Design Problem

Problem: Design a nominally pinned base plate for a 254 x 254 UC 73 column (S355) supporting an axial compression of NEd = 1,750 kN and a nominal shear VEd = 85 kN. The base plate sits on a C30/37 concrete pad foundation with a grout layer.

Design codes: BS EN 1993-1-8:2005 + UK NA, BS EN 1992-1-1:2004 (concrete design), SCI P398.

Materials:

• Base plate: S275JR to BS EN 10025-2, fyp = 275 MPa, fup = 410 MPa
• Column: S355JR, fy = 355 MPa
• Concrete: C30/37, fck = 30 MPa
• Grout: 30 mm nominal thickness, compressive strength >= concrete
• Holding-down bolts: M24, Class 8.8 (fyb = 640 MPa, fub = 800 MPa)
• gamma_M0 = 1.00, gamma_M2 = 1.25 (UK NA)

Step 1: Base Plate Dimensions — Preliminary Sizing

Per SCI P398 guidance for nominally pinned bases:

Plate plan area from concrete bearing:

Design bearing strength of concrete under the base plate (EN 1992-1-1 Clause 6.7, assuming a square plate of side a1 = 500 mm):

fjd = (2/3) x fcd x sqrt(a1 x b1 / a x b) where the loaded area enhancement depends on the foundation size.

For a concentrically loaded base plate on a foundation extending well beyond the plate edges, the joint coefficient beta_j = 2/3, and the foundation joint material coefficient alpha = 1.0 (EN 1993-1-8 Clause 6.2.5(7)):

fcd = fck / gamma_c = 30 / 1.5 = 20 MPa fjd = (2/3) x 20 = 13.3 MPa (conservative, without enhancement)

Required plate area:

A_required = NEd / fjd = 1,750 x 10^3 / 13.3 = 131,600 mm2

For a square plate: sqrt(131,600) = 363 mm side. Round up to common UK plate sizes: use 450 x 450 mm.

Check concrete bearing with loaded area enhancement:

For a 450 x 450 mm plate on a foundation where the foundation plan dimensions (b1, d1) are at least 3x the plate dimensions:

alpha_d = min(b1/bp, d1/dp, 3) = 3.0 approximately Enhancement factor = sqrt(alpha_d) = sqrt(3.0) = 1.732

kj = sqrt(a1 x b1 / a x b) but limited to 5.0 (EN 1992-1-1 Clause 6.7)

Effective bearing: fjd = 2/3 x 20 x 1.732 = 23.1 MPa (with enhancement).

Actual bearing stress: NEd / A = 1,750,000 / (450 x 450) = 8.64 MPa << 23.1 MPa — acceptable.

Step 2: Base Plate Thickness — Equivalent T-Stub Method

Per EN 1993-1-8 Clause 6.2.6.11, a nominally pinned base plate transmits compression through bearing. The bending in the plate between the column and the edge is checked using the equivalent T-stub in compression model.

Geometry:

• Plate dimensions: b = 450 mm (parallel to column flange), d = 450 mm
• Projection beyond column: c = (450 - 254.6) / 2 = 97.7 mm each side
• Projection beyond column depth: (450 - 254.1) / 2 = 98.0 mm

Bending moment in plate projection (cantilever model per SCI P398):

The bearing pressure on the projection acts as a UDL:

w = NEd / (b x d) x b_per_unit_width = 8.64 MPa x 1 mm = 8.64 N/mm (per mm width)

M_Ed = w x c2 / 2 = 8.64 x 97.72 / 2 = 41,240 N·mm/mm width

Plastic moment resistance per unit width:

M_pl,Rd = (tp2 x fyp / gamma_M0) / 4 = tp2 x 275 / 4 = 68.75 tp2 N·mm/mm

For M_Ed <= M_pl,Rd: tp,min = sqrt(41,240 / 68.75) = sqrt(600) = 24.5 mm

Use 25 mm base plate (standard UK plate thickness).

Step 3: Anchor Bolt Design — Tension Check

While the column is primarily in compression, minimum holding-down bolts are required per UK practice and BS EN 1993-1-8 Clause 6.2.6.12.

Use 4 x M24, Class 8.8 holding-down bolts (fyb = 640 MPa, fub = 800 MPa per BS EN ISO 898-1).

Tensile resistance of a single bolt (EN 1993-1-8 Table 3.4):

Tensile stress area for M24: As = 353 mm2 Ft,Rd = 0.9 x fub x As / gamma_M2 = 0.9 x 800 x 353 / 1.25 = 203.3 kN

For a nominally pinned base with 4 bolts, the uplift capacity is: Ft,total = 4 x 203.3 = 813.2 kN

This provides adequate robustness against accidental uplift and satisfies the UK NA requirement for minimum 4-bolt base plates on all column bases (nominal 2-bolt bases are permitted only for secondary members where the engineer certifies no uplift).

Step 4: Shear Transfer

The nominal shear VEd = 85 kN must be transferred to the foundation. Three mechanisms are available:

Option A — Friction (preferred for pinned bases):

With pre-compression from NEd and a friction coefficient between steel and grout mu = 0.2 (EN 1993-1-8 Clause 6.2.2(6)):

V_friction = mu x NEd = 0.2 x 1,750 = 350 kN

350 kN >> 85 kN — shear is transferred entirely by friction. No shear key needed.

Option B — Shear key (if friction insufficient):

If friction were inadequate (e.g., for a base plate with significant shear and low compression), a shear key — typically a short UB or UC stub welded to the underside of the base plate and cast into the foundation — would be designed to EN 1993-1-8 Clause 6.2.2(3).

Option C — Anchor bolts in shear:

Not recommended for pinned bases in UK practice per SCI P398. Anchor bolts in shear should be avoided where possible — the oversized holes (typically +6 mm for M24 per BS EN 1090-2) prevent all bolts from bearing simultaneously, and combined shear+tension interaction per Table 3.4 significantly reduces capacity.

Step 5: Weld — Column to Base Plate

Per SCI P398, a 6 mm or 8 mm continuous fillet weld all around is standard for UK column base plates where the axial load is transferred primarily by bearing (machined column end or fitted bearing).

For a column end bearing on the base plate (common UK practice), the weld is nominal — it provides location, resists incidental moments during erection, and transfers the nominal shear into the base plate.

Use 6 mm CFW (continuous fillet weld), E42 electrodes to BS EN ISO 2560.

Weld capacity check at 6 mm leg:

Fw,Rd = fu / (sqrt(3) x beta_w x gamma_M2) = 410 / (1.732 x 0.85 x 1.25) = 223 MPa

Weld throat: a = 0.7 x 6 = 4.2 mm

Resistance per mm run: Fw,Rd_per_mm = 223 x 4.2 = 936 N/mm

Perimeter of UC 254 weld: approximately 2 x (254.6 + 14.2) + 2 x (254.1 - 2 x 14.2) = 990 mm (approximate)

Total weld resistance: 990 x 936 / 1000 = 926 kN (axial shear capacity).

The nominal 6 mm weld is adequate (NEd = 1,750 kN transferred by bearing, not weld).

Step 6: Grout Bed Requirements

Per BS EN 1090-2 (Execution of steel structures), the grout bed under the base plate must:

• Be at least 25 mm thick (30 mm nominal specified)
• Achieve compressive strength >= fck of foundation concrete within 7 days
• Be fully compacted without voids under the full plate area
• Extend at least 25 mm beyond the plate edges on all sides (or be trimmed flush and sealed)

For UK practice, cementitious grout to BS EN 1504-6 Class R4 is standard. Epoxy grout may be specified where early strength or chemical resistance is required.

Summary of Base Plate Design

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Frequently Asked Questions

When is a moment-resisting base plate required instead of a pinned base?

A moment-resisting (fixed) base is required when: (1) the frame relies on base fixity for sway stability (portal frames with fixed bases); (2) significant cantilever moments exist at the column base (crane columns, sign gantries); (3) the column is part of a moment frame and base rotation would invalidate the analysis assumptions. SCI P398 provides complete design procedures for moment-resisting bases including extended end plates with bolts outside the column flanges.

What is the minimum base plate thickness in UK practice?

SCI P398 recommends a minimum 15 mm for plates in S275 supporting UC sections up to 254 mm depth, and 20 mm for larger columns. Plates thinner than 15 mm may warp during fabrication and are prone to corrosion at the grout interface. BS EN 1090-2 mandates a minimum plate flatness of h/100 after welding. The UK NA does not specify a minimum thickness directly, but UK industry practice generally uses 20 mm as a practical minimum for primary columns.

How are base plates designed for combined axial load and moment?

For combined loading, the bearing pressure distribution under the base plate is non-uniform. The T-stub in compression model from EN 1993-1-8 Clause 6.2.6.11 is used with a reduced effective bearing area. The tension side bolts are designed per Table 3.4. SCI P398 provides a step-by-step procedure including the iterative calculation of the neutral axis depth. For UK practice, the moment-axial interaction should be checked at both ULS and SLS to prevent excessive grout crushing or bolt yielding.

What anchor bolt edge distance is required for UK base plates?

BS EN 1993-1-8 Table 3.3 specifies minimum edge distances for bolt holes: 1.2d0 from the centre of the hole to the plate edge in the direction of load transfer (where d0 is the hole diameter). For M24 bolts with 26 mm clearance holes, the minimum edge distance is 1.2 x 26 = 31.2 mm. UK practice uses a practical minimum of 50 mm to allow for fabrication tolerances and grout cover. SCI P398 recommends 2.0d = 48 mm to the bolt centre line as a practical minimum.

Related Pages

• UK Base Plate Design Guide — EN 1993-1-8 overview
• UK Bolt Capacity Tables — Class 8.8 and 10.9 bolt resistances
• UK Anchor Bolts — Holding-down bolt design
• UK Steel Grades — S235, S275, S355 UK grades
• UK Connection Design Guide — EN 1993-1-8 bolted and welded joints
• UK Weld Sizes — Fillet weld design guide
• EN 1993 Base Plate Design — European code reference

Educational reference only. All design values are per BS EN 1993-1-8:2005 + UK NA, BS EN 1992-1-1:2004, and SCI P398. 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.