Ultimate and serviceability limit states

The two questions

Will it carry?

The ultimate limit state check. The consequence is severe — collapse, danger to life — so the safety margin here is large.

It covers strength, stability, overturning, sliding and failure of the connections.

Is it usable?

The serviceability limit state check. The consequence is discomfort or material damage, not danger to life. So the safety margin is smaller and, in exchange, the requirement can be stricter.

It covers deflection, vibration, cracking and visible deformation.

Why the second is stricter in light gauge steel

Because the stiffness of the structure is limited: little material, large spans, relatively slender members.

Typical situations:

• A floor: it passes the strength check comfortably, but the vibration would be noticeable. Floor vibration.
• An external wall under wind pressure: it carries, but a rigid finish would crack.
• A lintel: its capacity is fine, but the window would stick.

So a significant part of the design revolves around providing stiffness, not strength.

The design decision

The serviceability limits are partly a matter of agreement: the standard gives recommended values, but the actual requirement depends on the sensitivity of the finish and the expectations of use.

So settle at the start of the project:

• what the finishes will be,
• what is expected of the “feel” of the floor,
• whether there is a special need — vibration-sensitive equipment, say.

What matters in this for the client

A structure can comply with the standard and still be unpleasant. If someone says “we calculated everything, it complies”, ask back: to what criteria were the serviceability checks made?

The point

Strength is about safety, serviceability about quality. The occupant will perceive the second — so it should not be left at the minimum.