Limit states of strength and serviceability

A limit state is a state beyond which the structure no longer performs as it was designed to. Limit state design keeps the probability of reaching any limit state acceptably small. IS 456:2000 cl. 35 lists the following.

Limit state of strength (collapse)

The structure, or any part of it, must not collapse, buckle, overturn or become unstable under the worst factored loads. It is checked for:

• flexure (bending), compression, shear and torsion (member level);
• overturning, sliding, loss of stability and rupture of critical sections (structure level).

It is checked with design loads ($\gamma_f\times$ characteristic load) and design material strengths ($f_{ck}/1.5$, $f_y/1.15$).

Limit state of serviceability

The structure must remain comfortable and usable under service loads. It is checked for:

• Deflection: excessive sag spoils the appearance and damages partitions and finishes. Limits: span/250 for the total deflection, and span/350 or 20 mm (whichever is less) after finishes and partitions are built.
• Cracking: crack width should generally not exceed 0.3 mm, and 0.2 mm where members are exposed to weather, moisture or aggressive conditions, to protect the appearance and the steel.
• Durability: adequate cover, cement content and water-cement ratio for the exposure condition.
• Vibration, fire resistance, fatigue and impact where relevant.

Other limit states

Fire resistance, special impact or explosion loads, or other limit states as required for the structure.