Two-Crane Tandem Lift of an Irregular Load
A 22 t air-handling unit, 9 m long and with its centre of gravity 6 m from the tail end, must be lifted over a 12 m high plant room wall and landed on a roof curb. Crane A (180 t) will take the head end at a 14 m radius; Crane B (130 t) will take the tail end at an 18 m radius. The rigger proposes a single sling per crane to a common lifting eye at each end. The wind is 8 m/s gusting to 12 m/s, and the load has a 6 m² sail area. The plant room roof is occupied by other trades.
1. Identify the load-distribution problem and explain how the centre of gravity affects the share taken by each crane.
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With the CoG 6 m from the tail and 3 m from the head, the load is not shared equally. Static share is inversely proportional to distance from the CoG: the head end takes roughly 22 t × 6/9 ≈ 14.7 t, and the tail takes roughly 22 t × 3/9 ≈ 7.3 t. Those are static figures only. During a tandem lift, the share changes continuously as the hook heights and radii change, as the load tilts, and as wind load acts on the 6 m² sail. A gust can transfer several tonnes from one crane to the other in seconds.
The plan must therefore state, for each crane, the maximum and minimum share expected through the whole lift path, and each crane's chart capacity at its worst-case radius must exceed its maximum share with an adequate margin - typically the same margin the site applies to any critical lift, and never less than the manufacturer's requirements. Both cranes must be capable of holding the whole load briefly if the other loses tension, or the lift must be engineered so that cannot occur. Single slings to end eyes are inadequate: use a spreader or equalising arrangement, rated shackles, and a documented rigging plan signed by a competent rigger and checked by the lift supervisor.
2. What control measures and hold points would you require before and during this lift?
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This is a critical lift: two cranes, an offset CoG, an occupied roof and wind. Before the lift, require a written lift plan and a documented risk assessment, a nominated lift supervisor with authority to stop, a pre-lift meeting with both crane operators and all riggers, and a dry run of the planned path with the load just clear of the ground. Confirm each crane's configuration and chart against the planned radii, and mark the slew limits physically so neither crane enters the other's exclusion zone.
Set wind hold points: agree a measured limit at the load, not at ground level, because wind increases with height. At 12 m/s gusting with 6 m² sail, the wind load and the pendulum effect on a long load are significant; if the limit is reached, lower the load to a safe position and stop. Clear the roof of non-essential trades, or stop their work in the landing zone. Maintain radio contact on a single channel with the lift supervisor controlling both operators; one person gives directions, not two. Tag lines from outside the fall zone. Abort criteria: wind above the agreed limit, loss of radio, any unexpected load shift, or any crane alarm.