Load chart interpretation with a lattice boom derrick and a long radius
A lattice-boom derrick is lifting prefabricated wall panels onto a building frame. The panel weighs 2.4 t, and the rigging (spreader beam, slings, and shackles) adds 0.35 t. The operator is working at a 26 m radius with the boom at 65 degrees. The load chart for that boom length shows 3.1 t at 24 m radius, 2.7 t at 26 m, and 2.4 t at 28 m. The chart is based on a freely suspended load with no wind. The site is exposed, and the forecast is for gusts to 12 m/s. The dogger wants to lift the next panel because the total load of 2.75 t is under the 3.1 t figure at 24 m, and the crane can be boomed in slightly once the panel is clear.
1. Assess the proposed lift against the chart and the conditions.
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The comparison must be made at the actual radius, not a radius the crane might reach later. At 26 m the chart allows 2.7 t. The lifted load is 2.4 t plus 0.35 t of rigging, which is 2.75 t, already above the 2.7 t figure before any wind or dynamic allowance. Booming in after the panel is clear does not help, because the critical moment is when the panel is picked up at 26 m, and the load is at its maximum then. The 3.1 t figure applies at 24 m and cannot be used.
The lift is therefore outside the chart at the planned radius. Options are to reduce the radius before the pick, reduce the load (smaller panel or lighter rigging), or use a crane with a higher capacity at that radius. Wind gusts to 12 m/s add a further reason to stop; many operators apply a wind limit well below that for large panels, and the chart assumes no wind. The lift should not proceed as planned.
2. The supervisor says the chart has a built-in safety factor, so being slightly over is acceptable. How would you respond?
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The chart already incorporates the design factors required by the standard, and those factors are consumed by the rated capacity. They are not spare capacity available for the operator to use. Being over the chart means the actual load exceeds the value the crane was designed and tested for in that configuration, so the margin is gone and the structure, ropes, and tipping stability are being asked to do more than their rated duty.
Slight overloads are also not reliably slight in effect. The load may be at 26 m on the chart but the boom tip can move outward under load deflection, increasing the radius and reducing capacity further. Wind and dynamic effects add load. The correct response is to treat the chart as a limit, not a target, and to re-plan the lift so the actual load at the actual radius is within the chart with margin for the conditions.