Asymmetric skid with offset centre of gravity
A 4.8 t skid is to be lifted from a truck tray using a two-leg sling assembly. The skid is 3.6 m long, 1.4 m wide and 1.9 m high. Its centre of gravity is marked 1.2 m from the left end, not at the geometric centre of 1.8 m. The lifting points are 3.0 m apart on the top frame, set 0.3 m in from each end. The rigger proposes one 8 t polyester round sling choked around the skid and a 4 t shackle at the hook, with no spreader. Because the hook will not sit over the centre of gravity, the sling angle to the horizontal is estimated at 35° on the left leg and 55° on the right leg. The load is to be landed on a concrete slab with a 150 mm upstand; the landing area has an exclusion zone, but two workers are guiding the skid by hand. The lift plan is a one-page 'standard lift' and does not show the centre of gravity offset.
1. Calculate the approximate vertical load share at each lifting point and the resulting sling tension. What does this show about the proposed 8 t sling and 4 t shackle?
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Take the left lifting point at 0.3 m from the left end and the right point at 3.3 m, so the span is 3.0 m. The centre of gravity is at 1.2 m from the left end, 0.9 m from the left point and 2.1 m from the right point. Summing moments about the left point: right reaction = 4.8 t × 0.9 m ÷ 3.0 m = 1.44 t. Left reaction = 4.8 t − 1.44 t = 3.36 t. So the left leg takes about 70% of the load.
Using the estimated angles to the horizontal, left sling tension = 3.36 t ÷ sin 35° ≈ 5.86 t. Right sling tension = 1.44 t ÷ sin 55° ≈ 1.76 t. The left leg is therefore the critical leg by a large margin.
The proposed 4 t shackle at the hook is already undersized for the 4.8 t total load, before any angle effect. The single 8 t round sling is also not a safe arrangement: a choked capacity is typically reduced below the straight-pull rating, and the left leg tension is close to 5.86 t. The sling would also tend to slide or load unevenly because it is not controlling the offset centre of gravity. The arrangement should be rejected.
2. Describe a safer rigging and landing arrangement, including how to control the offset centre of gravity and the upstand.
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I would use two independent slings from a rated master link or spreader beam, with each sling and shackle rated for its calculated leg tension. The hook or master link should be positioned over the centre of gravity, or the sling lengths should be adjusted so the skid hangs level. A spreader beam can keep the legs at a known angle and reduce inward load on the top frame. The left leg needs particular attention: its tension is about 5.86 t, so a 4 t shackle is not acceptable.
Before the main lift I would carry out a trial lift of about 100 mm to check that the skid hangs level and that the load share is as predicted. Tag lines would be attached at two corners so the load can be controlled without workers touching it. The exclusion zone should keep everyone clear of the suspended load and the landing area.
For landing, I would place timber packers or sleepers on the slab so the skid sits above the 150 mm upstand and is not landed on it. Workers would use tag lines or push-pull poles, not their hands, to guide the skid. The load would be lowered slowly, checked for stability on the packers, and only then would the rigging be slackened. The lift plan must be revised to show the actual centre of gravity, sling angles, tensions and landing method.