Re-checking capacity when the load has to go further out
You are loading a 2 100 kg steel beam onto a truck tray using a vehicle loading crane rated at 10 tonne-metres. The truck is set up on a level concrete hardstand with all outriggers fully extended. The beam must be picked from a stack 5 m from the crane's slew centre and placed on the tray at 7.5 m. The load chart shows: at 5 m radius, 2 600 kg with the boom at the required length; at 7 m, 1 750 kg; at 8 m, 1 450 kg. The beam is 9 m long and slung from two points 6 m apart, with the centre of gravity slightly off-centre toward one end. A dogger tells you the beam 'only weighs a bit over two tonne' and suggests lifting it at the stack and slewing with the load close in, then booming out over the tray. The tray deck is 1.3 m high and the beam must clear the headboard.
1. Assess whether the lift can be completed as planned, and identify where in the lift the crane is most heavily loaded.
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The critical radius is not the pick-up point - it is wherever the load is furthest from the slew centre while suspended. Placing the beam on the tray at 7.5 m means the crane must handle 2 100 kg at 7.5 m. Interpolating between 7 m (1 750 kg) and 8 m (1 450 kg): at 7.5 m capacity is about 1 600 kg. The load exceeds that by roughly 500 kg, about 31% over. The lift as planned cannot be done.
Even the pick-up at 5 m (2 600 kg capacity) is not the governing case, because the boom must travel outward to reach the tray. Booming out with the load suspended increases radius and reduces capacity simultaneously - the most dangerous phase. Slewing with the load close in is fine in itself, but it does not help once the boom extends.
The off-centre centre of gravity adds a second issue: with slinging points 6 m apart and the CoG offset, the beam will hang out of level, one sling will carry more than half the load, and the effective radius to the CoG will differ from the radius to the hook. That must be allowed for in the capacity check.
Options: reposition the truck so the tray is within about 6 m of the slew centre, or use a larger crane, or break the load down. I would not proceed on the dogger's estimate.
2. The dogger argues that because the crane is rated at 10 tonne-metres, a 2.1 t load is obviously fine. Explain why that reasoning is wrong and what you would do next.
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The tonne-metre rating is a single summary figure, not a capacity. It describes the crane's load moment, and the actual allowable load depends on radius, boom length, boom angle, whether the jib or fly section is fitted, and the structural or stability limit that governs at that geometry. On many cranes the chart is limited by strength at short radius and by stability at long radius, so the same machine can be comfortable at 5 m and overloaded at 7.5 m with an unchanged load. Quoting the rating without the chart is meaningless.
I would show the dogger the actual chart figures for 5, 7 and 8 m and the interpolation at 7.5 m, and explain the margin: 2 100 kg against roughly 1 600 kg. I would also point out that the chart assumes level ground, correctly extended outriggers, no side pull, and a freely suspended load - all of which we have, so the chart is the binding limit.
Next steps: reposition the truck so the placement point falls within a radius where capacity exceeds 2 100 kg with margin - around 6 m or less - and re-verify the chart at the new geometry. Confirm slinging so the beam hangs level and the load is shared predictably, and check headboard clearance at the reduced radius. If no position works, arrange a larger crane or a different delivery method. The lift plan and chart check get recorded before the lift starts.