fy (Yield Strength of Steel)

Yield strength (fy) is the stress at which a steel reinforcement bar transitions from elastic deformation (recoverable) to plastic deformation (permanent). For Indian HYSD bars per IS 1786:2008, fy is determined by the 0.2% proof-stress method on a tensile test specimen — the stress corresponding to 0.2% permanent strain after unloading. This 0.2% offset is necessary because cold-twisted and TMT bars do not exhibit a sharp yield point like plain mild steel. Common Indian grades: Fe415 (fy = 415 MPa), Fe500 (500 MPa), Fe500D (500 MPa with ductility add-ons), Fe550 (550 MPa), Fe550D (550 MPa ductile), Fe600 (600 MPa).

In limit-state design per IS 456:2000 Cl. 38.1, the design yield strength for ULS is 0.87 fy (the 1.15 partial safety factor on material). So Fe500 designs use 435 MPa, Fe550 uses 478 MPa. For balanced section design, the stress-strain curve is idealised as elastic-perfectly-plastic with yield at 0.87 fy and infinite strain capacity (capped at 0.0035 by concrete crushing). Modulus of elasticity Es = 2 × 10⁵ MPa is universal for all steel grades.

Higher fy reduces steel quantity for the same design moment, but increases the cracking width at service loads (because the bar is closer to yield at SLS). IS 456 Cl. 35.3 caps maximum permissible crack width at 0.3 mm for moderate exposure, achievable up to Fe550 with proper bar spacing. For Fe600 and beyond, crack width may govern over strength — explicit serviceability check is essential. Higher fy also increases bond stress demand at the same area, requiring longer development and lap lengths per IS 456 Cl. 26.2.1.