A lifting lug is the smallest piece of engineered steel on most erection packages and the one with the least margin for being wrong. It is welded to a member that was designed for its final position, it carries the whole weight of that member in a position nobody designed it for, and when it fails it does so in a fraction of a second, above people.
Two documents are commonly used to check one: the AISC Specification, which treats a padeye as a pin-connected plate, and ASME BTH-1, which is written for below-the-hook lifting devices and asks for larger design factors. They do not give the same answer, and it is worth knowing why before choosing one.
First the proportions, then the strength
The strength equations for a pin-connected plate assume a plate of reasonable shape. AISC 360 states that shape as proportions, and a lug that fails them is outside the equations however much capacity they appear to show.
The limit states that govern
At the hole the plate is checked for tension rupture across the net section, shear rupture in the steel beyond the hole, and bearing of the pin on the plate. Pin bearing depends only on the plate thickness, the pin diameter and the steel's yield strength, so a thicker plate helps it and a wider one does not. Below the hole the lug is a short cantilever: the base section and the weld to the member carry the sling force and, once the sling is inclined, the moment of its horizontal component about the base. On an inclined lift either of those can govern.
| Limit state | AISC 360-16 (ASD) | ASME BTH-1-2020 |
|---|---|---|
| Tension rupture through the hole | Eq. D5-1, Ω = 2.00 | Eq. (3-45), divided by 1.20 N_d |
| Shear rupture / tear-out beyond the hole | Eq. D5-2, Ω = 2.00 | Eqs. (3-49) and (3-50), divided by 1.20 N_d |
| Pin bearing | Eq. J7-1, Ω = 2.00 | Eq. (3-53), divided by N_d |
| Base section | §D2 with §H1, Ω = 1.67 | Eq. (3-1), divided by N_d |
| Base weld | §J2.4, Ω = 2.00 | Eq. (3-55), divided by 1.20 N_d |
Why BTH-1 asks for more
BTH-1 designs to a factor N_d chosen by the Design Category of the lift, and adds a further 1.20 on the fracture and connection limit states. A structural specification written for a building in service does not know whether the lift is a controlled, repeated operation or a one-off pick of an awkward piece in the wind.
The standard also sorts lifters by Service Class, from the number of load cycles they will see: Service Class 0 covers up to 20,000 cycles. An erection padeye used for a handful of lifts is Service Class 0. A lifting beam that will lift a module every day for years is not, and needs a fatigue check the static equations do not provide.
What the calculation does not see
- A sling out of the plane of the plate. The equations above assume the sling lies in the plane of the lug. Out of plane, the base bends about its weak axis; orient the lug to the rigging, or check that bending.
- The shackle, the pin and the member. The lug is one link. The shackle's working load limit, the pin, and the local capacity of the flange or web the lug is welded to are separate checks.
- Hole clearance. A loose pin in a large hole concentrates bearing. BTH-1 penalises clearance directly; AISC 360-16 limits it only where the pin moves under load.
- The weld as built. A fillet the drawing calls for and the shop does not deliver is the most common real failure. The lug belongs on the fabrication drawing with its weld size and inspection, not on a site sketch.
Choosing between the two
Where the project's lifting procedure names BTH-1, use it: it was written for this. Where it does not, running both is cheap and informative: the checks that govern usually differ, and a lug that passes BTH-1 Category B with its proportions met has little left to argue about. Our lifting lug calculator checks one padeye to both codes side by side and names the governing check of each; the erection engineering itself is part of our steel structures work.
Frequently asked questions
Which Design Category should an erection lug use?
That is a judgement about the lift, not about the plate. BTH-1 describes Category A as predictable loads in controlled service and Category B as loads that are not precisely defined or conditions that are severe. A one-off lift of an irregular piece on an open site is usually closer to B.
Does a wider plate help pin bearing?
No. Bearing depends on the plate thickness, the pin diameter and the yield strength. Width helps tension and tear-out beyond the hole; thickness or a cheek plate helps bearing.
Why does the base weld govern on an inclined lift?
Because the horizontal component of the sling force acts at the height of the hole, and its moment about the base is resisted by the weld group and the base section. For two fillet welds of length L along the base, elastic line-weld theory puts the bending stress at 6·h·tanθ / L times the direct stress, where h is the height of the hole. With the hole 150 mm above a 200 mm base, a 15° sling angle adds about 1.2 times the direct stress: the weld stress roughly doubles.
Is AISC 360-22 different from 360-16 for padeyes?
Yes, in one respect: AISC 360-22 adds a clearance-dependent factor C_r to the shear rupture equation D5-2. A check labelled 360-16 does not include it.
