Clause 8.4 gives the design compressive stress fcd for columns and compression members based on the limit state of flexural buckling. The method uses buckling curves (a, b, c, d) with imperfection factors to account for initial crookedness, residual stresses, and section type. The design compressive stress depends on the non-dimensional slenderness ratio λ.
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Key Requirements
•fcd = (fy/γm0) / [φ + √(φ² − λ²)] ≤ fy/γm0, but shall not exceed fy/γm0
•φ = 0.5 × [1 + α(λ − 0.2) + λ²]
•λ = √(fy / fcc) where fcc = π²E / (KL/r)²
•E = 2 × 10⁵ MPa (modulus of elasticity of steel)
•Selection of buckling curve (a, b, c, d) depends on section type and axis of buckling — Table 10 of IS 800
Reference Tables
Imperfection Factor α for Buckling Curves (Clause 8.4)
Buckling Curve
α
a
0.21
b
0.34
c
0.49
d
0.76
Curve 'a' is for least imperfections (hot-rolled I about strong axis, t ≤ 40 mm). Curve 'd' is for maximum imperfections (cold-formed, welded built-up sections).
✓For standard ISMB columns with both ends fixed (K = 0.65), KL/r about the minor axis almost always governs. Always check both axes — the lower fcd controls.
✓Most Indian rolled I-sections use buckling curve 'b' about the major axis and 'c' about the minor axis (tf ≤ 40 mm). Angles and channels use curve 'c'.
✓For KL/r > 180, the column capacity drops dramatically. IS 800 Cl. 3.8 recommends maximum KL/r = 180 for compression members (250 for tension members).
Common Mistakes
⚠Using the wrong buckling curve — rolled I-sections and welded I-sections have different curves. Check Table 10 carefully.
⚠Forgetting to check buckling about both axes — the minor axis (y-y) usually governs for open sections like I-beams.
⚠Using KL/r exceeding 180 without justification — IS 800 limits slenderness to 180 for compression members (Cl. 3.8).