--- title: "Merits of limit state over working stress" subject: "Design of Steel and Timber Structure" chapter: 3 key: "semester_6/design_of_steel_and_timber_structure:why-lsm-better-wsm" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/why-lsm-better-wsm site: BCT Engineering --- # Merits of limit state over working stress Design of Steel and Timber Structure · Chapter 3: Limit State Design Method - Asked in: 2073 Magh, 5 marks; 2076 Bhadra, 6 marks; 2070 Bhadra, 6 marks (asked 3 times) - Video: [Working Stress Method V/S Limit State Method | ReactionTest](https://www.youtube.com/watch?v=X00-ri4GmqQ) — Reaction Test - 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/why-lsm-better-wsm - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-3 ## Question Explain why the limit state design method is better (more popular) than the working stress design method. ## Short answer LSM uses partial safety factors based on load variability and statistical reliability, considers post-elastic strength and redistribution, provides uniform reliability, checks serviceability explicitly, and yields lighter, more economical sections than WSM's single factor approach. ## Answer The Limit State Method (LSM) is now preferred (IS 800:2007) because it is more rational and economical than the Working Stress Method (WSM). 1. **Realistic behaviour:** WSM assumes linear elastic behaviour only. LSM considers the actual behaviour up to failure, including post-elastic strength, local and lateral buckling, and redistribution. 2. **Separate partial safety factors:** WSM uses one factor of safety for everything. LSM applies $\gamma_f$ to loads (different for DL, LL, WL) and $\gamma_m$ to materials and connections, so each uncertainty is treated according to its own variability. 3. **Uniform reliability:** the probability of failure is nearly the same for all members, whereas WSM gives unknown, non-uniform safety margins. 4. **Economy:** because dead loads vary less than live loads, LSM uses smaller factors where uncertainty is less and gives lighter sections. 5. **Serviceability check:** LSM checks deflection, vibration, durability and fire explicitly, along with strength. 6. **Probabilistic basis:** characteristic values are statistical (e.g. the strength exceeded by 95% of test results, i.e. the 5% fractile), allowing improvement as more data become available. 7. **Compatible with other codes:** IS 456 (RCC), IS 1893 and international codes (Eurocode, AISC LRFD) all use limit states, so one design philosophy suits composite and mixed structures. 8. **Direct check of ultimate capacity:** design strength is compared directly with the factored load, giving a clear margin against failure. Thus LSM gives safer, more uniform and generally more economical designs than WSM. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do the safety factors in LSM differ from WSM? A. LSM uses a single factor on stress B. WSM applies factors to loads and materials separately C. LSM applies gamma_f to loads and gamma_m to material, each by its own variability D. Both use the same single factor Answer: C. LSM treats each uncertainty with its own partial factor, while WSM has one overall factor. (Key point: Safety factors) 2. What is the safety advantage of LSM? A. Unknown safety margins B. Nearly uniform probability of failure across members C. Higher stresses at working load D. No need for serviceability check Answer: B. LSM gives nearly the same probability of failure for all members, whereas WSM has non-uniform margins. (Key point: Reliability) 3. Why does LSM give economical sections? A. It uses a larger load factor on dead load B. It ignores live load C. Dead load variability is smaller than live load, so lower factors apply where uncertainty is less D. It reduces steel grade Answer: C. Different factors for DL and LL give lighter sections where uncertainty is less. (Key point: Economy) 4. What does LSM check explicitly in addition to strength? A. Only the cost B. Deflection, vibration, durability and fire C. Only slenderness D. Nothing else Answer: B. LSM includes explicit serviceability checks. (Key point: Serviceability) ## Related in Design of Steel and Timber Structure - [Working stress vs limit state method](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Awsm-vs-lsm-difference) — Design of Steel and Timber Structure, chapter 3, asked 3× - [Basic assumptions of working stress method](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Awsm-basic-assumptions) — Design of Steel and Timber Structure, chapter 2 - [Different limit states in steel design](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Atypes-of-limit-states) — Design of Steel and Timber Structure, chapter 3 ## Related in other subjects - [Working stress vs limit state method](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aworking-stress-vs-limit-state-method) — Design of RCC Structure, chapter 1, asked 6× - [Why Kaiser window is better than fixed windows](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Awhy-kaiser-better) — Digital Signal Analysis and Processing, chapter 5, asked 2× - [Preventive maintenance is better than corrective](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Apreventive-better-than-corrective) — Construction Management, chapter 9, asked 4× - [T/F: radial more reliable than network](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-radial-more-reliable-network) — Transmission and Distribution System Design, chapter 5, asked 2×