Annex B (Clause 46) covers the Working Stress Method (WSM), which was the primary design method before IS 456:2000 introduced the Limit State Method. Though LSM is the preferred method, WSM is still widely used for: (a) checking serviceability, (b) water-retaining structures (IS 3370), (c) design of non-critical structures, and (d) legacy building verification.
Key Requirements
•Permissible stress in concrete (compression): σcbc from Table 21
•Permissible stress in concrete (direct compression): σcc from Table 21
•Permissible stress in steel (tension): σst from Table 22 — 140 MPa (Fe250), 230 MPa (Fe415), 275 MPa (Fe500)
•Modular ratio m = 280 / (3 σcbc) — used for transformed section analysis
•Permissible bond stress: τbd from Table 21 (for WSM)
Reference Tables
Table 21 — Permissible Stresses in Concrete (WSM)
Grade
σcbc (Bending) MPa
σcc (Direct) MPa
M15
5
4
M20
7
5
M25
8.5
6
M30
10
8
M35
11.5
9
M40
13
10
M45
14.5
11
M50
16
12
Table 22 — Permissible Stresses in Steel Reinforcement (WSM)
Steel
σst Tension (MPa)
σsc Compression (MPa)
Fe250 (mild steel)
140
130
Fe415 (TMT)
230
190
Fe500 (TMT)
275
190
Modular Ratio m = 280 / (3 σcbc)
Grade
σcbc
m
M15
5
18.67
M20
7
13.33
M25
8.5
10.98
M30
10
9.33
M35
11.5
8.11
M40
13
7.18
Formulas
m = 280 / (3 σcbc)
Modular ratio for transformed section
m = Modular ratio (Es/Ec equivalent for WSM)σcbc = Permissible bending compressive stress in concrete (MPa)
Practical Notes
✓WSM is still used for water-retaining structures (IS 3370) because crack width control is paramount — WSM's lower stresses inherently limit cracking.
✓Many government PWD departments and older design offices still use WSM. It's also the exam syllabus for GATE and SSC JE.
✓The modular ratio m is NOT Es/Ec — it's a semi-empirical value (280/3σcbc) that accounts for creep. This is a common source of confusion.
Common Mistakes
⚠Mixing WSM stresses with LSM calculations — σst = 230 MPa (WSM for Fe415) vs 0.87fy = 361 MPa (LSM). Using WSM stresses in LSM design is overly conservative.
⚠Using m = Es/Ec = 200000/Ec instead of 280/(3σcbc) — the WSM modular ratio is different from the elastic modular ratio.
⚠Forgetting that WSM uses unfactored (service) loads — no load factors of 1.5 as in LSM.