Load Chart Selection at Maximum Radius
A self-erecting crane with a maximum capacity of 4 t is being used to lift a prefabricated plant room module weighing 1.85 t. The module is 3.2 m long, 2.4 m wide and 2.6 m high, and its centre of gravity is offset 400 mm from its geometric centre because of an internal pump set. The lift point is a single central lifting eye on the module's frame. The crane's load chart shows 2.2 t at 16 m radius and 1.7 t at 20 m. The intended landing position is 18.5 m from the slew centre. Two polyester roundslings are to be used in a two-leg bridle from the single eye, each leg at 30° to the vertical. The ground is firm and level, and the crane is at its full free-standing height.
1. Determine whether the lift is within the crane's capacity at the intended radius, and state the capacity figure you would use.
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The intended radius is 18.5 m. The chart gives 2.2 t at 16 m and 1.7 t at 20 m, so the capacity at 18.5 m must be interpolated, not taken from the nearer figure.
Linear interpolation: capacity falls 0.5 t over 4 m, i.e. 0.125 t per metre. From 16 m to 18.5 m is 2.5 m, so reduction = 2.5 × 0.125 = 0.3125 t. Capacity at 18.5 m ≈ 2.2 − 0.31 = 1.89 t.
However, interpolation is not a substitute for the manufacturer's actual chart values - many charts are stepped, and the correct approach is to use the tabulated value for the radius band that covers 18.5 m, which may be lower than the interpolated figure. Use the chart's stated value for the applicable radius, and if the chart does not list 18.5 m, use the next longer radius (20 m, 1.7 t) as the conservative capacity.
The module weighs 1.85 t. Against 1.7 t at 20 m it is over capacity; against the interpolated 1.89 t the margin is only 0.04 t (about 2%), which is not acceptable once the offset CoG and dynamic effects are considered. The lift should not proceed at 18.5 m without re-checking against the true chart value and reducing the radius.
2. Explain how the offset centre of gravity and the sling angle affect the actual load on the crane and on each sling leg, and what you would do about it.
Read the worked answer
Offset CoG: the module's CoG is 400 mm from its geometric centre, but the single lifting eye is central. When lifted, the module will hang tilted until the CoG comes under the hook. That tilt changes the effective radius (the load swings outward or inward) and introduces a horizontal component that the crane and sling must resist. It also means the load is not balanced, so one sling leg takes more than the other and the module is harder to control during landing.
Sling angle: two legs at 30° to the vertical. The load factor per leg = 1 / (2 × cos 30°) = 1 / (2 × 0.866) = 0.577, so each leg carries 0.577 × total load. For 1.85 t, each leg ≈ 1.07 t - but because of the offset CoG the loaded leg will carry more, so the sling SWL must be checked against the worst leg, not the average. A 2 t SWL sling per leg would be marginal; a 3 t sling gives sensible margin.
Actions: use a properly designed multi-point lifting arrangement or a spreader/balance beam so the module hangs level; recalculate the radius with the actual hanging position; verify sling SWL against the worst-loaded leg with the offset included; and confirm the lifting eye and frame are rated for the angled bridle. If the crane cannot achieve the required radius with margin, reduce the radius by repositioning the crane or the landing point.