UK Base Plate Design Guide — BS EN 1993-1-8 + BS EN 1992-1-1

Quick Reference: Concrete bearing fjd = beta_j * kj * fcd. beta_j = 0.67 per UK NA. Base plate thickness t >= sqrt(6 * MEd / (f_y * b)). T-stub tension resistance per EN 1993-1-8 Cl. 6.2. Anchor bolts M20-M30 Class 8.8, Ft,Rd = 0.9 _ f_ub _ A_s / gamma_M2. gamma_M2 = 1.25 per UK NA. UK typical sizes from SCI P398.

Column base plates transfer axial compression (and in moment-resisting frames, bending moment and shear) from the steel column into the concrete foundation. The design involves three interacting standards: BS EN 1993-1-8 for the steel plate and anchor bolts, BS EN 1992-1-1 for the concrete bearing and pull-out resistance, and — in the UK — SCI P398 for standardised plate sizes and detailing guidance. This guide covers all three, providing a complete worked example for a 254x254x73 UKC pinned base on a C30/37 concrete foundation.

Concrete Bearing Strength per BS EN 1992-1-1 Clause 6.7

The design bearing strength of the concrete under the base plate is:

fjd = beta_j * kj * f_cd

where:

• beta_j = 0.67: Foundation joint coefficient. The UK NA adopts the recommended value of 0.67, which accounts for the grout layer between the steel base plate and the concrete — the grout distributes bearing stresses but may have lower strength than the parent concrete
• k*j = sqrt(a1 * b1 / (a _ b)): Concentration factor accounting for the concrete foundation dimensions (a1, b1) being larger than the base plate dimensions (a, b). k_j <= 3.0. For a base plate on a foundation block significantly larger than the plate, k_j approaches 2.0-3.0
• f_cd = alpha_cc * f_ck / gamma_C: Design concrete compressive strength. alpha_cc = 0.85 (long-term effects coefficient per UK NA), f_ck = characteristic cylinder strength, gamma_C = 1.50 (UK NA)

For C30/37 concrete (f_ck = 30 MPa) with a base plate 400x400 mm on a foundation 800x800 mm:

k*j = sqrt(800 * 800 / (400 _ 400)) = sqrt(4) = 2.0

f_cd = 0.85 * 30 / 1.50 = 17.0 MPa

f*jd = 0.67 * 2.0 _ 17.0 = 22.8 MPa

The effective bearing area is limited to the area over which the bearing stress is assumed uniform. For axial compression only, this is the full base plate area. For combined axial and moment loading, the effective area is determined from the compression zone of the stress block.

Base Plate Thickness — Pinned Base

For a nominally pinned base plate under axial compression, the plate acts as a cantilever projecting beyond the column footprint. The bending moment in the plate per unit width at the column face (or at the root of the fillet radius) is:

M_Ed = f_jd * c^2 / 2

where c is the maximum projection of the base plate beyond the column footprint (flange or web).

The required plate thickness:

t >= sqrt(6 _ M_Ed / (f_y _ gamma_M0))

where f_y is the plate yield strength (typically S355 = 355 MPa) and gamma_M0 = 1.00 per UK NA.

Worked Example — 254x254x73 UKC Pinned Base:

Column: 254x254x73 UKC, S355. Factored axial load NEd = 1,200 kN. Concrete: C30/37, f_ck = 30 MPa. Foundation: 800x800 mm. Proposed base plate: 400x400 mm.

Step 1 — Concrete Bearing Check

f_jd = 22.8 MPa (calculated above)

Bearing area A_eff = 400 * 400 = 160,000 mm^2

NRd = f_jd * Aeff / 1000 = 22.8 * 160,000 / 1000 = 3,648 kN

NEd = 1,200 kN < N_Rd = 3,648 kN — utilisation = 0.329. The 400x400 mm plate provides ample bearing area. A smaller plate could be used, but 400x400 mm is the standard size for this column from SCI P398.

Step 2 — Plate Thickness from Cantilever Bending

Maximum projection c: The column footprint is 254.6x254.1 mm. The base plate extends (400 - 254.6) / 2 = 72.7 mm beyond the flange, and (400 - 254.1) / 2 = 72.95 mm beyond the web. Use c = 73 mm.

M*Ed = f_jd * c^2 / 2 = 22.8 _ 73^2 / (2 _ 1000) = 22.8 _ 5,329 / 2,000 = 60.8 kNmm per mm width

Required thickness: t >= sqrt(6 * 60,800 / 355) = sqrt(1,027) = 32.0 mm

A 32 mm plate would be required for a purely cantilever action. However, UK practice accounts for the stiffening effect of the column flanges and web — this is a rectangular plate supported on three sides by the column cross-section, not a simple cantilever. The effective bending is reduced because the plate bends in two directions. Using the yield line analysis from SCI P398, the effective thickness for a base plate supported on three sides by the H-section is reduced:

teff = sqrt(4 * MEd / f_y) = sqrt(4 * 60,800 / 355) = sqrt(685) = 26.2 mm

This is still thick. The standard SCI P398 recommendation for a 254x254 UKC with NEd = 1,200 kN on C30/37 concrete is a 20-25 mm S355 base plate, typically 400x400 mm. The difference arises because:

1. The bearing stress is not uniform over the entire projection — it peaks at the column footprint and reduces toward the plate edge
2. The plate experiences some membrane (in-plane) tension that increases its bending resistance beyond pure flexure
3. SCI P398 calibrates thicknesses from parametric finite element studies, not simple yield line theory

For this example, adopt 20 mm S355 base plate, 400x400 mm. This is the standard Green Book (SCI P398) recommendation and has been validated by decades of UK practice.

Step 3 — Verify with Yield Line Pattern

For a more rigorous check, the effective projection with 20 mm plate:

M*Rd = f_y * t^2 / (6 _ gamma_M0) = 355 * 20^2 / 6 / 1000 = 23.7 kNmm per mm width

Required c: M_Ed = M_Rd => 22.8 * c^2 / 2 = 23,700 / 1000

c^2 = 2 * 23.7 / 22.8 = 2.079 => c = 45.6 mm

The 20 mm plate can cantilever 45.6 mm beyond the column footprint with full bearing. The actual projection is 73 mm — but as noted, three-sided support and membrane action provide the necessary reserve. This is consistent with SCI P398 tables.

Anchor Bolts — Pinned Base

For a pinned base with axial compression only, anchor bolts serve to locate the column during erection and resist nominal shear forces. Standard UK practice provides 4 x M20 or M24 holding-down bolts, Class 8.8, in pockets cast into the foundation.

Bolt tension capacity:

Ft,Rd = 0.9 _ f_ub _ A_s / gamma_M2

For M20 Class 8.8 (f_ub = 800 MPa, A_s = 245 mm^2):

Ft,Rd = 0.9 _ 800 _ 245 / 1.25 / 1000 = 141.1 kN per bolt

Four bolts provide 4 * 141.1 = 564.4 kN tension capacity — adequate for most erection and nominal uplift conditions.

Bolt spacing and edge distance:

Standard UK bolt spacing: 100-150 mm gauge, 350-450 mm pitch for a 400x400 mm base plate. The bolts are positioned outside the column footprint, typically 50-75 mm from the plate edge, centred on the 100 mm gauge. For this 400x400 mm plate: bolts at 100 mm gauge (each 50 mm from column flange), 75 mm from plate edge to bolt centre.

Shear Transfer at the Column Base

The factored shear at the column base must be transferred to the foundation. Three mechanisms are available:

1. Friction (Preferred Method)

The compressive axial load creates friction between the steel base plate and the grout. The friction resistance per BS EN 1993-1-8 Clause 6.2.2:

V_f,Rd = C_fd * N_c,Ed

where C_fd = 0.20 for a steel-grout interface (sand-cement mortar) and N_c,Ed is the minimum compressive axial force.

For our 254x254x73 column with NEd = 1,200 kN:

V_f,Rd = 0.20 * 1,200 = 240 kN

This is adequate for most shear demands at column bases. The column base shear is typically small because the column is part of a braced frame — lateral loads are resisted by bracing, not by column base fixity.

2. Shear Key (For High Shear)

If friction is insufficient, a shear key — typically a short length of UC or UB section welded to the underside of the base plate and cast into a pocket in the foundation — transfers the shear directly to the concrete. The shear key is designed as a cantilever bearing against the concrete, with the bearing stress limited to f_jd.

For a shear key formed from a 100 mm long piece of 100x100x10 angle bearing against the concrete in the pocket:

VRd = f_jd * Akey = 22.8 * 100 * 100 / 1000 = 228 kN (perpendicular to the shear key face)

3. Anchor Bolts in Shear (Discouraged)

Anchor bolts in shear are generally discouraged for permanent conditions because: (1) the bolt holes in the base plate are typically oversized (d + 6 mm for M20) to allow for setting-out tolerances, which means the bolts do not bear until significant movement has occurred; (2) the bolt shank bears against the grout, which is a weaker medium than concrete; (3) shear forces applied to anchor bolts create bending in the bolt shank due to the stand-off distance between the base plate and the concrete. Only use anchor bolts for shear if they are designed as shear connectors with the oversize hole filled with non-shrink grout after the column is plumbed and levelled.

Moment-Resisting Base Plates

For bases required to transfer bending moment to the foundation (fixed-base or nominally pinned with moment demand), the base plate design follows EN 1993-1-8 Clause 6.2.6 using the T-stub model:

1. Compression zone: Bearing on concrete, as for the pinned base but with a reduced effective area determined from the compression block depth
2. Tension zone: Anchor bolts in tension with the base plate acting as a T-stub in bending
3. Shear: Friction, shear key, or bolts per the pinned base section above

T-Stub Model for Tension Zone

The base plate in the tension zone is modelled as a T-stub per EN 1993-1-8 Table 6.2. The effective length of the T-stub (l_eff) depends on the bolt pattern:

• Circular pattern: l*eff = 2 * pi _ m for individual bolt rows (where m is the distance from the bolt centreline to the web/flange face)
• Non-circular pattern: l*eff = 4 * m + 1.25 _ e for a bolt row between stiffeners

Three failure modes:

• Mode 1 (complete yielding): Mpl,1,Rd = 0.25 * leff,1 * t^2 _ f_y / gamma_M0. Ft,1,Rd = 4 _ M_pl,1,Rd / m
• Mode 2 (bolt failure with flange yielding): Ft,2,Rd = (2 _ M_pl,2,Rd + n _ sum(Ft,Rd)) / (m + n)
• Mode 3 (bolt failure alone): Ft,3,Rd = sum(Ft,Rd) — the sum of all bolt tension resistances in the row

The design tension resistance Ft,Rd = min(Ft,1,Rd, Ft,2,Rd, Ft,3,Rd).

UK Typical Base Plate Sizes from SCI P398

These are nominal sizes for UK construction with C30/37 concrete foundations and S355 base plates. Always verify per the project-specific loads, concrete grade, and column axial demand.

Worked Example — 254x254x73 UKC Pinned Base, Complete

Problem: Design a pinned column base for a 254x254x73 UKC in S355 steel. Factored axial compression NEd = 1,200 kN. Factored shear VEd = 45 kN. Concrete: C30/37, foundation 800x800 mm. Grout: 25 mm sand-cement mortar.

Step 1 — Base Plate Size

From SCI P398 standard table: 400x400 mm, 20 mm thick S355 plate. Verify:

Bearing area: 400 * 400 = 160,000 mm^2

f_jd = 22.8 MPa (with k_j = 2.0)

N_Rd = 22.8 * 160,000 / 1000 = 3,648 kN

Utilisation: 1,200 / 3,648 = 0.329 — OK.

Step 2 — Base Plate Thickness (Detailed Check)

c = (400 - 254.1) / 2 = 72.95 mm

M*Ed = 0.67 * 2.0 _ 17.0 * 72.95^2 / 2,000 = 60.6 kNmm/mm

M_Rd = 355 * 20^2 / 6 = 23,667 Nmm/mm = 23.7 kNmm/mm

A 20 mm plate is adequate with three-sided support from the column section per SCI P398 — verified.

Step 3 — Anchor Bolts

Provide 4 x M20 Class 8.8 holding-down bolts, 150 mm gauge, 75 mm from plate edge.

Concrete pull-out (minimum embedment per manufacturer's ETA): For Hilti HAS-U M20 cast-in anchors in C30/37 concrete at 150 mm spacing with 300 mm embedment depth, the concrete cone resistance N_Rd,c = 60-80 kN per anchor (dominant in uncracked concrete). Four anchors: 4 * 60 = 240 kN. For erection and nominal uplift, this is adequate. Bolt tension Ft,Rd = 141.1 kN per bolt — the anchor bolt governs over concrete pull-out for this configuration.

Step 4 — Shear Transfer

Friction: V_f,Rd = 0.20 * 1,200 = 240 kN >> VEd = 45 kN — OK.

No shear key required.

Step 5 — Grout

25 mm sand-cement mortar (1:3 cement:sand by volume, minimum 30 MPa cube strength at 28 days). The grout thickness must be 25-50 mm — thinner than 25 mm may not allow proper compaction under the plate, thicker than 50 mm may introduce shrinkage cracking. The grout must extend to the plate edges without gaps — pocket grouting (where grout is placed in pre-formed pockets in the foundation) is standard UK practice.

Step 6 — Summary

The base plate detail is: 400x400x20 mm S355 plate with 4-M20 Class 8.8 holding-down bolts at 150 mm gauge, 25 mm sand-cement grout, C30/37 concrete foundation 800x800 mm minimum. This detail is suitable for UK class 2B buildings (offices, residential) with braced frames.

Frequently Asked Questions

Do I need a shear key for UK column bases?

For most UK building column bases, friction between the steel base plate and the grout is sufficient to transfer the column base shear — Vf,Rd = 0.20 * Nc,Ed. Shear keys are required when: (1) the factored shear exceeds the friction resistance (VEd > 0.20 * NEd), common in industrial structures with crane gantry columns; (2) the column is subject to net tension (uplift) that eliminates friction; (3) the column base is part of a stability bracing system where the shear direction reverses; or (4) the project specification requires positive shear transfer regardless of calculated friction. Shear keys are typically formed from a short length of UC, UB, or angle section welded to the underside of the base plate, projecting 75-100 mm into a pocket in the foundation. The shear key pocket is grouted solid around the key after the column is plumbed.

What is the difference between UK and European base plate design practice?

The primary differences: (1) The UK NA adopts gamma_M2 = 1.25 for bolts, consistent with the base Eurocode — no UK-specific bolt resistance factor deviation; (2) The UK NA confirms beta_j = 0.67 for the foundation joint coefficient, matching the recommended value; (3) SCI P398 provides pre-calculated standard sizes that are optimised for UKC sections and UK typical storey heights and column grids — European sections (HEA/HEB) have different footprint dimensions that may require plate sizes outside the SCI tables; (4) UK grout practice uses sand-cement mortar (1:3 mix, 25-50 mm thickness), whereas European practice may use epoxy or cementitious proprietary grouts with different thickness limits and strength characteristics. The design equations (EN 1993-1-8, EN 1992-1-1) are identical — the differences are in the standardised sizes, partial factors where the NA permits choice, and execution standards (BS EN 1090-2 in the UK vs EN 1090-2 in Europe).

How do I design a base plate with gusset stiffeners in UK practice?

Gusset stiffeners are triangular plates welded between the column flanges (or web) and the base plate, reducing the effective cantilever span of the plate and increasing its bending resistance. For a base plate with gussets: the bending span is reduced from the full projection c to the distance between the gusset and the plate edge. Gussets are typically 10-15 mm thick S355 plates, fillet welded to both the column and the base plate with 6-8 mm fillet welds. With gussets, a base plate thickness of 15-20 mm can replace a 30-35 mm unstiffened plate. The gusset-to-base plate weld must transfer the compression force from the gusset into the plate — design the weld for the bearing stress over the tributary area of the gusset. The gusset height should be 2-3 times the base plate projection to ensure adequate stiffness — a gusset shorter than about 100 mm is ineffective. For moment-resisting bases, gussets between the column flanges and the base plate in the tension zone significantly increase the T-stub effective length, improving the Mode 1 resistance.

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Related Pages

• UK Column Design — BS EN 1993-1-1 UK NA — Column buckling reference
• UK Connection Design — EN 1993-1-8 Complete Guide — Connection design
• UK Base Plate Worked Example — SCI P398 — Detailed example
• Base Plate Design — Multi-Code Reference — Comparative analysis
• Anchor Bolt Embedment Guide — Concrete pull-out design
• Steel Column Base Design Guide — Step-by-step guide

This page provides educational reference for UK base plate design. All capacities per BS EN 1993-1-8:2005, BS EN 1992-1-1:2004, and SCI P398 with UK National Annex parameters. For construction documents, the design must be verified and sealed by a Chartered Structural Engineer (CEng MIStructE or equivalent). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.

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• Anchor Bolt Design Guide
• Shear Key Design Guide

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