Lifting a 9.2 t skid with a two-leg sling: leg loads, off-centre centre of gravity and crane capacity
A 9.2 t compressor skid with four lifting lugs, spaced 3.0 m apart in the lifting direction, must be lifted from a truck and set on a foundation 6 m away. The rigging plan calls for a two-leg sling, each leg a 6×37 wire-rope sling with a 5.0 t working load limit, with an included angle of 70° between the legs; each leg ends in a 4.75 t screw-pin shackle. The actual centre of gravity sits 400 mm toward lug A. The crane's load chart shows 10.0 t at the 12 m working radius, and the hook block weighs 1.2 t. The crew intends to add the slings (0.2 t total) to the load and lift. The near sling has three broken outer wires in one lay and a kinked section; the shackle pin is worn flush with the body.
1. Compute the leg loads and the crane load, and state whether the planned lift is acceptable.
Read the worked answer
Crane load: 9.2 t (skid) + 0.2 t (slings) + 1.2 t (hook block) = 10.6 t against a chart capacity of 10.0 t at 12 m. That is 106 % of the chart value, before any dynamic allowance. Not acceptable; the chart already represents the limit, and a lift that starts and stops adds load.
Sling geometry: included angle 70°, so each leg is 35° from vertical; cos 35° = 0.819.
Symmetric case: 9.2 / (2 × 0.819) = 5.62 t per leg, already above the 5.0 t WLL.
With the centre of gravity 400 mm toward lug A, the vertical share on the near leg is 9.2 × (1.5 + 0.4) / 3.0 = 5.83 t, and on the far leg 9.2 × (1.5 − 0.4) / 3.0 = 3.37 t. Converting to sling tension:
- Near leg: 5.83 / 0.819 = 7.1 t
- Far leg: 3.37 / 0.819 = 4.1 t
With a 1.1 dynamic factor the near leg reaches about 7.8 t, i.e. roughly 156 % of the 5.0 t WLL. The 4.75 t shackle on that leg is also exceeded. The lift must not proceed.
2. What must be changed before the lift, and how do you deal with the damaged sling and shackle?
Read the worked answer
Re-rig, do not micro-manage the numbers. Options: use a spreader beam so the legs hang vertically (tension then equals the vertical share, about 5.8 t near leg, still needing a larger sling), use four legs sized for the off-centre share, or use two legs of at least 10 t WLL at an included angle no greater than about 60°. Each option must be recalculated, not guessed, and the lugs checked against the intended sling direction - lugs designed for vertical pull may not be rated for a 35° offset.
Crane: reduce the working radius or use a crane whose chart at the actual radius exceeds the total load with margin. Recheck the chart at the radius that the final placement requires, not the pickup radius.
Sling: a kink or bird-cage is grounds for immediate discard regardless of the broken-wire count, and three broken outer wires in one lay add to that. Tag it and remove it from service; do not cut it up and leave it in the gang box.
Shackle: a pin worn flush with the body, or without a functioning retaining device, means the WLL is no longer valid. Discard it.
Plan: the revised lift needs a written plan approved by a competent person, with the centre of gravity marked on the skid, the lug capacity verified, the sling and shackle certificates checked, and the exclusion zone defined before the trial lift of 100–200 mm.
3. What is your authority and duty at the moment the crew says the load is 'only slightly over', and what post-lift checks apply?
Read the worked answer
An inspector has stop-work authority on a nonconforming lift, and it should be used without negotiation. Say clearly that the lift is stopped: the near leg is at roughly 156 % of its WLL, the crane is over its chart and the rigging hardware is damaged. Record the calculations, the chart reading and the condition of the sling and shackle in the site log, and inform the lifting supervisor and the client in writing. 'Slightly over' is not a category in lifting: a WLL is a limit, not a target, and the failure mode is sudden and not proportional to the overload.
The common pressures - crane already mobilised, truck blocking the gate, night shift - are exactly the conditions under which overloads happen. If the client overrides, escalate through the site safety officer and the regulator rather than signing anything.
After a compliant lift: inspect the slings for new broken wires, crush or heat damage; check the lugs and the skid for deformation or cracking at the lifting points; check the crane rope for slack or twist; confirm the load is stable on its foundation and the temporary supports are removed; and confirm that the lift record, including the revised rigging plan and the load test, is filed.