Yield Strength (fy)

Yield strength (fy) is the stress at which a material transitions from elastic (recoverable) to plastic (permanent) deformation. For steel, yield strength is the most-quoted material property and the primary design parameter. For Indian structural steel and reinforcement, yield strength values are: Fe-410 (250 MPa), Fe-490 (330 MPa), Fe-540 (380 MPa), Fe-590 (460 MPa) for hot-rolled structural sections per IS 2062:2011. For reinforcement bars per IS 1786:2008: Fe-415 (415 MPa), Fe-500 (500 MPa), Fe-500D (500 MPa ductile), Fe-550 (550 MPa), Fe-550D, Fe-600 (600 MPa).

Yield strength is determined by tensile testing per IS 1608:2018 — a specimen is loaded in tension while strain is recorded; the yield point is identified by either a sharp upper-yield-lower-yield discontinuity (mild steel) or by the 0.2% offset method (HYSD bars, cold-formed steel). For HYSD reinforcement bars, the rounded stress-strain curve does not show a sharp yield point, so the 0.2% proof stress is taken as the conventional yield strength. Modulus of elasticity E = 2 × 10⁵ MPa is universal for all steel grades and is independent of yield strength.

In limit-state design per IS 800:2007 Cl. 5.4.1 + IS 456:2000 Cl. 38.1: design strength = fy / γm, where γm = 1.10 (steel for tension and bending) or 1.25 (steel ultimate strength). For Fe-500 reinforcement, design strength = 500/1.15 = 435 MPa; for Fe-410 structural steel, design strength = 250/1.10 = 227 MPa. Higher yield strength reduces required cross-sectional area for the same design force, lowering steel weight. However, higher yield strength does NOT improve stiffness (E is the same), so deflection-governed designs benefit from higher yield only marginally. Higher fy also means tighter control on cracking and slenderness — higher-strength steel is more sensitive to local instability and brittleness.