Clause 3.7 classifies steel cross-sections into four classes based on the width-to-thickness ratios of their compression elements. The classification determines whether the section can develop full plastic moment, is limited to elastic capacity, or requires reduced effective section properties due to local buckling.
Key Requirements
•Class 1 (Plastic): Can form a plastic hinge with sufficient rotation capacity for plastic analysis
•Class 2 (Compact): Can develop full plastic moment but with limited rotation capacity — elastic analysis only
•Class 3 (Semi-compact): Can reach yield stress at extreme fibre but local buckling prevents full plastic moment — design to elastic section modulus Ze
•Class 4 (Slender): Local buckling occurs before yield — use effective section properties per Clause 7.3.2
•Width-to-thickness limits depend on the element type (outstand flange, internal element, web) and the steel grade via ε = √(250/fy)
ε = √(250/fy). For E250: ε = 1.0; E300: ε = 0.91; E350: ε = 0.845; E410: ε = 0.781. Sections exceeding Class 3 limits are Class 4 (Slender).
Formulas
ε = √(250/fy)
Yield stress ratio used to adjust width-to-thickness limits for different steel grades
ε = Yield stress ratio (dimensionless)fy = Yield stress of steel in MPa
Practical Notes
✓Most standard Indian hot-rolled sections (ISMB, ISLB) in E250 fall in Class 2 or Class 3. Very few standard sections are Class 1 — plastic design is not common in Indian practice.
✓When using E350 or higher grade steel, the same section may shift from Class 2 to Class 3 (or even Class 4) because ε decreases and the limiting ratios become tighter.
✓For Class 3 sections, use the elastic section modulus Ze (not Zp) in moment capacity calculations. This can reduce beam capacity by 10–15% compared to a Class 2 section.
Common Mistakes
⚠Ignoring section classification and using Zp for all sections — this is unsafe for Class 3 and Class 4 sections where local buckling limits capacity.
⚠Not recalculating classification when changing steel grade — a section that is Class 2 in E250 may become Class 3 or 4 in E350.
⚠Using plastic analysis (Clause 4.5) with Class 2 sections — only Class 1 sections are permitted for plastic analysis.