Limit States Design
General Concepts and Procedure


The limit states, or performance-based, design approach is common in Europe and in other industries. The approach involves defining one or more limit states and their associated performance criteria and design checks in a precise and quantitative manner. The basic format can be summarized as follows:

  • explicit statement of performance goal or objective
  • explicit statement of performance criteria, and usually, a stated acceptable probability of exceeding the criteria
  • direct relation between goal and what you will check (and how you will check it)
  • explicit recognition of randomness and uncertainty (e.g. LRFD format for steel design)
Limit State
Objectives
Performance Criteria
Probability of Exceeding Performance Criteria
Limit 1
Goal you are trying to achieve
Response parameter(s) measured
x1% in y1 years
Limit 2
"
"
x2% in y2 years

For each limit state, it is necessary to recognize and manage randomness and uncertainty. It isn't adequate to simply say that the average demand is less that the average capacity - the probability of failure must be less than a specified amount.

This failure probability is usually designated as x% probability of failure in y years where y is generally taken to be the service life of the structure. For example, the Load and Resistance Factor Design (LRFD) format uses load factors on the demand and resistance factors on the capacity to provide a certain probability of failure. The basic design inequality is stated as:

gDAVE < fCAVE

and therefore

DAVE / CAVE > g / f          for a given probablility of failure