Two-Leg Pick of an Off-Center Stair Assembly
A 4,800-lb prefabricated stair assembly, 12 ft long, is to be set on the third floor. The center of gravity is 4 ft from the left end. The rigger proposes two pick points 2 ft and 10 ft from the left end, using two 8-ft wire-rope slings in a choker hitch, each with a 2-ton vertical working load limit, and a hook height that puts the slings at about 45° from horizontal. The crane chart shows 9,400 lb capacity at the 38-ft radius with outriggers fully extended. Rigging and shackles weigh 260 lb. One outrigger will sit on 4 ft of compacted backfill over a utility trench. The crew wants to make the pick before the morning concrete trucks arrive, and the wind is 12 mph with gusts to 20 mph.
1. Calculate the load at each pick point and the sling tension at 45°. Does the proposed rigging have adequate capacity?
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Treat the assembly as a beam. The pick points are 8 ft apart, and the center of gravity is 2 ft from the left pick point and 6 ft from the right. Taking moments about the right pick point: R_left × 8 = 4,800 × 6, so R_left = 3,600 lb and R_right = 1,200 lb. The load is not shared equally, so equal-length slings will let the assembly hang tilted until the center of gravity is under the hook, and the left sling will carry much more than half the load.
For a conservative check, use the larger reaction and the sling angle: T = R_left / sin 45° = 3,600 / 0.707 ≈ 5,090 lb. With a choker hitch, the sling efficiency is about 0.8, so a 2-ton (4,000-lb) vertical-rated sling provides only about 3,200 lb in choker configuration. That is well below the required tension. The proposed rigging is not adequate. The fix is not simply a bigger crane: relocate the pick points closer to the center of gravity, use unequal sling lengths or a spreader so the assembly hangs level, and select slings and shackles based on the actual tension at the actual angle, including the choker derating.
2. The crane chart shows capacity well above the load. What other conditions could make this pick unacceptable?
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Chart capacity is only one part of the lift. The total suspended load is 4,800 + 260 = 5,060 lb, about 54% of the 9,400-lb chart value, so the crane is not the limiting factor at that radius. The outrigger ground is. One outrigger is on 4 ft of compacted backfill over a utility trench, which can settle or collapse under outrigger load. The crane manufacturer's outrigger loading, the trench location, and the soil condition must be evaluated, and matting or an engineered pad may be required. The lift should not proceed until the ground support is accepted.
Wind is the second issue. A 12-ft stair assembly has significant sail area, and gusts to 20 mph can swing or rotate the load. The crane manufacturer's wind limit and the project lift plan govern; if the gust limit is exceeded, the pick stops. The pick also needs tag lines, a pre-lift meeting, and a check that the hook, latches, and shackles are rated and compatible. A chart margin does not justify taking a lift with an unaddressed ground or wind hazard.
3. Describe the rigging configuration and lift controls you would require before the pick.
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First, place the pick points symmetrically about the center of gravity or use a spreader beam with adjustable legs so the stair hangs level and each sling carries a predictable share. Calculate the tension in each leg at the actual angle, not at 60° by habit, and apply the correct hitch derating. Select wire-rope slings, shackles, and hooks with working load limits above those tensions, and verify the hook has a functioning latch.
Protect the stair edges with softeners at every contact point, and use two tag lines so the load cannot spin. Confirm the crane's load chart for the actual radius and configuration, and resolve the outrigger ground condition with matting or an engineered pad before setup. Hold a pre-lift meeting covering the path, the landing, communication, and the wind limit. Make a pre-lift test: raise the load a few inches, stop, and check for shift, sling adjustment, and crane stability. If anything changes, lower the load and re-plan.