Two-Leg Choker Lift of an Offset-CG Skid
A 6,000 lb skid-mounted pump package must be set on a mezzanine 22 ft above grade. The only crane available is a 30-ton hydraulic truck crane; at the final set point the load radius is 26 ft and the capacity chart (outriggers fully extended, no jib) shows 9,400 lb. Two 3/4-inch wire rope slings, each rated 4,000 lb in a vertical hitch, will be choked to lifting lugs 60 inches apart on the skid. The skid's center of gravity is marked 10 inches from the midpoint between the lugs, toward the north lug. The rigging plan shows both legs at 60 degrees to horizontal. The operator intends to boom out and swing the load over an active, in-service pipe rack to reach the mezzanine.
1. Compute the load each sling leg actually carries and determine whether the two slings are adequate.
Read the worked answer
Step 1 - Reactions from the offset CG (statics, not a 50/50 split).
- Lug spacing L = 60 in. The CG is 10 in toward the north lug from the midpoint.
- North lug to CG = 30 - 10 = 20 in. South lug to CG = 30 + 10 = 40 in.
- R_north = 6,000 lb x (40/60) = 4,000 lb
- R_south = 6,000 lb x (20/60) = 2,000 lb
Check: 4,000 + 2,000 = 6,000 lb.
Step 2 - Leg tension at 60 degrees to horizontal.
- T = R / sin 60 deg = R / 0.866
- North leg: 4,000 / 0.866 = 4,619 lb
- South leg: 2,000 / 0.866 = 2,309 lb
Step 3 - Sling capacity in the choke.
- A choker hitch derates a sling rated in a vertical hitch by about 25%.
- 4,000 lb x 0.75 = 3,000 lb WLL per sling
Conclusion: not adequate.
The north sling is asked to carry 4,619 lb against a 3,000 lb working load limit, roughly 154% of rating. The south sling is lightly loaded by comparison. The failure mode is a broken or damaged sling on the north side, and the imbalance also means the skid will hang tilted. Checking only the total load against the sum of the ratings would have missed this entirely.
2. The lift is on hold. What re-rigging and verification would you require before it proceeds?
Read the worked answer
Fix the geometry and the capacity, in this order:
- Re-rig for the real load path. Options, best first:
- A spreader beam with vertical legs, which removes the angle penalty and puts each leg back near its vertical rating.
- Larger slings selected from the choked rating at the actual angle, sized for the 4,000 lb north reaction, not the 3,000 lb average.
- Reposition the lugs so the pick is closer to the CG, if the skid design allows.
- Recompute after re-rigging. Any change to sling type, hitch, angle, or lug position changes the numbers, so redo the reaction and tension calculation and re-check the choke derate and the D/d ratio around the lug.
- Verify the hardware. Confirm the lifting lugs are rated, welded per drawing, and free of cracks; shackles and hooks rated at or above the leg tension with the pin sized to the lug; no knots, kinks, or broken wires in the rope.
- Verify crane capacity for the whole path. The 9,400 lb at 26 ft covers only the set point. Check the pick point and every point in between, confirm the chart is for full outriggers on the actual ground bearing (matting under the floats), and confirm the load plus rigging weight is well inside the chart. Add rigging weight to the 6,000 lb.
- Control the load. Tag lines on both ends, a spotter who is not under the load, and a pre-lift meeting.
3. The operator says he will just boom out and swing the skid over the active pipe rack. Is that acceptable as described, and what would you require?
Read the worked answer
Not as described. A swing over an in-service pipe rack is a lift-plan decision, not an operator preference.
Issues to resolve before the lift:
- The route is a hazard. Loss of load, a sling failure, or a two-blocking event over live piping can rupture a line, release product, or injure people below. The rack owner and the plant's lifting authority must agree to the route.
- Exclusion zone. A barricaded area below the swing path with no personnel, and the area treated as a drop zone for the entire move.
- Line-of-fire and crane configuration. Longer boom at greater radius reduces capacity and increases deflection. Confirm the chart for that exact configuration, and confirm the anti-two-block and load-moment indicator are functioning.
- Alternatives. Can the crane set up on the mezzanine side and pick straight in? Can the load be moved with a different method or at a lower-risk time?
- Documentation. This looks like a critical or engineered lift given the offset CG and the swing over live equipment; it needs a written, reviewed lift plan and a competent person's sign-off, not a verbal plan at the truck.
Answer: the swing is only acceptable after the route is approved, the exclusion zone is in place, the crane configuration is verified against the chart for the whole path, and the lift plan is written and signed. Otherwise, find another route.