Four-Leg Bridle on a Rigid Skid: The Per-Leg Number on the Plan Sheet Is Wrong
You are handed a lift plan for a 12,400 lb steel skid with four symmetric lifting lugs. The plan calls for a four-leg bridle of 3/4-in wire rope slings, each rated 8,000 lb in a vertical hitch and 6,000 lb in a choker. The legs are 10 ft long and the hook will sit about 7 ft above the lugs, so each leg runs at approximately 45 degrees to horizontal. The plan sheet shows 3,100 lb per leg, well under the 8,000 lb rating, and is signed off. When you walk the load you find one lug has been bent closed by a previous handling and cannot be rigged vertically, so that leg will be choked around the lug base. The skid is a welded box structure and is effectively rigid.
1. The plan's 3,100 lb per leg comes from dividing the load by four. What is wrong with that assumption?
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Dividing by four assumes all four legs share the load equally. On a rigid load with four fixed attachment points and a single hook, they cannot.
- The load is a welded box; it does not flex to let all four legs come up equally taut.
- In practice the hook finds the position where the geometry balances, and two legs carry the load while two go slack. Which two depends on tiny differences in leg length, lug position, and how the hook seats - you cannot predict it, so you cannot credit the two good legs.
- Even with a perfectly matched set, the standard safe assumption for a multi-leg bridle on a non-equalizing load is two legs take it all, unless the assembly genuinely equalizes the load between legs (a spreader beam with a proper equalizer, or a matched master link and equalizer assembly).
So the plan's 3,100 lb is the number you get if the load were a flexible chain. It is not the number for a rigid skid.
2. Recompute the worst-case leg tension, including the 45 degree angle, and check both the vertical and the choker leg against their ratings.
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Step 1 - worst-case vertical share per leg.
Two effective legs carry 12,400 lb:
- Vertical share = 12,400 / 2 = 6,200 lb per leg
Step 2 - apply the sling angle.
Leg tension = vertical share / sin(angle to horizontal). At 45 degrees, sin 45 = 0.707:
- Tension = 6,200 / 0.707 = 8,770 lb per leg
Step 3 - compare to ratings.
| Leg | Rating | Demand | Result |
|---|---|---|---|
| Vertical hitch legs | 8,000 lb | 8,770 lb | Over by ~10% - fails |
| Choker leg | 6,000 lb | 8,770 lb | Over by ~46% - fails badly |
The plan's 3,100 lb understated the real demand by nearly a factor of three.
The angle is not even the fixable part. Look at what the two-leg assumption does on its own: 6,200 lb per leg before you apply any angle. A 6,000 lb choker rating is already insufficient at 90 degrees - that is, no leg length and no hook height can rescue the choker leg. Steepening the angle helps the vertical legs but does nothing for a component that is under-rated before the angle is applied.
3. Give at least two ways to make this lift safe, and say what you would do about the bent lug.
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Option A - upsize and keep the four-leg bridle.
Pick slings whose vertical rating exceeds 8,770 lb, and whose choker rating also exceeds 8,770 lb (not 6,000). You also have to check the master link, shackles, and crane hook at 12,400 lb plus the angle effects, and confirm the hook and block can take the resultant. Confirm every leg physically fits and the eyes fit the hardware.
Option B - make the arrangement actually equalize.
Use a spreader beam of adequate capacity so the legs hang closer to vertical and the beam takes the horizontal component. Or use a matched equalizer so load share can be credited. Note that a spreader still does not turn four points into four credited legs on a rigid load unless the geometry genuinely equalizes - you may end up with a two-point pick hanging from a beam, and that is fine as long as the numbers were run that way.
Option C - reconfigure to two legs.
Two properly sized slings on two sound lugs, with the load's center of gravity between them and a plan for the load not tipping about that axis. Simple, honest, and easy to inspect.
The bent lug: do not rig it, and do not straighten it with a torch or a hammer and call it good. A deformed lifting lug is a rejected lifting point until someone qualified evaluates it - that lug was bent by a load it was not designed for, and the weld and base metal around it are now suspect. If the lift needs four points, the lug gets repaired or replaced under engineering direction, or you switch to two points plus a positive means of controlling rotation and tilt.