Off-Centre Precast Panel Lift
A precast wall panel 3.6 m high by 2.4 m wide, weighing 1,850 kg, is to be lifted from a flatbed and set on a foundation wall. The lift plan shows a two-leg chain sling with the master link at the panel's top centre, but the panel's centre of gravity is 300 mm off the geometric centre because of an integral pilaster. The included angle between the legs at the hook is 120 degrees. Each chain leg is tagged 1,000 kg WLL for a single vertical lift. The crane's capacity at the 9 m working radius is 4,200 kg; the rigging weighs 120 kg. Wind is 32 km/h gusting to 45 km/h, and the panel has no wind openings. The operator cannot see the landing zone, and the radio to the signal person cuts out intermittently. The crew proposes one tag line and plans to 'walk it in' by hand.
1. Calculate the load on each sling leg and compare it with the crane and sling ratings. What does the result tell you?
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Total load = 1,850 kg panel + 120 kg rigging = 1,970 kg. Crane utilisation = 1,970 / 4,200 = 47%, so the crane's static capacity is not the controlling problem. Sling geometry is. With a 120-degree included angle, each leg is 60 degrees from vertical, and the tension per leg is T = W / (2 cos 60) = 1,850 / (2 x 0.5) = 1,850 kg. Each leg is tagged 1,000 kg WLL, so each leg is at 185% of its vertical rating before any off-centre effect. Because the centre of gravity is 300 mm off centre, the load will not be shared equally; one leg will take more than 1,850 kg. The lift is therefore not acceptable with this rigging. The crane may have capacity, but the rigging does not.
2. Would you allow this lift to proceed? Identify the controlling hazards and the changes required.
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Do not proceed. The controlling hazards are the overloaded slings, the off-centre centre of gravity, the wind, and the loss of communication. Change the rigging first: use a certified spreader beam or four-leg sling arranged so the load is shared and each leg is within its rated angle, or use a single leg with a properly rated shackle if the panel has a designed lifting point. Recalculate the tension at the actual angles and check the shackles, master link, and hooks. The wind is 32 km/h gusting to 45 km/h on an 8.64 m2 solid panel; that is a sail, and the manufacturer's wind limit may be lower. Stop if the gust limit is exceeded. The operator is blind and the radio is intermittent, so a qualified signal person must be in continuous communication, by radio with a backup hand-signal method, and the landing zone must be barricaded. A trial lift and a hold point are required.
3. How should the off-centre centre of gravity and the communication failure be managed before the panel is moved?
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The off-centre centre of gravity must be corrected before the panel leaves the flatbed: adjust the pick points or use a spreader beam so the panel hangs level, mark the centre of gravity, and use two tag lines from opposite corners so the panel cannot spin. One tag line is not enough to control a 2.4 m by 3.6 m panel. For communication, do not accept an intermittent radio. Test the radio, assign a backup signal person or hand signals, and stop the lift if communication is lost. The operator must not move the load on a guess. A pre-lift meeting should confirm the load weight, rigging, wind limit, signal sequence, and the exclusion zone.