Eurocode 3 Cross-Section Classification (Class 1–4) Explained

A steel cross-section is an assembly of thin plates: flanges and webs. When such a plate is compressed, it can buckle locally: it ripples out of its plane long before the whole member fails. Classification answers a single physical question: how far into the plastic range can the section be pushed before local buckling cuts the process short?

• Class 1 (plastic): the section can develop a full plastic hinge and then keep rotating at the plastic moment. This rotation capacity is what allows plastic global analysis with moment redistribution.
• Class 2 (compact): the section still reaches its full plastic moment resistance, but local buckling limits the rotation capacity. Plastic resistance yes, plastic analysis no.
• Class 3 (semi-compact): the extreme compression fibre can just reach the yield strength, but local buckling prevents the plastic stress block from spreading through the depth. Resistance stops at first yield.
• Class 4 (slender): local buckling occurs before yield is reached anywhere. Part of the compressed plate sheds its load, and only an effective portion of the section can be counted on.

The class number is thus a measure of deformation capacity, from ductile hinge behaviour down to elastic buckling of a slender plate.

Classification compares the width-to-thickness ratio c/t of every compressed (or partially compressed) plate element against limits from Table 5.2 of EN 1993-1-1. The width c is the flat part of the plate, measured between the fillets or corner radii, not the overall width. Two families of elements are distinguished:

• Internal parts are supported along both longitudinal edges: the web of an IPE or HEA, or the walls of an RHS/SHS. Two supported edges resist buckling well, so the limits are generous.
• Outstand parts are supported along one edge only and free along the other: the half-flange of an I- or H-section, or an angle leg. A free edge buckles easily, so the limits are much stricter.

The limits are scaled by the material factor ε = √(235/f_y). Higher yield strength means a smaller ε and therefore stricter limits: the same geometry can drop a class when you switch to a stronger steel grade, because yielding is postponed while the buckling stress of the plate stays the same.

Circular hollow sections (CHS) are a special case: the whole tube wall is one curved element, classified by the d/t ratio against limits in ε² rather than ε.

Each compressed element gets its own class; the cross-section class is the least favourable (highest) class of its elements. A section with Class 1 flanges and a Class 3 web is a Class 3 section (with one useful exception discussed below).

The class decides which section modulus enters the resistance check (with γ_M0 = 1.0 in most National Annexes):

1. Classes 1 and 2 use the plastic section modulus W_pl: the bending resistance is the full plastic moment M_pl,Rd = W_pl·f_y/γ_M0, with rectangular plastic stress blocks fully developed.
2. Class 3 uses the elastic section modulus W_el: resistance is the elastic moment M_el,Rd = W_el·f_y/γ_M0, reached when the extreme fibre first yields. The difference between W_pl and W_el (the shape factor) is simply thrown away.
3. Class 4 uses effective section properties (A_eff, W_eff) computed with the effective-width method of EN 1993-1-5: the buckled central portions of slender compressed plates are removed, and only the strips near the supported edges are counted. For a member in compression this can also shift the centroid of the effective section, generating an additional bending moment that must be added to the check.

In practice this is why classification comes first in every EC3 member design: it selects the whole resistance model, not just a coefficient.

The c/t limits depend on the stress distribution in the element, so a section does not have one class: it has a class for each internal force. The clearest case is the web of an I-section:

• In bending, only half the web is in compression and the stress varies linearly, so the limit for an internal part in bending is generous (the 72ε / 83ε / 124ε row of the table).
• In pure compression, the entire web is uniformly compressed and the limits collapse to the much stricter 33ε / 38ε / 42ε row.

A typical slender rolled beam can therefore be Class 1 in bending yet Class 4 as a column. For combined compression and bending, classification is done for the actual stress distribution (via the α or ψ parameters of Table 5.2); a quick conservative shortcut is to classify as if in pure compression.

The class can likewise differ between major-axis and minor-axis bending, since different elements end up in compression, and a section that changes class under N+M interaction is verified with the rules of its worst applicable class.

• Cold-formed hollow sections are often Class 4 in compression. Thin-walled SHS/RHS optimise weight by stretching c/t, and the uniform-compression limits are strict. Re-check the class whenever a "beam" section is reused as a compression chord or column.
• Copying the bending class into a compression check. As shown above, the web row of the table changes completely between bending and uniform compression: classification is per load case, not per profile.
• Using the overall width instead of the flat width c. For rolled sections c excludes the root radius; for cold-formed tubes it excludes the corner radii. Taking the full width is conservative but wrong; taking too little is unsafe.
• Forgetting ε when changing steel grade. Upgrading the grade raises f_y, shrinks ε, and can silently demote a section by a class: more strength on paper, but a weaker resistance model.
• Using ε instead of ε² for CHS. Tube limits are written in d/t against ε², so grade changes hit circular sections twice as hard.
• Missing the web exception. EN 1993-1-1 allows a section with Class 1 or 2 flanges and a Class 3 web to be treated as an effective Class 2 section with a reduced effective web, often recovering plastic bending resistance.

On crosssections.app you do not have to run the table by hand: every profile page (for example in the IPE, HEB, SHS or CHS families) includes a calculator that shows the cross-section class for your selected steel grade, separately for bending and compression.