UK Steel Fy and Fu Values — S275 and S355 per BS EN 10025-2 + UK NA
Nominal yield strength (fy) and ultimate tensile strength (fu) for UK structural steel grades S275 and S355 per BS EN 10025-2:2019, with thickness brackets per EN 1993-1-1 Table 3.1 and UK National Annex modifications. Essential reference for all EN 1993-1-1 member design, connection design per EN 1993-1-8, and material procurement verification for UK construction projects.
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Nominal Yield Strength fy by Thickness
The nominal yield strength fy depends on the nominal thickness of the element (flange or plate thickness). EN 1993-1-1 Table 3.1 defines the thickness brackets used in design:
| Grade | t ≤ 16 mm | 16 < t ≤ 40 mm | 40 < t ≤ 63 mm | 63 < t ≤ 80 mm | 80 < t ≤ 100 mm |
|---|---|---|---|---|---|
| S275 | 275 | 265 | 255 | 245 | 235 |
| S355 | 355 | 345 | 335 | 325 | 315 |
Values in MPa. For t > 100 mm, refer to BS EN 10025-2 Table 7 (reduced values apply).
Ultimate Tensile Strength fu Range
The tensile strength fu is specified as a range rather than a single value. The minimum fu is used for connection design per EN 1993-1-8 (bearing, weld resistance):
| Grade | fu Range (MPa) | Min fu (Connection Design) |
|---|---|---|
| S275 | 410-560 | 410 |
| S355 | 470-630 | 470 |
For S275, fu = 410 MPa minimum is used for EN 1993-1-8 bearing checks (Fb,Rd) and weld throat calculations. For S355, fu = 470 MPa minimum applies.
Yield Strength Ratio and Ductility
BS EN 10025-2 requires minimum elongation after fracture (L0 = 5.65√S0):
| Grade | t ≤ 16 mm | 16 < t ≤ 40 mm | 40 < t ≤ 63 mm | t > 63 mm |
|---|---|---|---|---|
| S275 | 23 % | 23 % | 22 % | 21 % |
| S355 | 22 % | 22 % | 21 % | 20 % |
The fy/fu ratio is also important for seismic design (EN 1998-1). For S355, the ratio fy_min/fu_min = 355/470 = 0.76, which satisfies the EN 1998-1 requirement of fy/fu ≤ 0.85 for capacity design.
UK National Annex Provisions
The UK NA to BS EN 1993-1-1 (NA.2.4) adopts the Table 3.1 values unchanged from the recommended values. Key UK-specific points:
- S275 is the default grade for general UK building construction per NA.2.4
- S355 is recommended for heavier structural frames, longer spans, and where section weight optimisation is desired
- Thickness brackets are identical to the recommended values (no modification)
- γM0 = 1.00 and γM1 = 1.00 per UK NA Table NA.1
- γM2 = 1.25 for connections (bolts, welds, plates in bearing)
Grade Equivalence to Historical UK Standards
| Historical BS 4360 / BS 449 | EN 10025-2 | fy (t ≤ 16 mm) | Min fu |
|---|---|---|---|
| Grade 43A / 43B / 43C / 43D | S275 | 275 | 410 |
| Grade 50A / 50B / 50C / 50D | S355 | 355 | 470 |
When assessing existing UK structures built to BS 449 or BS 4360, use these equivalences as a starting point. Always verify through tensile coupon testing for critical assessments.
Worked Example — Design Strength for 610×229 UB 125
Given: Section 610 UB 125, flange thickness 19.6 mm, grade S355JR. From Table 3.1: t = 19.6 mm falls in 16 < t ≤ 40 mm bracket, fy = 345 MPa, fu = 470 MPa.
Design bending resistance (EN 1993-1-1 Clause 6.2.5): M_c,Rd = W_pl,y × fy / γ_M0 = 3229 × 345 / 1.0 × 10⁻³ = 1114 kNm
Connection bearing resistance (EN 1993-1-8 Clause 3.6): F_b,Rd uses f_u = 470 MPa (minimum) for the plate bearing check with γ_M2 = 1.25.
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 is the nominal yield strength of S355 steel for a 15 mm thick flange?
For t ≤ 16 mm, the nominal yield strength fy = 355 MPa. This applies to S355JR, S355J0, S355J2, and S355K2 identically. For a 20 mm flange (16 < t ≤ 40 mm), fy reduces to 345 MPa. For a 50 mm plate, fy = 335 MPa. This stepped reduction reflects the lower yield strength in thicker sections due to the rolling process.
What tensile strength fu should I use for EN 1993-1-8 connection design?
Use the minimum specified tensile strength: fu = 410 MPa for S275 and fu = 470 MPa for S355. These are the 5 % fractile values from BS EN 10025-2. The actual fu is typically 10-15 % higher than the minimum, but design must use the guaranteed minimum. For fu/fy ratio checks (welded joints, seismic), use the minimum values conservatively.
Does the UK National Annex modify the fy thickness brackets?
No. The UK NA to BS EN 1993-1-1 adopts the EN 1993-1-1 Table 3.1 thickness brackets without modification. The brackets are: t ≤ 16 mm, 16 < t ≤ 40 mm, 40 < t ≤ 63 mm, 63 < t ≤ 80 mm, and 80 < t ≤ 100 mm. The UK NA also adopts γM0 = 1.00 and γM1 = 1.00 without modification.
What are the equivalent grades for pre-2000 UK steel?
BS 4360 Grade 43A/B/C/D corresponds to S275JR/J0/J2. BS 4360 Grade 50A/B/C/D corresponds to S355JR/J0/J2. Grade 43 had fy = 275 MPa and fu = 410-530 MPa. Grade 50 had fy = 355 MPa and fu = 490-630 MPa (slightly narrower range than modern S355). For assessment of existing structures, tensile coupon testing is recommended to establish actual properties.
Related Pages
- EN 1993 Steel Design Overview
- European Steel Properties
- EN 1993 Beam Design Guide
- EN 1993 Column Buckling
- EN 1990 Load Combinations
- UK Steel Chemical Composition
- UK Steel Charpy Values
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.