Merits of limit state over working stress

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.