Introduction

The performance of a structure and its components is described using limit-state functions that separate desired states from undesired states. The physical effects of exceedance of a limit state may be either reversible or irreversible. For the reversible case, removal of the cause of the exceedance allows the structure to return to a desired state. For the irreversible case, the same is not true and certain consequences, such as damage, may occur depending on the nature of the limit state. The consequences may, in turn, be either recoverable or unrecoverable from the deformed state. For example, if the damage is limited, say, in the form of a localized permanent set in a case where the same is not desired, the condition may be repairable, for example, by replacing the affected parts.

As discussed in Chapter 3, limit states are usually classified into four types:

1. Serviceability limit states (SLS) that represent criteria governing normal functional or operational use.

2. Ultimate limit states (ULS) that represent the failure of the structure and its components usually when subjected to extreme values of actions or action effects.

3. Fatigue limit states (FLS) that represent damage accumulation (leading to cracking when certain limits are exceeded) under repetitive actions.

4. Accidental limit states (ALS) that represent situations of accidental or abnormal events.

In limit-state assessment, such various limit states are considered against different target safety levels; the target to be attained for any particular type of limit state is a function of the consequences and ease of recovery from that state.