How does a Class 1 (plastic) section differ from a Class 2 (compact) section?

Class 1 reaches Mp with high rotation capacity, Class 2 reaches Mp but with limited rotation

Class 2 can form a full mechanism, Class 1 cannot

What happens in a Class 3 (semi-compact) section?

The extreme fibre reaches fy but local buckling prevents the plastic moment

Local buckling occurs before yield at the extreme fibre

It carries Mp with limited rotation

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Classification of steel sections by buckling

Short answer

IS 800:2007 classifies sections as plastic (Class 1), compact (2), semi-compact (3) or slender (4) using width-to-thickness ratios and ε=√(250/fy). Plastic sections allow full plastic rotation; compact reach Mp with limited rotation; semi-compact only elastic yield at extreme fiber; slender buckle before yield, needing effective section.

Classify and describe steel sections according to their local buckling behaviour.

IS 800:2007 (cl. 3.7, Table 2) classifies cross-sections by how their plate elements (flanges, web) behave under local buckling, using the width-to-thickness ratios $b/t_f$ and $d/t_w$ and $\varepsilon=\sqrt{250/f_y}$ .

Class	Name	Behaviour	Plastic hinge / rotation capacity
1	Plastic	Can form a plastic hinge and rotate enough for plastic redistribution of moment	Full plastic moment $M_p$ , high rotation
2	Compact	Can reach $M_p$ but local buckling limits rotation	$M_p$ , limited rotation
3	Semi-compact	Extreme fibre reaches $f_y$ , but local buckling prevents the plastic moment	Elastic moment $M_y = Z_e f_y$
4	Slender	Local buckling occurs before the yield stress is reached in the extreme fibre	Less than $M_y$ ; effective section used

Typical limits (rolled I-section, outstand flange $b/t_f$ ): Class 1 $\le 9.4\varepsilon$ , Class 2 $\le 10.5\varepsilon$ , Class 3 $\le 15.7\varepsilon$ . For the web of an I-section in bending (neutral axis at mid-depth): $d/t_w \le 84\varepsilon$ (plastic), $105\varepsilon$ (compact), $126\varepsilon$ (semi-compact).

Design use: Class 1 and 2 sections use the plastic section modulus $Z_p$ (with $\beta_b=1.0$ ); Class 3 uses the elastic modulus $Z_e$ ; Class 4 uses effective properties after deducting the buckled width.

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