Rated capacity at the lift radius
You are operating a portal boom crane on a wharf, lifting a 7.2 t prefabricated steel module from a truck parked 14 m from the slew centre. The crane's load chart for the current boom length and configuration shows 8.0 t at 14 m, 6.6 t at 16 m, and 5.4 t at 18 m, all with the outriggers fully extended on the specified pad size. The wharf deck is rated for the crane's maximum outrigger load only when the pads are on the 1.2 m spreader mats, and two mats are missing. The supervisor says the module is "only just over seven tonnes" and asks you to lift it at the 14 m position without the mats, then slew 90 degrees to place it. Wind is 12 knots gusting to 18, and the load has a 1.5 m2 sail area.
1. Using the chart figures given, explain whether the planned lift is within rated capacity and identify every factor that changes the effective capacity or load.
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The bare numbers look acceptable: 7.2 t against 8.0 t at 14 m leaves 0.8 t, or 10 percent margin. That margin is not real for this lift.
First, the 14 m figure applies only at the stated configuration. The missing spreader mats mean the outrigger load is spread over a smaller area than the chart's ground-bearing assumption, so the crane is no longer in the configuration the chart describes. The chart value cannot be used until the mats are fitted and the deck checked against the actual outrigger load.
Second, radius is not fixed. Slew 90 degrees and the boom tip moves; any settlement, boom deflection under load, or truck movement changes the radius. If radius grows to 16 m, capacity drops to 6.6 t - below the 7.2 t load. The 2 m band between 14 and 16 m is where this lift fails.
Third, add the rigging. Slings, shackles and spreader beam weight count against capacity. Even 0.3 t of rigging brings the total to 7.5 t, cutting the margin at 14 m to 0.5 t and making the 16 m case worse.
Fourth, wind on a 1.5 m2 sail area at 18 knots produces side load and increases the effective radius and swing. The chart figures assume still-air conditions unless a wind derating is specified; check the manufacturer's wind limit and stop-work threshold.
Conclusion: do not lift. Fit the mats, verify deck loading, re-measure the radius at the worst-case position after slewing, add rigging weight, and confirm the wind limit before proceeding.
2. The supervisor argues that because the load is under the chart figure, no lift plan is needed. How do you respond, and what would you document?
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Being under the chart figure is necessary but not sufficient. The chart is one input into a lift study, not a substitute for one.
A load at 90 percent of capacity with a non-standard ground condition, a changing radius and wind is a lift that needs a written plan. The plan should record: the calculated load including rigging; the worst-case radius after slewing and the corresponding capacity; the ground-bearing check with mats fitted; the wind limit and monitoring method; the slew path and exclusion zone; and the communication method with the dogger.
Document the refusal and the reason in the site diary or pre-start record, note the missing mats as a defect, and escalate to the crane supervisor or safety officer if the instruction is repeated. The duty is on the operator to stop when the lift is not adequately planned, and a written record protects both the operator and the site.