ULS (ultimate limit state)
ULS is the design check against structural failure (collapse, fracture, instability) under AS/NZS 1170. Engineers size members to resist factored loads at ULS.
Ask Chalkline about this →ULS (Ultimate Limit State) is the engineering design check against structural failure of a building or member: collapse, fracture, loss of stability, or overturning. The structure must remain standing and safe at the ULS check. ULS is the “don’t fall down” check; it pairs with SLS (Serviceability Limit State), the “looks and feels okay” check that addresses deflection, vibration, and crack control under normal service loads.
Under AS/NZS 1170.0 (general principles of structural design) and AS/NZS 1170.1 (load actions), engineers size structural members to resist factored loads at ULS. The factors are applied to dead, live, wind, snow, and earthquake loads:
Typical ULS load combinations (residential, AS/NZS 1170.0):
| Combination | Description |
|---|---|
| 1.35G | Permanent (dead) load alone |
| 1.2G + 1.5Q | Permanent + variable (live) load |
| 1.2G + Wu | Permanent + ultimate wind load |
| 0.9G + Wu | Uplift case (light dead, ultimate wind) |
| G + Eu | Permanent + ultimate earthquake |
(G = permanent load, Q = imposed/variable load, Wu = ultimate wind action, Eu = ultimate earthquake action.)
ULS vs SLS:
| Aspect | ULS | SLS |
|---|---|---|
| What it checks | Failure (collapse, fracture, instability) | Service performance (deflection, vibration, crack width) |
| Load factors | Factored (1.2, 1.5, etc.) | Unfactored (working loads) |
| Probability of exceedance | Very low (in normal life) | Once per service life acceptable |
| What it sets | Member size, strength capacity, connections | Stiffness, member geometry, finishes tolerance |
A typical residential lintel must satisfy both checks. A lintel might pass ULS (it doesn’t break) but fail SLS (deflection is 30 mm and the wall above cracks). Both checks are required; pass either alone is not enough.
For the builder: the engineer’s design certificate confirms ULS and SLS have both been satisfied. The builder cannot informally override the result by “the timber looks fine to me”, because failure of either ULS or SLS is invisible until the load case happens.
Common confusion:
- Wind ratings (N1 to C4 per AS 4055) are inputs to ULS load combinations, not separate checks themselves.
- A structure passing one ULS load combination must pass them all; the worst-case is the design constraint.
- “Load width” in residential span tables is part of the ULS calculation; underestimating load width breaks both ULS and SLS.
Also known as: ultimate state; collapse-state design; strength limit state.
Category: Structure.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.