Two-Crane Tandem Lift of an Irregular Tank
Two 15-ton rough-terrain cranes must set a 9,000 lb fiberglass tank onto a foundation. The tank is 22 ft long with its CG marked 8 ft from the left end. Crane A will pick the left end, Crane B the right end, with pick points 20 ft apart. The rigging plan shows each crane taking 50 percent. The site foreman says the load is light and the cranes are big, so no load chart check is needed. Wind is gusting to 18 mph, and the foundation is 30 ft from the pick point, requiring a short travel with the load suspended. The signal person will be on the far side of the tank, out of the operator's view.
1. Compute the actual share each crane will carry and explain why the 50/50 assumption is wrong.
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Take moments about Crane A's pick point. The CG is 8 ft from the left end, and the pick points are 20 ft apart. The share on B equals load × (distance from A to CG)/(distance between picks) = 9,000 × 8/20 = 3,600 lb. The share on A is 9,000 − 3,600 = 5,400 lb. So the split is 60/40, not 50/50. A 50/50 plan would have Crane A sized for 4,500 lb when it actually sees 5,400 lb - a 20 percent underestimate. Tandem lifts also transfer load as the hook heights change: if B is raised first, more weight shifts to A, and the share can approach the full load if one crane goes slack. The plan must state the maximum share any crane can see (here, assume up to the full 9,000 lb in a worst case) and check each crane's chart at the actual radius and boom configuration, including the deduction for the block and rigging. The foreman's "big crane" reasoning ignores that capacity depends on radius, not just nominal tonnage.
2. List the planning and communication controls this tandem lift needs before it starts, given the travel and the blind signal person.
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Tandem lifts require a written plan and a designated lift director who controls both operators. The plan should fix the load's orientation, the order of operations, the maximum radius for each crane, and a stop condition if either crane approaches its chart limit. Because the load must travel 30 ft suspended, verify ground bearing and levelness along the path, keep the load as low as practical, and use tag lines from outside the swing zone. Wind at 18 mph on a 22 ft tank creates sail area; check each crane's chart for the wind allowance and stop if gusts exceed the manufacturer's limit. For signals, a blind signal person is not acceptable: either relocate the signal person to the operator's view, use a relay signal person, or use radios with a dedicated channel and a confirmed communication check. One person must give signals to each operator, and all stop signals must be obeyed from anyone. Brief the crew, establish the exclusion zone, and confirm the foundation is ready before the pick.