on one side to tilt the tank so the CG sits under the hook; use a lifting beam with adjustable pick points; or add certified lugs positioned so the sling geometry is symmetric about the CG. After re-rigging, re-check that all legs are taut at a low trial lift, confirm the tank hangs level, and only then complete the lift. Document the revised rigging and re-brief the team.