--- title: "Safety and serviceability requirements" subject: "Design of Steel and Timber Structure" chapter: 3 key: "semester_6/design_of_steel_and_timber_structure:safety-serviceability-requirements" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/safety-serviceability-requirements site: BCT Engineering --- # Safety and serviceability requirements Design of Steel and Timber Structure · Chapter 3: Limit State Design Method - Asked in: 2077 Chaitra, 8 marks; 2072 Asoj, 4 marks (asked 2 times) - Video: [Steel Design - Serviceability](https://www.youtube.com/watch?v=bsPLGXulADc) — The Bridge Tutors - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/safety-serviceability-requirements - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-3 ## Question What are the safety and serviceability requirements of (steel) structures? Explain. ## Short answer Safety requires stability, strength (design strength ≥ factored loads), ductility, fire resistance, and durability. Serviceability covers deflection limits (e.g., span/300 for floors), vibration control, and durability under unfactored loads with appropriate protective measures. ## Answer 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a safety (strength) requirement? A. Vibration frequency check B. Deflection below span/300 C. Stability against overturning, sliding and uplift under factored loads D. Appearance of finishes Answer: C. Overall stability under factored loads is a safety requirement. (Key point: Stability) 2. Per IS 800 Table 6, the deflection limit for purlins and girts with elastic cladding is: A. span/300 B. span/150 C. span/360 D. span/500 Answer: B. Purlins and girts with elastic cladding are limited to span/150. (Key point: Deflection limit) 3. Which load level is used for strength and for serviceability checks? A. Factored for both B. Unfactored for both C. Unfactored for strength, factored for serviceability D. Factored for strength, unfactored (characteristic) for serviceability Answer: D. Strength uses factored loads and serviceability uses unfactored characteristic loads. (Key point: Load level) 4. Why is ductility required? A. To increase stiffness B. So that failure is gradual with warning C. To reduce deflection D. To avoid corrosion Answer: B. Ductile failure, e.g. yield before rupture, gives warning before collapse. 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