Four-Leg Bridle With a Damaged Chain and Unknown Sharing
A 14,000 lb welded steel skid is to be lifted by four corner lugs on a 10 ft square pattern. The plan is to hang the hook 7 ft above the lug plane when taut, using a 4-leg chain sling with 10 ft legs, which puts each leg at about 45 degrees from vertical. The rigger says each leg only sees 3,500 lb because the load is 14,000 lb, so the 4-leg sling tag reading 12,600 lb working load limit at 90 degrees included angle is fine. One leg has a link with a deep gouge and a slight twist; the rigger says it is not in the load path because the other three will take the weight. The shackles at the lugs are 3/4 in screw-pin with a 4.75 ton working load limit, and one has no load-rated tag. The lugs are rated 5 tons each, vertical only. The crane's chart capacity at the planned radius is 18,000 lb, and the hook block and slings weigh 900 lb.
1. Compute the actual leg tension and evaluate the rigger's 3,500 lb per leg figure.
Read the worked answer
The 3,500 lb figure is simply 14,000 divided by four, which assumes all four legs share equally. That assumption is not available here: the sling legs are not a verified matched set, and equal sharing on a four-leg bridle cannot be demonstrated without proof of equal length and a load stiff enough to enforce it.
Geometry: the half diagonal of a 10 ft square is 7.07 ft. With the hook 7 ft above the lug plane, each leg is at 45 degrees from vertical, so the vertical component of each leg's tension is its tension times cosine 45, or 0.707.
Design on two legs sharing the load, which is standard practice where equal sharing cannot be proven. Each leg carries 14,000 / 2 = 7,000 lb vertically, so tension = 7,000 / 0.707 = 9,900 lb. Even crediting three legs gives 6,600 lb per leg, far from 3,500 lb.
At 9,900 lb, the 4.75 ton (9,500 lb) shackles are overloaded, and the sling is being asked to work at an included angle its tag does not cover.
2. List each deficiency in the rigging and the disposition you would direct for it.
Read the worked answer
- Damaged chain leg: a gouged and twisted link is removal criteria. It cannot be left in the sling, and it cannot be treated as a non-load-bearing leg. Tag it out of service, remove the sling from the job, and do not field-repair. Chain repairs belong to a qualified chain repair facility, and the sling must come back with a load-rated tag.
- Working load limit at the wrong angle: the tag reads 12,600 lb at 90 degrees included angle. This lift is at 90 degrees, but that rating assumes load sharing the damaged leg cannot provide. Use a matched sling whose rating is stated for this geometry and confirm it against the two-leg basis.
- Undersized shackles: 3/4 in screw-pin at 4.75 tons is 9,500 lb against 9,900 lb of computed tension. Step up to a shackle rated above the tension with margin, and confirm the pin is fully seated and moused.
- Untagged shackle: no load rating means out of service, full stop.
- Lugs rated vertical only: the 45 degree leg angle introduces a horizontal component into each lug. Confirm with the skid manufacturer or the engineer that the lugs and their welds can take it, or use a spreader beam so the legs pull vertically.
3. The crane has capacity. What must the lift plan show before you approve the pick, and what is the crane's actual utilization?
Read the worked answer
Utilization: 14,000 lb load plus 900 lb of hook block and slings is 14,900 lb against 18,000 lb, or 83 percent. So this is not a comfortable pick even before the rigging is fixed; it triggers the critical lift threshold under the 75 percent policy, which means a written plan, a pre-lift meeting, and hold points.
Before approval, the plan must show:
- The revised rigging arrangement, with a matched sling and shackles sized against a stated assumption for how many legs share the load, and the tension at the actual leg angle.
- Confirmation from the skid manufacturer or engineer that the lugs can take the horizontal component, or a spreader beam that eliminates it.
- Radius measured on the ground, verified against the chart for the exact boom and counterweight configuration, with the rigging weight included.
- The rigging hardware certificates and current inspection records, and the damaged sling documented as removed from service.
- Tag lines, a barricaded drop zone, and a defined hold point for the operator to confirm the load is level before the swing.
The damaged sling is a stop-work item. Fix the rigging, then revisit whether 83 percent is where you want this crane to be.