UK Cold-Formed Steel — EN 1993-1-3 Design Guide

This reference covers cold-formed steel design for UK practice per EN 1993-1-3:2006 and UK NA. Cold-formed sections are widely used in UK construction for purlins, side rails, mezzanine floors, and light steel framing.

Design requirements, worked examples, and practical design guidance are provided for common design office applications.

Code Reference: EN 1993-1-3:2006 and UK NA

Material Grades for UK Cold-Formed Sections (BS EN 10326)

Zinc coating classes: Z140 (140 g/m²), Z180, Z225, Z275, Z350. Standard for internal: Z180. External: Z275.

Section Types and Applications

Effective Width Method (EN 1993-1-5, EN 1993-1-3 Clause 4)

Cold-formed sections have slender elements that buckle locally before reaching yield. The effective width method accounts for local buckling:

[ \rho = \frac{\bar{\lambda}_p - 0.055(3 + \psi)}{\bar{\lambda}_p^2} \leq 1.0 ]

Where (\bar{\lambda}p = \sqrt{f_y / \sigma{cr}}) and ψ is the stress ratio.

For internal compression elements (web under uniform compression): [ \bar{\lambda}p = \frac{b/t}{28.4 \varepsilon \sqrt{k\sigma}} ]

Typical Section Properties — C-Sections (S350GD+Z)

Effective properties after allowing for local buckling. Values are approximate — consult manufacturer's data for exact values.

Purlin Design to EN 1993-1-3

For UK purlin design, the key checks are:

1. Bending moment resistance (Clause 6.1.4)
2. Shear resistance (Clause 6.1.5)
3. Combined bending and shear
4. Web crippling at supports (Clause 6.1.7)
5. Deflection (serviceability)
6. Lateral-torsional buckling (Clause 6.2, typically braced by sheeting)

Worked Example — Purlin Design

Given:

• C200×75×2.5 in S350GD+Z (fy = 350 MPa)
• Span: 6000 mm, simply supported
• Spacing: 1800 mm
• Loading: dead 0.5 kN/m², snow 0.75 kN/m² (combination factor ψ0 = 0.7)
• Sheeting provides full lateral restraint to top flange

Step 1 — Design loads: Ultimate load: qEd = 1.35 × 0.5 + 1.5 × 0.75 = 1.80 kN/m² Per purlin: qEd = 1.80 × 1.8 = 3.24 kN/m

Step 2 — Bending moment: MEd = qEd × L² / 8 = 3.24 × 6.0² / 8 = 14.58 kNm

Step 3 — Moment resistance (simplified, assuming fully effective section): Mc,Rd = Wel,y × fy / γM0 = 36.2 × 10³ × 350 / 1.0 × 10⁻⁶ = 12.67 kNm

UT = 14.58 / 12.67 = 1.15 — Not satisfactory. Try deeper section.

Step 4 — Try C250×90×3.0: Mc,Rd = 63.4 × 10³ × 350 / 1.0 × 10⁻⁶ = 22.19 kNm UT = 14.58 / 22.19 = 0.66 — Satisfactory

Step 5 — Deflection check (SLS, characteristic load): qser = 0.5 + 0.75 = 1.25 kN/m²; per purlin = 1.25 × 1.8 = 2.25 kN/m δ = 5 × 2.25 × 6000⁴ / (384 × 210,000 × 792.3 × 10⁴) = 15.4 mm L/200 = 6000/200 = 30 mm → 15.4 < 30 — OK

Step 6 — Web crippling check (support reaction): VEd = 3.24 × 6.0 / 2 = 9.72 kN Per EN 1993-1-3 Clause 6.1.7, web crippling resistance depends on bearing plate length: For 100 mm stiff bearing: Rw,Rd ≈ 12 kN (simplified for C250 at 3.0 mm) — OK (check manufacturer's data)

Design Resources

• UK Steel Properties — Material data
• UK Framing Systems — Structural systems overview
• UK Deflection — Serviceability limits
• UK Steel Beam Sizes — Hot-rolled section data
• UK Composite Beam — Composite construction
• All UK References

Frequently Asked Questions

How does EN 1993-1-3 apply to UK cold-formed steel?

EN 1993-1-3 covers cold-formed members and sheeting. UK NA specifies γM0 = 1.00, γM1 = 1.00. The effective width method per EN 1993-1-5 is used for local buckling of slender sections, where the cross-section is reduced to account for post-buckling reserve. Distortional buckling (a unique mode in cold-formed sections where the flange-lip assembly rotates about the web-flange junction) is checked per Clause 5.5. EN 1993-1-3 also specifies minimum thickness limits: t ≥ 1.2 mm for structural members, t ≥ 0.7 mm for sheeting.

What steel grades are used for UK cold-formed sections?

Common grades: S320GD+Z (fy = 320 MPa), S350GD+Z (fy = 350 MPa), S390GD+Z (fy = 390 MPa) per BS EN 10326. Zinc coating classes Z180 to Z350 for corrosion protection. S350GD+Z is the most common grade for UK purlin applications. The coating class is specified by the designer based on environmental exposure: Z180 for internal (offices), Z275 for covered external (car parks), Z350 for fully exposed (coastal).

What are the design implications of cold-formed vs hot-rolled steel?

Cold-formed sections (C, Z, sigma) are typically: (a) thinner (1.5-4.0 mm vs 5-25 mm), (b) higher strength-to-weight ratio, (c) more prone to local and distortional buckling, (d) designed using effective width method rather than plastic section modulus, and (e) have thinner material that requires corrosion protection. Connections for cold-formed sections use self-drilling screws, blind rivets, or powder-actuated fasteners rather than standard bolts.

How are cold-formed purlins designed for continuous spanning?

Z-purlins are designed as continuous beams over multiple supports using the "sleeved" or "overlapping" system. Sleeved purlins use an internal sleeve at supports to transfer negative moment. Overlapped purlins extend the Z-section over the support with nested sections. The design to EN 1993-1-3 accounts for: (a) moment redistribution at supports (typically 10-20% reduction applied to peak moments), (b) web crippling at internal supports, and (c) lateral-torsional restraint at support locations.

What are the connection methods for cold-formed steel in UK practice?

Connections in cold-formed steel use: self-drilling screws (most common, to BS EN 14566), blind rivets (BS EN ISO 15977), power-actuated fasteners (for connecting to hot-rolled steel or concrete), and bolted connections (M8-M12, typically Category A bearing type). The design resistance of screw connections follows EN 1993-1-3 Clause 8.3. Minimum material thickness for screw connections: 1.5 mm. Edge distance for screws: minimum 2d (loaded), 1.5d (unloaded). Typical screw spacing: 300 mm for sheeting fixing.

Reference only. Verify all values against the current edition of EN 1993-1-3:2006 and UK NA. This information does not constitute professional engineering advice.