Two-Part Line and an Unknown Load
You are asked to lift a bundle of steel reinforcement from a delivery truck to the deck. The bundle is banded but the bands are partly broken, and the site engineer estimates the mass at 'about 2.5 t'. The crane is a hammerhead with a maximum capacity of 8 t at 20 m radius, falling to 3.2 t at 45 m. The truck is parked at 38 m radius. You have a two-leg chain sling rated at 4 t per leg in a straight pull, at 60° included angle between the legs. The rigger proposes a two-part line (reeving the load line through a snatch block on the hook) to 'double the capacity'. The load chart in the cab is for single-part reeving. The wind is calm and the deck is clear.
1. Calculate the approximate capacity available at 38 m and assess the rigging proposal.
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At 38 m the chart capacity is between 8 t at 20 m and 3.2 t at 45 m. Interpolating linearly is not strictly correct - the chart must be read at the actual radius - but the trend shows capacity is well below 8 t and approaching 3.2 t. At 38 m, a reasonable estimate from the stated end points is roughly 4 t, but the operator must read the actual chart figure for 38 m, not interpolate. The estimated 2.5 t load may be within capacity, but the estimate is unverified.
The two-part line proposal is wrong. Reeving through a snatch block increases the line pull available at the hook but does not increase the crane's rated capacity at that radius; the load chart is based on the structural and stability limits of the crane, and the chart shown is for single-part reeving. Using a different reeving changes the line speed and the load on the line, and may invalidate the chart. The capacity at radius is unchanged. The proposal must be rejected unless the manufacturer's chart for that reeving is used.
2. What must happen before the lift proceeds?
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Obtain a verified mass - weigh the bundle or get a documented figure, not 'about 2.5 t'. Read the actual load chart at 38 m and confirm the load plus rigging is within capacity. Inspect the sling and reject any damaged or unrated components; check the chain sling rating at the actual included angle, since a 60° included angle reduces each leg's vertical share. Re-band or contain the bundle so it cannot spill. Confirm the rigging arrangement is the one covered by the chart, and record the lift plan. If any of these cannot be confirmed, do not lift.