Tying a Cage That Will Be Lifted
You are prefabricating a column cage on the ground for a lift into position. The column is 400 mm square and 3.6 m tall, with eight N20 vertical bars and N10 ties at 200 mm centres. The crane is booked for 10 am and the rigger wants the cage ready by 9:30. You have tied the cage with single tie wire at every fourth intersection to save time. The lift points are two slings at the top of the cage. A colleague warns that a cage tied this way can distort or shed bars during the lift. The ground is uneven and the cage will be lifted over a walkway.
1. Evaluate the tying pattern and decide whether the cage is safe to lift.
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The cage is not safe to lift as tied. Single tie wire at every fourth intersection is too sparse for a prefabricated cage that will be lifted and slung. The cage must act as a rigid unit during the lift, and the ties transfer the lift forces between bars.
With sparse tying, the cage can distort, the ties can slip, and individual bars or ties can shed. Lifting over a walkway makes any dropped item a serious hazard. The uneven ground also means the cage may not sit flat before lifting, adding racking loads.
The correct action is to add ties, generally at every intersection or as specified, using double ties or appropriate tie wire, and to add temporary bracing or a lifting frame if the cage is slender. The rigger and supervisor should inspect the cage before the lift. Do not rush the tying to meet the crane booking.
2. Describe how you would prepare the cage for the lift and what you would check immediately before the crane takes the load.
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Prepare the cage by completing the ties, adding diagonal bracing or a spreader frame to keep the cage square, and fitting lifting points designed for the cage rather than relying on slings around the top bars. Protect the bars at the sling contact points.
Immediately before the lift, check that all ties are secure, the bracing is in place, the lifting points are correctly located, and the slings are rated and correctly attached. Check the cage is clear of the ground and that no loose bars or tools remain on it.
Then conduct a trial lift of about 100 mm, stop, and inspect for distortion or movement before lifting to height. Keep the walkway clear during the lift. If anything shifts, lower the cage and re-tie before continuing.