Working Stress Method (WSM)

Working Stress Method (WSM, also called allowable-stress method) is the older structural-design philosophy where the structure is designed using service loads (unfactored) and material strengths reduced by an arbitrary factor of safety. Material strength is divided by FoS to give 'permissible stress'; computed stresses must not exceed permissible. Indian Standard IS 456:2000 Annex B retains WSM for water-retaining structures; IS 3370 Part 2:2009 mandates WSM for concrete water tanks. Most other applications now use the modern Limit State Method (LSM) per IS 456 + IS 800:2007.

WSM characteristics: (a) single safety factor on material (typically 3.0 for concrete in compression, 1.85 for steel in tension); (b) permissible stresses below yield: e.g., for Fe415 steel in tension σst = 230 MPa (below yield 415); for M25 concrete in compression σcc = 8.5 MPa (below 0.45 fck = 11.25); (c) elastic stress-strain assumed throughout, no plastic redistribution; (d) deflection and crack-width checks with the same load level (not separate ULS/SLS as in LSM); (e) overall structure has 1.7-2.5× safety margin between design and ultimate, similar to LSM.

LSM superseded WSM in India for general construction in 2000 (with the IS 456:2000 revision). Reasons: (a) WSM's single safety factor doesn't distinguish material variability from load uncertainty; (b) WSM cannot adequately handle reliability-based design; (c) WSM is conservative for ordinary structures and overly aggressive for unusual structures. WSM is retained for: (1) Water-retaining concrete tanks per IS 3370 Part 2 — crack-width control critical, WSM gives more controllable design; (2) Specialty applications where LSM's plastic redistribution assumptions don't apply; (3) Older structure analysis — to compare with original WSM design. For routine modern Indian construction, LSM is the only correct design philosophy.