How do I choose a service class?

From an expected number of load cycles over the device's life, not from a feeling about how often it gets used. On the worked lift point the step from the lowest class to the next doubled the pin bearing check, from 89 percent to 177, and going further up the classes changed nothing more. So the expensive decision is whether the device is used repeatedly at all, and the classes above the first cost the same as each other here.

Should I use the heavier design category for an important lift?

No, and this is the commonest misuse of the scheme. The category exists for loads that are not well known, not for lifts that feel significant. A one-off critical lift of a precisely weighed item is exactly the case the lighter category was written for; choosing the heavier one adds steel and addresses a risk that is not present. What does justify the heavier category is a general-purpose device that will lift whatever arrives, because its loads are unknown by definition.

Which checks does the service class actually affect?

The ones that respond to repeated loading, which on a padeye is pin bearing. A pin bearing repeatedly against a hole is a wear and fatigue problem rather than a strength one, so that is where the derating lands. On the worked lug every other check - net section, fracture, shear-out and the weld - was identical across all five service classes, which means a device failing on service class fails in one place and can usually be fixed there with a larger pin, a thicker plate at the hole, or a cheek plate.

Do the design category and service class need to be recorded?

On the calculation and on the device. On the calculation, because without them the allowable stress has no traceable basis, and making that basis visible is the whole point of the scheme. On the device, because a lifting device that goes back into a rack will meet loads its designer never saw, and a device with no category or class marked is one nobody can assess for reuse.

ASME BTH-1 design category and service class

Two declarations, and what each one is about

One is about knowledge and one is about wear. They are independent, they are both required, and neither of them is a judgement about how important the lift feels.

The below-the-hook design standard asks a designer for two things before any allowable stress can be established.

The design category is a statement about how well the loading is known. The lighter category is for a device whose loads are predictable and whose service is controlled: known weights, known geometry, an environment somebody manages. The heavier category is for a device whose loads are less certain, or whose service conditions are not controlled to that standard. Nothing about the steel changes between them; what changes is how much of the material's strength you are permitted to use.

The service class is a statement about how many load cycles the device will see. A one-off lift and a device that runs hundreds of times a shift are different problems, and the difference enters through the checks that care about repeated loading.

Two properties are worth carrying:

They are independent. A one-off lift with poorly known loads and a repetitive lift with precisely known ones both exist, and the two declarations answer different questions.

Where an allowable comes from on this route

F_{t} = \frac{F _{y}}{N _{d}}

$F_{y}$: is the material's yield strength
$N_{d}$: is the design factor the standard ties to the declared design category

That is the tension allowable; every other allowable on the route is derived from the same design factor in the same way. The values of the design factor belong to the standard and are not reproduced here. What matters for reading the numbers below is the shape: the category divides every allowable, so it moves every check that uses one by the same ratio.

They are declarations, not calculations. Somebody with authority decides them, and the acceptance follows. That is the useful part of the scheme: it makes the reason for a margin visible instead of burying it in a factor nobody can trace.

01How well is the load known?
Weighed, or calculated from a controlled model, with the environment managed
The lighter design category is defensible. The claim being made is about knowledge and control, and it should be written down as a claim.
Estimated, variable, or the service conditions are not controlled
The heavier design category. This is the default for most general-purpose lifting devices, and it is the correct default.
02How many load cycles will the device see?
A one-off or occasional lift
The lowest service class. Static behaviour governs and no fatigue derating applies.
Regular repeated use
A higher service class, chosen from the expected cycle count. On the worked lug this step alone doubled the bearing check, and it is a step rather than a gradual change.
03Who is making the declaration?
Somebody who owns the duty
Record it on the calculation and on the device. A device marked with its category and class is a device whose reuse can be assessed by whoever finds it in a rack in five years.
Nobody in particular
That is the problem to fix before the calculation. An undeclared category means the allowable stress came from nowhere.

What the design category costs

About half again, on every check that uses an allowable stress. The same lug, the same load, the same steel.

The worked lift point: a 250 kN attachment, 35 mm plate in S355, 240 mm wide, a 58 mm hole on a 54 mm pin, 85 mm edge distance, welded all round with a 16 mm fillet 240 mm long, sling arriving 10 degrees off the plate axis.

Both declarations live behind the second option. Selecting the below-the-hook route is what makes a design category and a service class exist as inputs at all; on the general mechanics route there is nothing to declare, because the allowables are yours and the standard is not supplying a design factor.

Lifting Lug Calculator · computed at page render

Design Category A, Service Class 0

The lighter category: loads known, service controlled.

Design load	250.0kN
Net-section tension	32.3%
Single-plane fracture	40.6%
Double-plane shear-out	37.8%
Pin bearing	59.6%
Fillet weld allowable	53.8%

Governing check: Pin bearing - BTH-1 §3-3.3.459.6% utilisationPass

Comfortable across the board, with bearing the closest check at 60 percent.

Open this example in the calculator

Lifting Lug Calculator · computed at page render

The same lug, Design Category B

One declaration changed. The steel, the geometry, the weld and the load are all identical.

Net-section tensionagainst 32.3%	48.5%
Single-plane fractureagainst 40.6%	60.8%
Double-plane shear-outagainst 37.8%	56.7%
Pin bearingagainst 59.6%	89.4%
Fillet weld allowableagainst 53.8%	80.7%

Governing check: Pin bearing - BTH-1 §3-3.3.489.4% utilisationPass

Every check moved by the same ratio, because the category acts on the allowable stress rather than on any one failure mode. Still passing, and with far less room.

Open this example in the calculator

The ratio is uniform, which is a useful diagnostic. If two calculations of the same device differ by a constant factor across every check, the difference is a category or an allowable, not a geometry change. If they differ unevenly, something about the model moved.

What the service class costs, and why it is a step

It touches the checks that care about repeated loading, and on this lug that is one check. Going from Class 0 to Class 1 doubled it; going from 1 to 4 changed nothing more.

Lifting Lug Calculator · computed at page render

Category B, Service Class 2

Regular repeated use declared. The design category and everything physical are unchanged from the previous example.

Pin bearingagainst 89.4% at Service Class 0 - failed	177.4%
Fillet weld allowableunchanged	80.7%
Double-plane shear-outunchanged	56.7%
Net-section tensionunchanged	48.5%

Governing check: Pin bearing - BTH-1 §3-3.3.4177.4% utilisationFail

One check moved and it doubled. Bearing at a pin hole is the check on this lug that responds to cycles, because a pin bearing repeatedly against a hole is a wear and fatigue problem rather than a strength one.

Open this example in the calculator

Sweeping every service class on the same lug:

Service class	Pin bearing	Fillet weld	Net-section tension
0	89.4%	80.7%	48.5%
1	177.4%	80.7%	48.5%
2	177.4%	80.7%	48.5%
3	177.4%	80.7%	48.5%
4	177.4%	80.7%	48.5%

Two readings.

It is a step at the first increment. Class 0 to Class 1 doubles the bearing check; Class 1 to Class 4 does not move it further on this lug. Declaring "some repeated use" and declaring "heavy repeated use" cost the same here, which means the expensive decision is whether the device is used repeatedly at all.

It touches one check. Everything else on the lug is unchanged across the whole sweep, so a device that fails on service class fails in one specific place and can usually be fixed there: a larger pin, a thicker plate at the hole, or a cheek plate.

The category moves everything by a constant ratio. The service class moves one check by a step. Knowing which of the two is biting tells you which fix will work.

Choosing them honestly

The category is about the load's uncertainty and the class is about the cycle count. Choosing either for any other reason produces a heavier device and no more safety.

Do not choose the heavier category because the lift feels important. The category exists for loads that are not well known. A one-off critical lift of a precisely weighed item is exactly the case the lighter category was written for, and choosing the heavier one addresses a risk that is not present while adding steel that is.

Do not choose the lighter category for a general-purpose device. A spreader beam that goes back into the yard and gets used for whatever comes next has, by definition, loads that are not known. Its category follows from that.

Declare the class from a cycle count, not a feeling. "Occasional" and "regular" are not inputs. An expected number of lifts over the device's life is.

Mark the device with both. A lifting device in a rack with no category or class marked is a device nobody can assess for reuse, and reuse is where devices meet loads their designers never saw.

Six ways these two declarations go wrong

Four are about choosing them for the wrong reason. Two are about not recording them.

1. The category chosen for importance rather than uncertainty. It buys a uniform reduction in allowable stress and addresses nothing about a well-known load.

2. The lighter category used for a general-purpose device. If the device will lift whatever arrives, its loads are not well known by definition.

3. The service class guessed. It should follow from an expected cycle count over the device's life.

4. A category factor lifted into another standard's scheme. The allowable stress derivation belongs to this standard and means nothing outside it.

5. Neither recorded on the calculation. The allowable stress then has no traceable basis, which is the specific thing this scheme was designed to prevent.

6. Neither marked on the device. A device in a rack with no category and no class is one nobody can assess for the next job, and the next job is where the trouble is.

Common questions

What is the difference between Design Category A and B?: How well the loading is known. The lighter category is for a device whose loads are predictable and whose service conditions are controlled; the heavier one is for a device whose loads are less certain or whose environment is not controlled to that standard. Nothing about the steel changes between them - what changes is how much of its strength you may use. On this article's worked lift point the change moved every check by about half again, uniformly, because the category acts on the allowable stress rather than on any one failure mode.
How do I choose a service class?: From an expected number of load cycles over the device's life, not from a feeling about how often it gets used. On the worked lift point the step from the lowest class to the next doubled the pin bearing check, from 89 percent to 177, and going further up the classes changed nothing more. So the expensive decision is whether the device is used repeatedly at all, and the classes above the first cost the same as each other here.
Should I use the heavier design category for an important lift?: No, and this is the commonest misuse of the scheme. The category exists for loads that are not well known, not for lifts that feel significant. A one-off critical lift of a precisely weighed item is exactly the case the lighter category was written for; choosing the heavier one adds steel and addresses a risk that is not present. What does justify the heavier category is a general-purpose device that will lift whatever arrives, because its loads are unknown by definition.
Which checks does the service class actually affect?: The ones that respond to repeated loading, which on a padeye is pin bearing. A pin bearing repeatedly against a hole is a wear and fatigue problem rather than a strength one, so that is where the derating lands. On the worked lug every other check - net section, fracture, shear-out and the weld - was identical across all five service classes, which means a device failing on service class fails in one place and can usually be fixed there with a larger pin, a thicker plate at the hole, or a cheek plate.
Do the design category and service class need to be recorded?: On the calculation and on the device. On the calculation, because without them the allowable stress has no traceable basis, and making that basis visible is the whole point of the scheme. On the device, because a lifting device that goes back into a rack will meet loads its designer never saw, and a device with no category or class marked is one nobody can assess for reuse.

Sources

Every document below is linked at its publisher or regulator. Xarpis reproduces no standard text; where a clause is named, the identifier is given so you can find it in your own copy.

ASME BTH-1Design of Below-the-Hook Lifting Devices
ASME · paid document
Structural, mechanical and electrical design criteria for below-the-hook lifting devices, used alongside ASME B30.20 which carries the safety requirements. The current edition is BTH-1-2023; Xarpis implements the 2020 edition and says so on every result.
ASME B30.20Below-the-Hook Lifting Devices
ASME · paid document
The safety half of the US below-the-hook pair: marking, construction, installation, inspection, testing, maintenance and operation. It requires the device to have been designed to BTH-1 and then governs everything that happens afterwards, which is why citing BTH-1 alone leaves half the obligation unstated.
ANSI/AISC 360Specification for Structural Steel Buildings
American Institute of Steel Construction · free to read
The US steel design specification, in both LRFD and ASD. AISC publishes it for free download, which makes it one of the few structural standards a reader can check the same afternoon they read about it.
29 CFR 1926.251Rigging equipment for material handling
US Occupational Safety and Health Administration · free to read
Inspection and safe-use requirements for chain, wire rope, fibre rope, synthetic webbing, shackles and hooks on US construction sites, including the requirement that rigging be inspected before each shift.

Run the check properly

Reading about a calculation is not the same as being able to hand one over. These tools produce the traceable record.

Spreader Beam Design Calculator Lifting Trunnion Design Calculator Vessel Upending & Tailing Lift Calculator

Something here wrong, or thinner than it should be? Tell us which paragraph and it gets rewritten. Articles carry the date they were last revised for exactly this reason.