Precast Panel With an Offset Centre of Gravity
At a precast yard, a 3.6 t wall panel measuring 4.8 m by 2.6 m is to be lifted from the casting bed onto a truck. It has two lifting inserts 2.4 m apart. Because of a blockout, the centre of gravity is 400 mm from the midpoint toward insert A. The rigging crew proposes two 2.0 t chain slings from each insert to a single hook centred over the panel's geometric centre; each leg will be about 30 degrees from vertical. The crew says each sling only carries half the load. A trial lift of 150 mm is made and the panel tilts, with the A side low. The load is still over the bed, and a tag line is attached. The operator must decide whether to continue, and what rigging change is needed.
1. Calculate the approximate vertical reaction at each insert if the panel were held level by a spreader beam, and explain why the proposed single-hook rigging does not produce a 50/50 share.
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The panel weight is 3.6 t, or about 35.3 kN. The inserts are 2.4 m apart, so the midpoint is 1.2 m from each insert. With the centre of gravity 0.4 m toward A, the distance from A to the CG is 1.2 - 0.4 = 0.8 m, and from B to the CG is 1.2 + 0.4 = 1.6 m. Taking moments about A: R_B x 2.4 = 3.6 x 0.8, so R_B = 1.2 t. Then R_A = 3.6 - 1.2 = 2.4 t. If a spreader beam holds the legs near vertical at 30 degrees from vertical, the sling tensions become approximately T_A = 2.4 / cos 30 = 2.77 t and T_B = 1.2 / cos 30 = 1.39 t. The horizontal components are about 1.39 t and 0.69 t, which are not equal, so a single hook cannot hold the panel level unless the hook is positioned over the CG and the leg angles differ. With the hook centred over the geometric centre, the panel rotates until the CG is below the hook. The A-side insert and sling therefore carry much more than half the load. The proposed 2.0 t slings are not adequate for the A-side tension, even before dynamic effects.
2. What rigging configuration and pre-lift checks would you require before lifting the panel to the truck?
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I would stop the trial lift, land the panel, and require a rigging plan that accounts for the actual CG. The preferred arrangement is a spreader beam or lifting beam with the hook positioned over the CG, or a certified equalizing system designed for an offset load, with each sling and insert rated for its calculated tension rather than half the panel weight. The insert capacity must be confirmed against the manufacturer's data, because the A insert is carrying about 2.4 t vertically and more along the sling. The sling angles, shackles, master link and hook latch must be checked, and softeners used where the slings contact the panel edge. Before the lift I would confirm the panel has no cracks around the inserts, attach two tag lines, clear personnel from the drop zone, and make a short trial lift to check level and rigging stability. If the panel still tilts, it must be landed and the rigging re-engineered; a tilting panel is not controlled by telling the signaler to 'hold it straight.' The final configuration and calculations should be recorded on the lift plan.