Two-Leg Lift of an Offset Column Weldment
A weldment consists of a 2.0 m × 1.5 m × 20 mm steel base plate and a 4.0 m length of 250 × 250 × 12 mm square hollow section column welded to it. The column centerline is offset 300 mm from the base plate centerline along the 2.0 m dimension. Two lifting lugs are welded on the base plate, 0.80 m either side of the centerline along the same 2.0 m dimension. The rigger plans a two-leg lift with slings at 60° to the horizontal, using 2 t polyester roundslings in a choker hitch. The load weighs about 830 kg. The crane operator says the load will 'hang level enough.' The shop floor is clear, but the load must pass over a 1.2 m high bench.
1. Calculate the center-of-gravity offset and the vertical load on each lug.
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Base plate volume = 2.0 × 1.5 × 0.020 = 0.060 m³; mass = 0.060 × 7850 = 471 kg. For the 250 × 250 × 12 SHS, outside area = 0.250² = 0.0625 m²; inside area = (0.250 - 2 × 0.012)² = 0.226² = 0.05108 m²; steel area = 0.01142 m². Volume for 4.0 m = 0.04570 m³; mass = 0.04570 × 7850 = 359 kg. Total mass = 471 + 359 = 830 kg. Take moments about the base centerline: CG offset = (359 × 0.300) / 830 = 0.130 m toward the column. Lugs are at ±0.80 m. Sum moments about the left lug: right reaction = 830 × (0.80 + 0.130) / 1.60 = 482 kg; left reaction = 830 - 482 = 348 kg. The right lug carries more load because the CG is closer to it.
2. Calculate the sling tension in each leg at 60° and assess the proposed 2 t choker slings.
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Sling tension = vertical reaction / sin(60°). Right leg = 482 / 0.866 = 557 kg; left leg = 348 / 0.866 = 402 kg. A 2 t polyester roundsling has a vertical working load limit of 2000 kg, but a choker hitch reduces capacity, typically to about 80%, or 1600 kg. On those numbers both legs are below the choker rating, but the conclusion is not automatic. The greater risk is the asymmetric CG: with equal sling lengths from one hook, the load will tilt until the CG is under the hook, so the sling angles and tensions change. The lift should use a spreader beam, adjustable slings, or a hook position calculated for the CG so the load stays controlled. Shock loading, edge contact, and the condition of the lugs must also be checked.
3. What rigging plan and pre-lift checks would you require before the load passes over the bench?
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I would require a documented lift plan showing the 830 kg weight, the 0.13 m CG offset, the lug locations, sling angles, tension in each leg, and the crane capacity at the required radius. The lugs must be inspected and confirmed as designed for the load direction. Use shackles of adequate rating, protect sling edges, and avoid choker damage at sharp corners. Because the load is asymmetric, use a spreader or adjustable rigging so the base remains level, and attach tag lines to control rotation. Check the hook height for the 1.2 m bench clearance, including the load and rigging. Perform a trial lift of about 100 mm, confirm the load hangs as planned, check for movement or sling slip, and keep personnel out of the exclusion zone before moving over the bench.