Tandem Lift with Unequal Load Share
A 36,000 lb vessel is to be tandem-lifted by two crawler cranes. Pick point A is 8 ft from the load CG; pick point B is 12 ft from the CG; the points are 20 ft apart. Crane A can lift 14,000 lb at the planned radius; Crane B can lift 18,000 lb. The rigger plans vertical slings and two tag lines. The load is 40 ft long and will be swung 90° during the lift. The site is level, wind is 8 mph, and the travel path is clear.
1. Calculate the static load share at each pick point.
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Use the inverse-distance relationship about the CG.
Load at A = 36,000 × (distance from CG to B / total span)
= 36,000 × (12 / 20) = 21,600 lb.
Load at B = 36,000 × (8 / 20) = 14,400 lb.
Check: 21,600 + 14,400 = 36,000 lb. Because the CG is closer to A, A carries the larger share.
2. Compare the calculated shares with the crane capacities and state whether the lift can proceed as planned.
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Crane A is rated 14,000 lb but would carry 21,600 lb, an overload of 7,600 lb. Crane B is rated 18,000 lb and would carry 14,400 lb, leaving 3,600 lb reserve. The lift cannot proceed as planned. Lowering or slacking Crane B does not solve the problem because the load share is controlled by geometry, not by operator effort. Options include using a larger crane at A, adding a third pick point, or relocating pick point A farther from the CG and recalculating. To bring A to 14,000 lb with the same 20 ft span, A would need to be at least about 12.2 ft from the CG, which may not be practical.
3. What controls are needed during the 90° swing?
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Treat the swing as a changing geometry, not a static pick. Radii change as the load rotates, so both cranes must be checked at their worst-case radii. Keep slings vertical where possible; angled slings introduce horizontal forces that can shift load between cranes. Use two tag lines controlled from outside the swing path, a single lead signal person, and agreed stop points during rotation. Monitor the load line and boom deflection. Stop immediately if the load shifts, a sling goes slack, or either crane approaches its capacity limit.