Tandem lift with unequal load share
Two 25 t mobile cranes are to tandem-lift a 12 t steel tank from a transport frame onto a foundation. Crane A is at an 8 m radius, where its chart allows 9.0 t; Crane B is at a 10 m radius, where its chart allows 7.0 t. The tank's centre of gravity is 1.2 m from its geometric centre, so the planned load share is not equal: Crane A is expected to take about 60% and Crane B about 40%. The rigging for each crane weighs 250 kg. Both cranes will use two-leg slings to lifting lugs, and the hook heights differ by 300 mm. The lift plan is a sketch showing 'tandem lift, 6 t each' and a radio channel. The ground is firm compacted gravel, but Crane B's outrigger is near a buried services trench. Wind is 8 m/s. The site wants the tank set in one continuous lift because the foundation shims are already placed.
1. Check the planned load share against the crane capacities. What is the controlling problem, and what should happen before the lift?
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The sketch is wrong. A 60/40 share of 12 t gives 7.2 t on Crane A and 4.8 t on Crane B. Adding each crane's 0.25 t rigging gives 7.45 t on A and 5.05 t on B. Crane A is then at 7.45 ÷ 9.0 = 83% of its chart capacity, and Crane B is at 5.05 ÷ 7.0 = 72%. Those figures are within capacity only if the 60/40 share is correct.
The controlling problem is that the load share is not fixed by the sketch. The offset centre of gravity and the 300 mm difference in hook height will change the share during the lift. If Crane A's hook is higher or its slings shorten, A could take more than 60%; it must not exceed about 8.75 t including rigging, or its chart is overloaded. Crane B's outrigger near the buried services trench also needs a ground-bearing check, because poor support could reduce stability or cause settlement.
The lift should stop before any load is raised until an engineered tandem-lift plan confirms the share, ground conditions, hook heights and communication. A trial lift with the load just clear of the frame should be used to verify the share, and the continuous set-down should not proceed without a planned hold point.
2. List the controls that must be in the revised tandem-lift plan and how you would verify the load share during the lift.
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The revised plan should be prepared or checked by a competent person and should show the actual tank weight, centre of gravity, lifting-lug positions, sling angles, rigging weights, radii and the specific load chart for each crane. It should state the permitted load share for each crane, the wind limit, the ground-bearing check for both outrigger positions, and the services scan for Crane B. Mats or spreaders may be needed near the trench.
The plan should nominate one lift director and one radio channel, with backup hand signals. Hook heights should be matched, or an equalising beam used, so the share is predictable. The load should be raised in stages: first a 100 mm trial lift, then a hold to check rigging, stability and load share before continuing. Load cells on each hook, or another agreed method, should verify the share. If either crane exceeds its calculated share, the lift should be stopped and the load lowered.
Other controls include an exclusion zone, tag lines, no personnel under the load, monitoring of outrigger settlement and wind, and a planned landing sequence. The foundation shims being in place is not a reason to make one uninterrupted lift; a hold point at the foundation is safer.