UK Base Plate Design — EN 1993-1-8 & UK NA

This reference covers base plate design for UK steel design per EN 1993-1-8:2005 Clause 6.2 and UK NA. Column base plates transfer axial compression and/or tension from the steel column to the concrete foundation through bearing on grout, bending of the plate, and tension in anchor bolts.

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

Code Reference: EN 1993-1-8:2005 Clause 6.2 and UK NA

Design Process Overview

The UK base plate design process follows these steps:

1. Determine applied loads (NEd, MEd, VEd) at column base
2. Calculate concrete bearing strength (EN 1992-1-1)
3. Determine required base plate dimensions (B × H)
4. Calculate plate thickness from T-stub bending
5. Design anchor bolts for tension and shear
6. Check shear transfer (friction, shear key, or bolts)
7. Design stiffeners if required

Concrete Bearing Strength (EN 1992-1-1 Clause 3.1.6)

[ f*{jd} = \beta_j \frac{F*{Rdu}}{b*{eff} l*{eff}} ]

Where:

• ( \beta_j = 2/3 ) (UK NA — unless verified by testing)
• ( F*{Rdu} = f*{cd} \times A_{c,eff} )
• ( f*{cd} = \alpha*{cc} f_{ck} / \gamma_c )
• ( \alpha_{cc} = 0.85 ) (UK NA to EN 1992-1-1)
• ( \gamma_c = 1.50 ) (concrete partial factor)

For C30/37 concrete (typical UK foundation): fcd = 0.85 × 30 / 1.5 = 17.0 N/mm² Design bearing strength: fjd = (2/3) × 17.0 = 11.3 N/mm²

Base Plate in Compression — T-Stub Model

The effective area under compression is determined by the spread of load through the plate at 45° from the column footprint:

[ c = tp \times \sqrt{\frac{f_y}{3 f{jd} \gamma_{M0}}} ]

Where tp is the plate thickness. The effective area is the column footprint expanded by c on each side. The compression resistance is then:

[ N*{c,Rd} = A*{eff} \times f_{jd} ]

Minimum Base Plate Thickness Recommendations

Thickness based on T-stub bending. Thicker plates may be required for moment-resisting bases. Based on typical UK office construction per SCI P398.

Anchor Bolt Design (EN 1993-1-8 Clause 6.2.6)

Tension resistance of anchor bolts: [ F*{t,Rd,anchor} = \frac{0.9 f*{ub} As}{\gamma{M2}} ]

Shear resistance (per anchor bolt): [ F*{v,Rd,anchor} = \frac{0.6 f*{ub} As}{\gamma{M2}} ]

Worked Example — Pinned Column Base

Given:

• Column: 203×203 UC 46 in S355
• NEd = 800 kN (compression, no significant moment)
• Concrete: C30/37, fck = 30 N/mm²
• Base plate: 350 × 350 mm, grade S275

Step 1 — Concrete bearing strength: fcd = 0.85 × 30 / 1.5 = 17.0 N/mm² fjd = (2/3) × 17.0 = 11.3 N/mm²

Step 2 — Required plate area: Amin = NEd / fjd = 800 × 10³ / 11.3 = 70,796 mm² Aplate = 350 × 350 = 122,500 mm² > 70,796 — OK

Step 3 — Plate thickness: Column footprint: 203.2 × 203.6 mm Additional projection: (350 - 203.2)/2 = 73.4 mm each side

c = 15 × √(275 / (3 × 11.3 × 1.0)) = 15 × 2.85 = 42.7 mm

Effective breadth = 203.2 + 2 × 42.7 = 288.6 mm ≤ 350 mm — OK Effective depth = 203.6 + 2 × 42.7 = 289.0 mm ≤ 350 mm — OK

Aeff = 289.0 × 288.6 = 83,405 mm² Nc,Rd = 83,405 × 11.3 × 10⁻³ = 942 kN > 800 kN — OK

Step 4 — Check bending stress in plate: The cantilever projection of 73.4 mm is subject to uniform bearing pressure: w = 800 × 10³ / (350 × 350) = 6.53 N/mm²

Moment at column face: M = w × projection²/2 = 6.53 × 73.4² / 2 = 17,587 Nmm/mm σ = M / (t²/6) = 17,587 × 6 / 15² = 469 N/mm² > 275 — Increase thickness

Re-calculate with tp = 20 mm: M = 17,587 Nmm/mm (same, independent of tp) σ = 17,587 × 6 / 20² = 264 N/mm² < 275 — OK

Use 20mm thick base plate.

Base Plate with Moment — Tension Side

For moment-resisting bases, the tension side uses the T-stub model (EN 1993-1-8 Clause 6.2.4):

[ F*{T,Rd} = min(F*{T,1,Rd}, F*{T,2,Rd}, F*{T,3,Rd}) ]

Where modes 1, 2, 3 represent:

• Mode 1: Complete flange yielding (thin plate, strong bolts)
• Mode 2: Bolt failure with flange yielding (intermediate)
• Mode 3: Bolt failure (thick plate)

[ F*{T,1,Rd} = \frac{4 M*{pl,1,Rd}}{m} \quad F*{T,2,Rd} = \frac{2 M*{pl,2,Rd} + n \Sigma F*{t,Rd}}{m + n} \quad F*{T,3,Rd} = \Sigma F_{t,Rd} ]

Shear Transfer Methods

Design Resources

• UK Column Design — Column buckling design
• UK Bolt Capacity — Bolt shear and tension
• UK Connection Design — Moment connection detailing
• UK Steel Properties — Material data
• UK Steel Beam Sizes — Section properties
• UK UB/UC Sections — Section dimensions
• All UK References

Frequently Asked Questions

How are UK base plates designed per EN 1993-1-8?

UK practice follows EN 1993-1-8 Clause 6.2 using the T-stub model. Concrete bearing strength fjd = βj FRdu / (beff leff). UK NA specifies βj = 2/3 unless verified by testing. The effective area under the column is determined by 45° load spread through the plate thickness, characterised by the dimension c = tp × √(fy / (3 fjd γM0)). The plate thickness is governed by T-stub flange bending at the column face.

What is the minimum base plate thickness per UK practice?

Minimum base plate thickness is typically 12 mm for pinned bases and 20 mm for moment-resisting bases per SCI P398. The plate thickness is governed by T-stub flange bending. For typical UK office buildings with 203×203 UC columns in S275 plate: pinned bases use 15-20 mm, moment-resisting bases use 25-30 mm. Thicker plates are required for higher steel grades or larger column sections.

How are anchor bolts sized for UK base plates?

Anchor bolts for pinned bases are typically sized for erection stability and shear transfer, not compression. Standard UK anchor bolts: M20 Grade 8.8 (fu = 800 MPa) in 300 mm embedment for moderate loads, M24 for heavy loads. Tension capacity per bolt: Ft,Rd = 0.9 × 800 × As / 1.25. Anchor bolts are designed to EN 1993-1-8 Clause 6.2.6 with UK NA partial factors.

What shear transfer methods are used in UK base plate design?

Four methods exist: (1) friction between grout and base plate (VRd ≈ 0.2 × NEd, adequate for most pinned bases with high compression), (2) anchor bolts in shear (when sufficient edge distance exists), (3) welded shear key (a short length of UC or plate welded to the base plate underside bears into the foundation), and (4) shear pocket (recess in foundation allows the column to bear directly). Shear keys are common for portal frame bases with high horizontal thrust.

When is gusset stiffening required on UK base plates?

Gusset stiffeners (triangular plates welded between column flange and base plate) are required when: (a) the base plate thickness exceeds 50 mm (practical fabrication limit), (b) moment-resisting bases with high bolt tension forces that produce Mode 1 T-stub failure (complete flange yielding), or (c) when the base plate projection exceeds the limit c calculated by the T-stub model. Stiffeners reduce the effective span for plate bending and allow thinner plates.

Reference only. Verify all values against the current edition of EN 1993-1-8:2005 Clause 6.2 and UK NA. This information does not constitute professional engineering advice.