Load Chart, Radius and the Extra Block
A self-erecting tower crane is set up on a compacted gravel pad beside a two-storey building. The operator has a load chart for the current jib configuration. The lift is a pallet of concrete pavers, estimated at 1,150 kg including the pallet. The rigger wants to use a four-leg chain sling with a spreader beam, and the hook block sits 6.2 m from the slew centre when the boom is at the required radius. The chart for that configuration shows a rated capacity of 1,400 kg at 6.0 m and 1,250 kg at 6.5 m. Wind is gusting to 12 m/s at ground level. The landing point is a balcony 5.8 m above the pad, and the operator cannot see the balcony from the cab. The site supervisor says the lift is fine because 1,150 kg is under 1,250 kg.
1. Assess whether the lift is within rated capacity, showing how radius and load estimation affect the answer.
Read the worked answer
The supervisor's comparison is invalid because it uses the wrong radius. The hook is at 6.2 m, not 6.5 m, so the capacity must be interpolated between 6.0 m and 6.5 m. Linear interpolation gives 1,400 − (0.2/0.5) × 150 = 1,400 − 60 = 1,340 kg. That is the gross rated capacity at 6.2 m. The 1,150 kg estimate is only the pallet and pavers; the rigging must be added. A four-leg chain sling with a spreader beam could reasonably add 60–120 kg, and the hook block and any below-the-hook device count too if the chart is a gross-capacity chart. At 1,150 + 100 = 1,250 kg the lift is still under 1,340 kg, but the margin is only about 90 kg, or under 7 percent. The operator must confirm the chart basis (gross or net), weigh or verify the pallet rather than rely on an estimate, and re-check the radius with the boom loaded, because jib deflection under load can increase radius. If the radius grows to 6.5 m the capacity falls to 1,250 kg and the lift is at or over the limit. The safe judgement is to re-rig closer, reduce the load, or use a lighter sling arrangement, and to treat the lift as marginal rather than 'fine'.
2. Explain how the blind landing and the wind gusts should change the plan.
Read the worked answer
A blind landing at a balcony 5.8 m up means the operator cannot place the load without a dedicated signaler who has continuous line of sight to both the load and the landing point. The signaler must be positioned so the operator can see them, and radio or hand signals must be agreed before the lift. The load should not be slewed over the balcony until the signaler confirms the landing area is clear and the load is oriented correctly. Wind at 12 m/s ground level is likely higher at boom height, and the crane's load chart or manual will specify a maximum wind speed for operation and a lower figure for handling large sail-area loads. A pallet of pavers has modest sail area, but the spreader beam and sling add some. The operator should check the crane's permitted wind limit, stop if gusts exceed it, and avoid the lift if the load cannot be controlled. The combination of a marginal capacity, a blind landing and gusting wind raises the risk enough that the lift should be replanned, not merely supervised.