Limit state of serviceability, requirements

The limit state of serviceability is the state beyond which the structure, though safe against collapse, becomes unfit for normal use because of excessive deflection, cracking, vibration or other defects. It is checked at service (unfactored) loads with a material partial factor $\gamma_m=1.0$.

Requirements (IS 456:2000, cl. 35.3, 23.2 and 26.3)

1. Deflection
   • Final deflection (including the effects of creep, shrinkage and temperature) $\le$ span/250.
   • Deflection after construction of partitions and finishes $\le$ span/350 or 20 mm, whichever is less.
   • Satisfied by the span/effective depth ratio: basic values 20 (simply supported), 26 (continuous) and 7 (cantilever), modified for steel stress and compression steel.
2. Cracking
   • Crack width generally limited to 0.3 mm (0.2 mm where members are exposed to weather, moisture or aggressive conditions).
   • Satisfied by minimum steel and by limiting the bar spacing (cl. 26.3.3): in beams, the clear distance between tension bars should not exceed the values of Table 15 (e.g. 280, 180 and 150 mm for Fe250, Fe415 and Fe500 with no redistribution); in slabs, main bars are spaced at not more than $3d$ or 300 mm.
3. Durability
   • Adequate nominal cover, minimum cement content, maximum water-cement ratio and minimum grade according to the exposure (mild to extreme).
4. Vibration
   • Slender members should not vibrate noticeably; limit the span/depth ratios.
5. Fire resistance: cover and member dimensions as per IS 456 Table 16A and Fig. 1.

These checks protect the appearance, comfort and long-term life of the structure.