Offset Center of Gravity in a Two-Leg Bridle
An 8,000 lb steel skid is to be lifted from a truck bed with a two-leg bridle. The skid is 8 ft long between lifting eyes. Its center of gravity is 3 ft from the left eye and 5 ft from the right eye because a pump is mounted on the left end. Each sling leg is 1/2 in. 6x19 IWRC wire rope with a tagged vertical working load limit of 4,000 lb. The rigger plans to use both legs at 60 degrees from horizontal and a single vertical tag line. The hook block weighs 650 lb and will be included in the crane's load chart deduction. The load chart at the planned 30 ft radius allows a gross capacity of 14,000 lb. The rigger says, 'Each leg only sees half the load, so 4,000 lb per leg is well under the 4,000 lb vertical rating.' The skid has sharp edges at the lifting eyes; no softeners are shown. The landing area is clear, but the skid will be swung over an open walkway.
1. Calculate the vertical reaction at each lifting eye and the tension in each sling leg at 60 degrees from horizontal. Does the rigger's 'half the load' assumption hold?
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No. The CG is not centered, so the vertical reactions are not equal. With an 8 ft span and the CG 3 ft from the left eye and 5 ft from the right eye:
- Left vertical reaction = 8,000 lb x (5 ft / 8 ft) = 5,000 lb
- Right vertical reaction = 8,000 lb x (3 ft / 8 ft) = 3,000 lb
For a two-leg bridle at 60 degrees from horizontal, the sling tension is the vertical reaction divided by sin 60 degrees (0.866):
- Left tension = 5,000 / 0.866 = about 5,774 lb
- Right tension = 3,000 / 0.866 = about 3,464 lb
The rigger assumed 4,000 lb per leg. The left leg is actually carrying about 5,774 lb of tension, which exceeds its 4,000 lb tagged vertical WLL. Even the vertical load allowed by that sling at 60 degrees is 4,000 x 0.866 = 3,464 lb, and the left eye is carrying 5,000 lb vertically. The left leg is overloaded and the plan is not acceptable.
2. What rigging changes would you require before the lift?
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The rigging must account for the offset CG. A spreader bar or adjusted pick points can move the hook line over the CG and equalize the leg loads, but the new geometry must be recalculated. Alternatively, use slings and hardware rated for the actual tension at the actual angle, with the left leg sized for at least 5,774 lb plus an appropriate design factor. Do not use the vertical WLL as if it applies at 60 degrees without checking the angle effect.
Add softeners or edge protection at the sharp lifting eyes, inspect the eyes for cracks or wear, and verify that shackles, master link, and hook latch are rated and properly sized. The tag line should be used to control rotation, not to support or steady the load vertically. Because the load will pass over an open walkway, barricade or reroute the walkway and keep personnel out from under the load. A qualified rigger should verify the final arrangement and the crane operator should confirm the total suspended load against the chart.
3. What total load should be compared with the crane's gross chart capacity?
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The chart comparison must use the total suspended load, not just the 8,000 lb skid. Add the hook block (650 lb), the slings, shackles, master link, and any spreader bar used. If the rigging weighs, for example, 150 lb, the total is 8,000 + 650 + 150 = 8,800 lb. The 14,000 lb gross chart capacity at 30 ft is then reduced by all load-handling devices to get the net capacity available for the skid. The operator should also confirm that 30 ft is the actual radius with the load suspended and that the chart is valid for the outrigger configuration and quadrant. The nominal crane rating is not the capacity; the chart value at the actual radius and configuration controls.