Why a Below-the-Hook Device Carries Its Own Rating

Why this matters

A rated load stamped on a lifting beam, a plate clamp, a coil hook or a vacuum lifter is not a general statement that the device is strong. It is a statement about one loading condition, and the data plate almost never spells that condition out. A crew that reads the number and stops there will happily hang a lighter load on the same device in a way that exceeds it, which is exactly what the worked example below does with a load 10 percent under the marked rating. This card is organised around what the rating deliberately leaves out, because the omissions are where people get hurt, and it names who owns each answer you cannot derive.

The rating is a number about a condition, not about a device

Every below-the-hook device was designed against a defined load case: the load applied at specified attachment points, at a specified spread, hanging free and plumb, with the bail on the centreline, at a specified temperature, in a specified duty cycle. The rated load is the answer to that case. Change any term and the answer changes, and the device does not tell you by how much.

That is not a defect in the marking. It is the same contract a sling tag makes and the same contract a crane load chart makes. The difference is that a sling tag prints an angle table and a load chart prints a radius column, so the reader can see the condition varying. A below-the-hook device prints one number, so the condition is invisible.

The device is also its own link in the chain, and the chain is only as strong as its weakest link. A 20 ton crane, 5 ton slings and a beam marked 4,000 lb give you 4,000 lb. Nothing upstream raises a below-the-hook rating, and the number of people who have argued otherwise at a crane is the reason this sentence is in the card.

What the rated load does not include

This is the part worth memorising, because each omission has been the cause of a real failure.

It does not include a different spread. On a lifting beam, the bail is a support and the lugs are load points, so the beam is in bending, and bending moment scales with the distance from the bail to the loaded lug. Widening the spread raises the moment for the same weight. A device with multiple lug holes is only rated at other holes if the manufacturer published a rating at those holes.

It does not include an off-centre load. The rating assumes the load's centre of gravity sits under the bail. Hang a load whose centre of gravity is off to one side and the beam tips until the bail is over the centre of gravity, which redistributes the lug reactions and can drive one leg well above half the weight while the assembly hangs at an angle. Where the centre of gravity is genuinely unknown, that is the finding, and the answer is to establish it, not to estimate it.

It does not include side pull. Drag a load sideways, or lift with the hoist rope out of plumb, and you apply a horizontal force nothing in the rating anticipated. Devices with jaws, clamps and tongs are the worst case here because side load can also open the gripping mechanism.

It does not include its own weight. The rated load is what the device may carry below itself. The crane above has to carry the device plus the load, so the device weight is a deduction from the crane's net capacity at the working radius, and it is counted once, in that deduction, never subtracted from the device's own rating.

It does not include shock. A design factor is computed for a static pull. Snatching the load, a jerky hoist start, or a load that swings and comes up short all apply a dynamic force greater than the weight, and the amount is not predictable from the ground. Shock loading spends the design factor rather than testing it.

It does not survive a repair or a modification. Welding a new lug, drilling a new hole, or straightening a bent member changes the member the design was based on. Only the manufacturer or a qualified person acting on the manufacturer's behalf can requalify a modified device.

The design factor is already inside the number

Before anyone applies a correction to a rated load, write down what the rated load already contains. A below-the-hook device designed under ASME BTH-1, in the edition your employer's rigging program or your contract adopts, carries a design factor against yield built into the published rated load, and BTH-1 assigns devices to design categories with different factors according to how predictable the loading and the environment were assumed to be. Which category a device was designed to is a statement about assumed predictability, not about how strong it is, and it is not something you can read off the plate.

The practical consequence: a correction you apply for angle, spread or off-centre loading is a re-basing of the load case, not an extra safety factor being layered on top of one that is already there. And you never round a rated capacity up, or a required clearance down, on the argument that there is a factor in there somewhere. That factor is what covers the things you did not think of, not the things you did.

Marking, proof test and inspection

For construction work, 29 CFR 1926.251(a)(4) requires that special custom-design grabs, hooks, clamps and other lifting accessories be marked to indicate their safe working loads and be proof tested to 125 percent of their rated load before use. That is a federal requirement in Part 1926 and it is the reason a home-made beam without a plate is not a rigging problem, it is a stop-work.

ASME B30.20, again in the edition adopted by your employer's program, your contract or your authority having jurisdiction, is the consensus standard that governs marking, inspection intervals and operating practice for below-the-hook devices, and it binds through that adoption rather than on its own.

An unmarked device is unrated, full stop. There is no field procedure that turns an unmarked beam into a rated one, and a tape measure and a strength table do not substitute for a design. Route it to the manufacturer for a replacement plate, or to a registered professional engineer for a rating, and set it aside until then.

Worked example: the lighter load that broke the rating

A lifting beam with a data plate reading rated load 4,000 lb at 10 ft spread, bail on the centreline. Beam weight from the plate: 350 lb. The job is a 3,600 lb fabricated frame whose two rated pick points are 14 ft apart, and whose centre of gravity has been established from the fabricator's drawing as centred between them.

The wrong read takes about two seconds: 3,600 is less than 4,000, so go.

The condition line the plate did not print. The rating is at 10 ft. The load is at 14 ft. Bending governs a beam with a centred bail, and for a centred bail the moment is the weight multiplied by the spread and divided by four.

• Rated condition: 4,000 lb x 10 ft / 4 = 10,000 lb-ft of bending moment
• Actual condition: 3,600 lb x 14 ft / 4 = 12,600 lb-ft of bending moment
• Ratio: 12,600 / 10,000 = 1.26

The lift is at 126 percent of the moment the device is rated for, on a load 10 percent under the marked capacity. Both facts are true at once and only one of them is on the plate.

Where the arithmetic stops. You can now see that you are over. You may not turn that same arithmetic around to get a permission. Rearranging gives 4 x 10,000 lb-ft / 14 ft, which is about 2,857 lb, and that figure is worth exactly nothing as an authorisation, because it assumes bending at the mid-span is the only thing that governs. It is not. Lug tear-out, weld capacity at the bail, lateral-torsional stability of the beam, and the manufacturer's own assumptions about lug position may all govern before mid-span bending does, and none of them scale the way the moment does. The arithmetic is licensed to stop a lift. It is not licensed to permit one. The manufacturer's published rating at the spread you intend to use is the only thing that permits one, and where the manufacturer has published nothing for that spread, the answer is a different device or a registered professional engineer, not a calculation done at the tailgate.

What the crane sees, once, not twice. Assume you find a beam correctly rated at 14 ft. The crane's suspended load is:

• Frame: 3,600 lb
• Beam: 350 lb
• Slings and shackles below the beam: 90 lb

Total suspended: 4,040 lb. That number is a deduction against the crane's gross chart capacity at the working radius, alongside the hook block and any stowed jib. The beam's 350 lb appears there and nowhere else; it is not also subtracted from the beam's own 4,000 lb rating, because the rating was already written as capacity below the device.

What getting this wrong looks like in the field. The beam does not usually snap on the pick. It takes a permanent set, a bow you cannot see against a 14 ft length, and it goes back on the rack looking normal. The next crew uses it at the marked 10 ft on a marked-capacity load and it fails there, at a condition that was well inside the plate, because the member has already been yielded. That is why a device that has been used outside its marked condition gets tagged and pulled rather than eyeballed and re-racked.

How to verify before the hook comes down

Do all of this with the device on the ground or on cribbing, unloaded, and with nobody positioned between the load and the machine.

• Read the plate and photograph it. Rated load, spread or configuration it applies to, device weight, serial number, and the manufacturer. If any of those is unreadable, the device is out of service until the manufacturer supplies the data.
• Match the plate to the lift you are about to make, term by term: spread, attachment points, bail position, and whether the load hangs plumb.
• Establish the centre of gravity from a document, not from where the load looks balanced. Where no document exists, that is the finding, and it changes the plan.
• Confirm the proof-test and inspection record exists for a custom-designed device before it is rigged, since 1926.251(a)(4) makes the proof test a precondition of use in construction, not a formality after the fact.
• Look for a bow, a twist, an elongated hole or a repaired weld on the unloaded device with it supported on cribbing, and tag it out at the first of those rather than deciding how much is acceptable.
• Add the device weight into the suspended load on the lift plan, and confirm it appears exactly once.

References

• 29 CFR 1926.251(a)(4), which requires custom-design lifting accessories used in construction work to be marked with their safe working load and proof tested to 125 percent of rated load before use
• 29 CFR 1910.184, slings, the general-industry standard covering the rigging attached above and below such a device
• ASME B30.20, below-the-hook lifting devices, and ASME BTH-1, design of below-the-hook lifting devices, in the edition adopted by your employer's rigging program, your contract or your authority having jurisdiction, which is how they bind
• Manufacturer documentation for the specific device, which owns the rated load, the spread it applies to, the device weight, and every removal-from-service criterion
• See related: What a Spreader Bar and a Lifting Beam Each Do; How to Write a Lift Plan Somebody Else Can Execute