Two-Crane Tandem Lift of an Off-Centre Load
Two cranes are to tandem-lift a 14 t prefabricated steel truss, 18 m long, from a laydown area onto bearing pads. Crane A is a 60 t slew crane at 9 m radius; Crane B is a 40 t slew crane at 8 m radius. The truss centre of gravity is 6.5 m from the left end, so the lift points are not symmetrical. The plan nominally splits the load 50/50. Crane A's chart at 9 m allows 12 t; Crane B's chart at 8 m allows 9 t. Both cranes will be on the same compacted pad, 4 m apart, and the truss must be slewed 90 degrees during the lift. The rigger proposes using the same two-leg sling assembly for both ends.
1. Evaluate whether the proposed 50/50 load split is safe, and calculate what each crane will actually carry.
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A 50/50 split is not safe and is wrong for an off-centre load. The share each crane takes is inversely proportional to its distance from the centre of gravity along the lift points. If the pick points are at the ends (0 m and 18 m) and the CoG is at 6.5 m from the left end, then Crane A (left) carries 18 minus 6.5 equals 11.5 m of the 18 m span, so its share is 11.5/18 of 14 t, about 8.9 t. Crane B carries 6.5/18 of 14 t, about 5.1 t. Total is 14 t, confirming the arithmetic.
On those figures Crane A is at 8.9 t against a 12 t chart limit, about 74%, and Crane B at 5.1 t against 9 t, about 57%. That looks acceptable, but the dynamic effects of slewing, the truss's own flexibility, and any loss of level will transfer load between cranes. The 50/50 assumption would have been dangerous if the pick points were closer to the CoG on one side. The lift plan must use the calculated shares, not an assumed equal split, and each crane must retain margin for load transfer during the slew.
2. What planning and communication controls must be in place before the truss leaves the ground?
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A single lift supervisor must control both cranes, with a documented tandem lift plan agreed by both operators and riggers. The plan must state the calculated load share for each crane, the permitted radius range, the slew path, the maximum wind limit, and the abort criteria.
Both cranes must be on ground proven for the combined outrigger loadings, and their outrigger pads must not interfere. The truss needs a spreader or equalising sling so the load is shared predictably and the truss is not overstressed; the same sling assembly on both ends may not suit the off-centre pick points and must be checked by a competent rigger.
Communication must be by one nominated signaler with a dedicated radio channel, plus a backup hand-signal method. A dry run at low height confirms the load shares and the truss's behaviour before it is taken over the bearing pads. If either crane approaches its limit, loses radio contact, or the load tilts, the lift is aborted and the truss is lowered to a planned safe position.
3. During the slew, Crane B's operator reports the load feels light and the truss begins to tilt. What is happening and what should be done?
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The truss is transferring load to Crane A as it tilts, meaning Crane B has lost its share and Crane A is taking more than its calculated 8.9 t. This can happen if Crane B hoists slightly faster, if the slew geometry changes the effective radius, or if the load's CoG shifts relative to the pick points. If Crane A exceeds its chart capacity the result is a tipping or structural overload.
The immediate action is to stop the slew, hold position, and lower the truss slowly to the ground under the supervisor's direction, with both operators following one voice. Do not attempt to correct the tilt by hoisting one crane, as that worsens the transfer.
Once grounded, the lift plan is reviewed: check the pick points and sling lengths, confirm the load share calculation, verify radii, and consider a spreader beam or a single-crane lift with a larger crane. The lift does not resume until the cause is understood and the plan is amended.