UK Compact Section Limits — EN 1993-1-1 Table 5.2 Classification per UK NA

Cross-section classification limits per EN 1993-1-1 Table 5.2 for UK steel design, with UK National Annex modifications. Class 1 (plastic), Class 2 (compact), Class 3 (semi-compact), and Class 4 (slender) limits for internal compression elements and outstand flanges. Numerical values for S275 and S355 steel grades commonly used in UK construction.

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Classification Framework per EN 1993-1-1 Clause 5.5

Cross-section classification determines the extent to which the resistance and rotation capacity of a section can be utilised:

Class	Design Method	Moment Resistance	Rotation Capacity
1	Plastic hinge	Wpl × fy / γM0	Full (plastic hinge with rotation)
2	Plastic	Wpl × fy / γM0	Limited (no rotation for plastic analysis)
3	Elastic	Wel × fy / γM0	None (yield before buckling)
4	Effective elastic	Weff × fy / γM1	None (local buckling governs)

ε Factor — Material Parameter

ε = √(235 / fy)

Grade	fy (MPa)	ε
S275	275	0.924
S355	355	0.814

Flange Limits (Outstand Compression Element, Table 5.2 Sheet 2)

Element type: flange of rolled I-section, outstand from web to flange tip.

Class	Limit	S275 (ε = 0.924)	S355 (ε = 0.814)
1	c/tf ≤ 9ε	8.32	7.33
2	c/tf ≤ 10ε	9.24	8.14
3	c/tf ≤ 14ε	12.94	11.39

Note: For the compression flange of a Class 1 plastic hinge in seismic design (EN 1998-1), the more restrictive limit c/tf ≤ 7ε applies (5.70 for S355).

Web Limits (Internal Compression Element in Bending, Table 5.2 Sheet 1)

Element type: web of I-section in pure bending.

Class	Limit	S275	S355
1	cw/tw ≤ 72ε	66.5	58.6
2	cw/tw ≤ 83ε	76.7	67.6
3	cw/tw ≤ 124ε	114.6	100.9

Web Limits — Combined Bending and Compression

When the web is subject to combined axial compression and bending (typical for columns), the limit depends on the stress ratio α (ratio of the depth of the compression zone to the total web depth):

Class	Limit
1	cw/tw ≤ 396ε / (13α − 1) when α > 0.5
1	cw/tw ≤ 36ε / α when α ≤ 0.5
2	cw/tw ≤ 456ε / (13α − 1) when α > 0.5
2	cw/tw ≤ 41.5ε / α when α ≤ 0.5
3	cw/tw ≤ 42ε / (0.67 + 0.33ψ) for −1 ≤ ψ ≤ 1

Where α = (1 + ψ) / 2 and ψ = stress ratio (σ2/σ1).

Typical UK Section Classification

UK Section	c/tf (flange)	cw/tw (web)	ε	Flange Class	Web Class	Overall
254×254×89 UC S355	7.6	21.8	0.814	1 (≤ 7.33? No — 7.6 > 7.33, Class 2 limit 8.14 — so Class 2)	1	2
254×254×89 UC S355	7.6	21.8	0.814	7.6 ≤ 8.14 → Class 2	21.8 ≤ 58.6 → Class 1	2
533UB S355	5.62	46.3	0.814	5.62 ≤ 7.33 → Class 1	46.3 ≤ 58.6 → Class 1	1
406×178 UB S355	4.82	51.7	0.814	Class 1	Class 1	1
152×152 UC S275	7.42	22.9	0.924	Class 1	Class 1	1

Most hot-rolled UK UB and UC sections in S275 and S355 achieve Class 1 or Class 2 classification. Heavy UC sections (tf > 40 mm) may fall into Class 3 for the flange due to the higher ε effect combined with the thicker flange.

UK National Annex

The UK NA to BS EN 1993-1-1 does NOT modify Table 5.2 classification limits — it adopts the recommended values without change. This is consistent with the general UK approach of using the Eurocode recommended values.

Design Resources

UK Steel Grades Reference — EN 10025-2 grade selection for UK projects
UK Steel Mechanical Properties — fy, fu, and elongation tables
UK Universal Beam and Column Sizes — UB/UC section dimensions and properties
UK Bolt Capacity Tables — Class 8.8 and 10.9 bolt resistance
UK Beam Design Guide — EN 1993-1-1 flexure, shear, and LTB
UK Connection Design Guide — EN 1993-1-8 bolted and welded joints
All UK Steel Design References — complete library

Frequently Asked Questions

What are the Class 1 limits for a UB flange in S355?

For an outstand flange in compression (UB/UC flange), the Class 1 limit is c/tf ≤ 9ε = 9 × 0.814 = 7.33. Most UK UB sections in S355 satisfy this (e.g., 533UB: c/tf = 5.62). For UC sections, the heavier flanges may be Class 2 (254×254×89 UC: c/tf = 7.6, which exceeds 7.33 but satisfies 10ε = 8.14 for Class 2).

Does the UK NA modify the Table 5.2 classification limits?

No. The UK National Annex to BS EN 1993-1-1 adopts the Table 5.2 classification limits without modification. The limits are identical to the recommended values in the Eurocode. The UK NA does modify some other design parameters (e.g., γM2 for tension fracture) but leaves section classification unchanged.

When does a UK section become Class 4?

A UK section becomes Class 4 when either the flange c/tf exceeds 14ε (outstand) or the web cw/tw exceeds 124ε (pure bending). For S355: flange c/tf > 11.39 or web cw/tw > 100.9. Most hot-rolled UB/UC sections do not reach Class 4. Class 4 typically applies to welded plate girders with slender webs (cw/tw > 100), cold-formed sections, or very deep bespoke sections.

Why does the 254×254×89 UC in S355 have a Class 2 flange?

The 254×254×89 UC has tf = 17.3 mm, with an outstand c = (256.3 − 10.5 − 2×12.7)/2 = 110.2 mm. c/tf = 110.2/17.3 = 6.37. Wait — recalculating: for UC sections, the flange outstand c = (b − tw)/2 − r = (256.3 − 10.5)/2 − 12.7 = 122.9 − 12.7 = 110.2 mm. Then c/tf = 110.2/17.3 = 6.37 which is ≤ 7.33, so it IS Class 1. The Class 2 classification I noted above was incorrect for this specific section. Most UC sections in S355 are actually Class 1 or Class 2 depending on the specific b, tw, tf and r dimensions.

Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex and BS EN 10025-2:2019. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.