Offset Center of Gravity and a Choked Two-Leg Bridle
A 14,000 lb air-cooled chiller with two top lifting lugs 10 ft apart must be lifted from a truck and set on a roof curb. The rigger plans a two-leg bridle using two 8 ft wire rope slings, each rated 10,000 lb in a vertical hitch, choked to the lugs, with the hook centered between them. The chiller's center of gravity is 1 ft off the midpoint toward one lug, and the manufacturer's instructions require the unit to be lifted level, within 5 degrees of horizontal, using a spreader bar. The crane is set up on the street with outrigger mats. The crane's chart at the required 45 ft radius is 18,000 lb. The landing area is a roof with a 6 in concrete deck on steel joists; the structural engineer's report allows 50 psf live load on the roof, and the chiller footprint is 5 ft by 12 ft.
1. Check the rigger's sling arrangement: sling angle, choker derate, and load share. Is it adequate?
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Geometry: half-spread = 5 ft, sling length = 8 ft, so vertical height = square root of (8 squared minus 5 squared) = 6.24 ft. Angle from horizontal = arctangent of (6.24 / 5) = 51.3 degrees, and sine 51.3 degrees = 0.781.
Choker derate: a wire rope sling in a choker hitch is rated at 0.8 of its vertical capacity, so each sling is good for 0.8 x 10,000 = 8,000 lb.
Centered load check, even before the offset: each leg would carry 7,000 lb vertical; tension = 7,000 / 0.781 = 8,963 lb, which already exceeds 8,000 lb.
With the 1 ft offset: the near lug is 4 ft from the CG and the far lug is 6 ft. Taking moments, the near leg carries 14,000 x (6 / 10) = 8,400 lb vertical. Tension = 8,400 / 0.781 = 10,755 lb, or about 134% of the choked capacity. The load will also tilt because the hook is centered on the lugs, not over the CG, violating the 5 degree limit.
Conclusion: the planned bridle is not adequate. Use a rated spreader beam with vertical legs as the manufacturer requires, size the slings for 8,400 lb and 5,600 lb vertical shares, and verify the beam's bending capacity.
2. Is the roof landing area adequate for the chiller? Show the check and state what you would require before the set.
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The uniform check is 14,000 lb / (5 ft x 12 ft) = 233 psf, nearly five times the 50 psf live-load allowance. But the uniform check is not the real issue: the unit sits on a curb, so the load is concentrated at the curb line and the supporting joists, not spread evenly over 60 sq ft.
The engineer's 50 psf is a general live-load allowance, not an approval for a 14,000 lb concentrated unit. Before setting, I would require the structural engineer to check the specific curb location, the joists, and the connections for the point loads, or require a temporary load-distribution platform or placement directly over a beam or column line.
I would also confirm the curb is designed for the chiller and that the roof can take the load during the set, not just in service. Until that is in writing, the chiller stays on the truck.
3. The rigger says he has done this 'a hundred times' and the chiller will be fine. What do you do?
Read the worked answer
Experience is not a substitute for the numbers. The math shows the near leg at about 134% of the choked sling capacity before any dynamic or wind effect, and the unit will not hang level, which the manufacturer prohibits. 'A hundred times' does not change either fact.
I would stop the lift, explain the calculation, and require:
- a rated spreader beam with the manufacturer's lifting diagram,
- sling capacities based on the actual 8,400 lb and 5,600 lb shares,
- verification that the beam and shackles are rated for the load,
- the engineer's approval of the roof location.
If the manufacturer's spreader is not available, get a written alternative from the manufacturer or a qualified engineer. The rigger can be part of the solution, but the decision rests on the plan and the capacity, not on his confidence.