Safety and serviceability requirements

A structure must be safe against collapse and fit for use throughout its design life (IS 800:2007 cl. 5.1, 5.2).

Safety (strength) requirements

• Stability: the structure as a whole must be stable against overturning, sliding and uplift under factored loads.
• Strength: every member and connection must resist the factored forces without yielding, buckling or rupture: design strength $\ge$ factored load effect.
• Fracture / fatigue: brittle fracture, fatigue and progressive collapse (lack of robustness) must be avoided.
• Ductility: failure should be gradual and give warning, e.g. yielding before rupture, ductile connections.
• Fire resistance for the required period and durability against corrosion.
• Partial factors: $\gamma_f$ on loads ($1.5$ for DL+LL, $1.2$ for DL+LL+WL), $\gamma_{m0}=1.10$, $\gamma_{m1}=1.25$.

Serviceability requirements

• Deflection under unfactored imposed loads within IS 800 Table 6, e.g. span/300 for floor and roof members not susceptible to cracking (span/360 if susceptible), span/150 for purlins and girts with elastic cladding, span/500 to span/1000 for crane gantry girders, and storey drift of about height/300 for multi-storey frames with elastic cladding.
• Vibration: floors and bridges must not cause discomfort; the natural frequency is checked.
• Durability: protection against corrosion by painting, galvanising or weathering steel.
• Fire: protection so that the structure remains serviceable for the required fire rating.
• Repair and maintenance access, and no excessive cracking or damage to finishes.

Serviceability is checked with unfactored (characteristic) loads and strength with factored loads.