It is both thicker and larger across the flats than a standard nut of the same thread.

The extra height gives more thread engagement, so it can develop the strength of a high-strength bolt without stripping.

The extra width spreads bearing load and gives the nut enough section to resist dilating under very high tension.

A nut under extreme load tries to expand radially, and a thin-walled one loses thread engagement as it does.

Both dimensions are specified in the relevant standard rather than being simply 'bigger'.

The thread itself is the same size as a standard nut's, so they are interchangeable on the bolt but not in the design.

Structural steel connections, and anywhere high preload is needed in a limited diameter.

Structural bolting develops tension close to the bolt's proof load, which a standard nut is not sized for.

Pressure flanges use them to achieve gasket compression with a bolt that fits the flange's hole pattern.

Machine foundations and anchor bolt assemblies use them for the same reason.

In each case the specification names the nut as part of an assembly rather than as a separate item.

For general assembly a standard nut is correct and a heavy one adds cost and clearance problems for no benefit.

Not in a structural assembly - bolt, nut and washer are specified and tested as a matched set.

The torque-tension relationship depends on the finish, lubrication and surface condition of the whole assembly.

Published installation methods assume the assembly they were developed with.

Substituting one component changes the relationship in a way nobody on site can measure.

Traceability requirements in structural work also mean the components must be documented as a set.

Outside structural bolting the constraint is looser, but property class matching still applies.

A heavy hex nut needs a larger socket, and in congested connections it may not fit.

Across-flats is larger for the same thread, so tooling sized for standard nuts is not sufficient.

Impact sockets are thicker walled than hand sockets, which reduces clearance further.

Clearance between adjacent bolts, and between a bolt and the web or flange of a member, is a detailing question resolved on the drawing.

Discovering it on site usually means a connection cannot be tightened properly.

Tension-control and direct-tension-indicating systems have their own tooling clearance requirements.

Only within the rules of the specific bolting system, and often not at all.

Some tightening methods take the assembly beyond its elastic range deliberately, which changes the parts permanently.

The lubricant applied by the manufacturer is consumed during the first installation.

Reuse rules are published by the system supplier and differ between methods and markets.

Where reuse is permitted it is usually limited to a stated number of cycles with a condition check.

The cost of a nut is trivial against the cost of an under-tensioned structural connection.

Structural assemblies specify washers, and their thickness and hardness are part of the design.

High-strength bolts embed themselves into commercial-quality washers, losing preload into that deformation.

Hardened structural washers resist that and are supplied with the assembly.

Slotted and oversized holes carry their own washer requirements.

Some tightening methods specify which face carries the washer and whether one or two are used.

Omitting a specified washer is a departure from the design rather than a simplification.

No - stacked nuts do not share load equally, and the arrangement is not equivalent.

In a stacked pair the nut nearer the joint carries most of the load, because thread engagement is loaded unevenly along its length.

The second nut adds relatively little capacity while adding height and clearance problems.

Two nuts do have a legitimate use as a jam arrangement for locking, which is a different purpose entirely.

Where more capacity is needed, a heavy hex nut or a larger bolt is the correct answer.

Structural specifications never call for stacked standard nuts as a substitute.