Ultimate Limit States (ULS) and Serviceability Limit States (SLS)

Limit states are defined as states beyond which the structure no longer satisfies an agreed relevant design criteria. Two basic groups of limit states considered are;

[1] Ultimate Limit States (ULS)

Ultimate limit states (ULS) are those associated with collapse or failure, and generally govern the strength of the structure or structural members because they correspond to the maximum load carrying resistance of such member. ULS also include loss of equilibrium or stability of the structure as a whole.

The Diagram Below is an example of Ultimate Limit State Failure

[2] Serviceability Limit States (SLS)

Serviceability limit states are states which correspond to conditions beyond which specified service conditions of a structure or structural member are no longer met. Serviceability limit states may be reversible or irreversible. They generally include issues like deformation, cracking and vibration which:

(1) Damage the structure or nonstructural elements (finishes, partitions, etc.) or the contents of buildings (such as machinery or sensitive equipment)

(2) Cause discomfort to the occupants of buildings

(3) Affect adversely appearance, durability or water tightness.

Serviceability limit state governs the stiffness of the structure and the detailing of reinforcement within it.

Below is Deflection of a Slab which show Serviceability Limit States Failure

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