A limit state is the condition at which a structure just fails to meet the requirements placed upon it. Beyond this boundary the structure no longer performs as intended, whether that means loss of strength, excessive movement or unacceptable damage. Limit states are defined in the Eurocodes and form the basis of modern structural design.
Types
There are two kinds of limit state:
| Limit state | Concerned with | Examples |
|---|---|---|
| Ultimate limit state (ULS) | Collapse, strength and stability | Failure of a member, overturning, buckling |
| Serviceability limit state (SLS) | Everyday performance and comfort | Deflection, cracking, vibration |
At the ultimate limit state the structure must not collapse or become unstable. At the serviceability limit state it must remain usable and undamaged in normal use.
Application
In structural design
- Loads are combined and factored, then checked against each relevant limit state
- The ULS governs the strength and stability of members and connections
- The SLS governs deflection, crack widths and vibration limits
- The applicable consequence class sets how severe the effects of failure would be, influencing the required safety margins
Related Terms
- Eurocode
- Load
- Collapse
- Consequence class
- Structure
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