Two-Crane Tandem Lift with Uncontrolled Load Share
An 18,000 lb horizontal vessel must be set on foundations using two cranes because one side is blocked by a pipe bridge. Crane A, a 50-ton truck crane, is rigged 12 ft from the vessel's center of gravity on the left side; Crane B, a 30-ton rough-terrain crane, is rigged 18 ft from the CG on the right side. The lift points are 30 ft apart. Crane A's chart capacity at the planned radius is 14,000 lb; Crane B's is 9,500 lb. The plan is for Crane A to take most of the load and Crane B to stabilize, with hand signals from a signal person who can see both operators. The vessel must be lifted 9 ft and moved 15 ft sideways. Wind is 12 mph gusting to 20 mph; the vessel has a 40 ft² sail area.
1. Calculate the static load share at each crane and compare it with the chart capacities.
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Static reactions from the 30 ft spacing: Crane A = 18,000 × 18 ÷ 30 = 10,800 lb; Crane B = 18,000 × 12 ÷ 30 = 7,200 lb. Add the rigging and block weights and any dynamic factor. At those static values, A is at 10,800 ÷ 14,000 = 77% and B at 7,200 ÷ 9,500 = 76% of chart capacity. The lift has little margin for load shift.
2. Why is the plan for Crane A to take most of the load and Crane B to stabilize unsafe, especially with the stated wind?
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The plan is unsafe because the load share is not fixed. If B lowers its hook, the vessel pivots about A's pick point and A can take nearly the entire 18,000 lb, exceeding its 14,000 lb capacity. Wind at 20 mph on 40 ft² will cause swinging and additional dynamic load. Stabilizing is not a defined load share; both cranes must be treated as lifting, not as one lifting and one tagging. A single signal person can direct both only with an agreed stop signal and clear line of sight or radio.
3. What controls and planning must be in place before the tandem lift starts?
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Before the lift, a qualified person should prepare a tandem-lift plan with calculated load share, rigging, radius, capacity, wind limit, and abort criteria. Both operators and the signal person must review it. Use a trial lift to confirm the load hangs level and the share is as predicted; keep personnel out from under the load; monitor the load and boom for movement; stop if wind exceeds the plan or if either crane approaches its limit. The lift director should control the sequence, and either operator must be able to stop the lift.