--- title: "Design process and basis of design" subject: "Design of Steel and Timber Structure" chapter: 1 key: "semester_6/design_of_steel_and_timber_structure:design-process-basis" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/design-process-basis site: BCT Engineering --- # Design process and basis of design Design of Steel and Timber Structure · Chapter 1: Steel Structures and their Analysis and Design - Asked in: 2075 Bhadra, 6 marks; 2075 Baisakh, 10 marks (asked 2 times) - Video: [Steel Structures Explained (Beginner to Pro Guide)](https://www.youtube.com/watch?v=4X6zV3MVd8o) — Rigid Academy - 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/design-process-basis - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-1 ## Question Write the design process and basis of design of steel structures. ## Short answer Design process: planning, load estimation, analysis, member and connection design, checks, and detailing. Basis: limit state design with partial factors γf on loads and γm on material (γm0=1.10 yielding, γm1=1.25 rupture) to ensure strength and serviceability limits per IS 800:2007. ## Answer #### Design process Structural design is an iterative process that goes from the client's need to a safe, serviceable and economical structure. 1. **Planning / functional requirements:** use of the building, spans, heights, architectural layout, site conditions. 2. **Structural system selection:** choose framing (portal frame, truss, braced or moment frame), material (steel grade) and joint types (bolted/welded). 3. **Estimation of loads:** dead load (IS 875 Part 1), imposed load (Part 2), wind (Part 3), earthquake (IS 1893 / NBC 105), and other loads (snow, crane, temperature). 4. **Structural analysis:** find axial force, shear force and bending moment in every member for the governing load combinations (IS 800:2007 Table 4). 5. **Member design:** select sections, check strength (tension, compression, flexure, combined), slenderness and deflection. 6. **Connection design:** design bolted/welded joints, splices, bases and gussets. 7. **Check and revise:** if a check fails, change the section or system and repeat. 8. **Detailing and drawings:** fabrication and erection drawings, bill of materials, specifications. #### Basis of design (IS 800:2007) - **Limit state design:** the structure must not reach any limit state during its design life. *Limit state of strength* (yielding, buckling, rupture, fatigue, overturning) and *limit state of serviceability* (deflection, vibration, durability, fire). - **Partial safety factors:** on loads $\gamma_f$ (e.g. 1.5 for DL+LL, 1.2 for DL+LL+WL) and on material $\gamma_{m0}=1.10$ (yielding), $\gamma_{m1}=1.25$ (rupture), $\gamma_{mb}=1.25$ (bolts), $\gamma_{mw}=1.25$ (shop weld), 1.50 (field weld). - **Design strength** $=$ characteristic strength $/\gamma_m$ and must be $\ge$ the factored action effect $\gamma_f \times$ characteristic load. - **Material:** structural steel to IS 2062 (E250, E350 etc.), with $E=2\times10^5$ N/mm², $G=0.769\times10^5$ N/mm², $\mu=0.3$. - **Load combinations:** DL+LL, DL+LL+WL/EL, DL+WL/EL, with the load factors of Table 4. - **Serviceability limits:** deflection limits of Table 6 (e.g. span/300 for floor beams, span/150 for purlins with elastic cladding), slenderness limits of Table 3. - **Other requirements:** stability against overturning and sliding, ductility, robustness, fire resistance and durability. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the design process, what is done immediately after estimating the loads? A. Detailing and drawings B. Structural analysis for governing load combinations C. Connection design D. Selecting the steel grade Answer: B. After loads are estimated, the structure is analysed to get axial force, shear and bending moment in each member before members and connections are designed. (Key point: Order of design process) 2. What is done when a member fails a strength or deflection check? A. Reduce the load factor B. Ignore the check if the failure is small C. Change the section or system and repeat the checks D. Switch to working stress design Answer: C. Design is iterative: a failed check means revising the section or structural system and rechecking. (Key point: Iterative nature) 3. Which set of material partial safety factors is correct as per IS 800:2007? A. 1.10 for yielding, 1.25 for rupture, 1.25 for bolts B. 1.25 for yielding, 1.10 for rupture, 1.50 for bolts C. 1.50 for yielding, 1.25 for rupture, 1.10 for bolts D. 1.10 for all material and connection checks Answer: A. gamma_m0 = 1.10 for yielding, gamma_m1 = 1.25 for ultimate (rupture) strength and 1.25 for bolts. (Key point: Partial safety factors) 4. What condition must be satisfied in limit state design? A. Characteristic load <= characteristic strength B. Working stress <= permissible stress C. Design strength >= factored action effect D. Factored load <= characteristic strength Answer: C. Design strength (characteristic strength / gamma_m) must be at least the factored action effect (gamma_f x characteristic load). 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