Critical Lift Plan for a Tandem Pick Near a Live Line
Two 50-ton mobile cranes will tandem-lift a 38,000 lb vessel from a truck to a foundation. Crane 1 will take 60 percent and Crane 2 will take 40 percent per the plan, but the rigging geometry places the hook of Crane 2 closer to the load's center, which tends to increase its share. The vessel must pass within 18 ft of a 13.8 kV overhead line; the utility states the line is de-energized but not grounded. Wind is forecast at 18 mph gusting to 25 mph. The lift plan was prepared by the crane rental company and signed by the project superintendent, but no lift director has been designated and the two operators have not met.
1. Assess the load-sharing assumption and what must be done before the tandem pick.
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Planned 60/40 sharing is an assumption, not a physical guarantee. Actual share depends on hook positions relative to the combined CG, boom stiffness, and rigging geometry. If Crane 2's hook is closer to the CG, its share rises above 40 percent and can approach or exceed 50 percent, and small changes in hook height transfer load between cranes. For a 38,000 lb vessel, a shift from 40 to 50 percent is 3,800 lb, which can consume the margin on the smaller-rated crane or its rigging.
Required before the pick:
- Designate a lift director with authority to stop the lift.
- Hold a joint pre-lift meeting with both operators, the rigger, and the signal person; confirm load shares and the sequence.
- Verify each crane's capacity at the actual radius, boom length, and configuration, including the percentage of capacity used; many critical lift plans cap at 75 percent.
- Use a load cell or dynamometer on at least one crane to monitor actual share, and establish a stop point if share deviates from plan.
- Confirm the vessel's CG and rigging attachment points; verify the spreader or equalizer geometry.
- Check ground bearing under outriggers for both cranes.
Judgement: do not proceed on a signed plan alone. The plan is a document; the lift director and joint briefing are the control.
2. Address the overhead line and wind conditions.
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Overhead line: 'de-energized but not grounded' is not a safe clearance basis. An ungrounded line can be re-energized or retain induced charge, and the utility's statement does not establish a verified isolation with visible air gap and grounds. Options: obtain a documented outage with grounds applied and visible isolation, or maintain the required minimum approach distance for 13.8 kV per the applicable standard and the utility's requirement, and use a spotter. If neither can be met, relocate the lift or reschedule. Do not rely on 'it's off.'
Wind: 18 mph gusting to 25 mph affects the vessel's sail area and the cranes' load charts. Many manufacturers require reducing capacity or stopping above certain wind speeds, and a tandem pick is more sensitive because gusts load one crane unequally. Calculate the wind area of the vessel and check the manufacturer's wind limits; if the gust limit is exceeded or the vessel has significant sail area, stop. Also account for the increased swing and loss of control near the line.
Judgement: the lift should not proceed until the line is positively isolated and grounded or the clearance is verified, and until wind is within the crane manufacturer's limits with a monitor on site. The combination of tandem sharing uncertainty, an ungrounded line, and gusting wind is a stop-work condition.