How to Define a Structural Limit State
The procedure for defining a limit-state function for structural reliability — identifying the resistance, the demand, the performance criterion and the safe/failure boundary.
When to use this guide
Use this guide when setting up a reliability analysis and needing to define the limit-state function that separates safe from failure states.
Step-by-step method
- Identify the failure mode: yielding, buckling, fatigue, fracture, displacement, leakage
- Identify the resistance (capacity): what is the structural quantity that must not be exceeded?
- Identify the demand (load): what is the applied quantity that drives the failure mode?
- Write the limit-state function: g = R - S, where R is resistance and S is demand
- Identify which variables in R and S are random
- Check that g > 0 means safe and g < 0 means failure — verify the sign convention
- If multiple failure modes exist, define a separate limit-state function for each
- Verify the limit-state function with a deterministic check: does g = 0 at the expected boundary?