European Base Plate Design — EN 1993-1-8 Guide

Complete reference for column base plate design per EN 1993-1-8:2005 Clause 6.2, covering concrete bearing strength [fjd] per EN 1992-1-1, equivalent T-stub in bending (Clause 6.2.4), anchor bolt tension and shear (Clause 6.2.6), and gusset/stiffener detailing. Includes a fully worked example for an exposed column base plate with S355 column and C30/37 concrete foundation.

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Code Reference: EN 1993-1-8:2005 Clause 6.2

EN 1993-1-8 Clause 6.2 covers pinned and moment-resisting column base plates. The key design checks are:

1. Concrete bearing (Clause 6.2.2) — bearing stress under the plate must not exceed fjd
2. Base plate bending (Clause 6.2.3) — equivalent T-stub in bending for the compression zone
3. T-stub in bending for tension (Clause 6.2.4) — anchor bolt tension with three failure modes
4. Anchor bolt resistance (Clause 6.2.6) — combined tension and shear per EN 1993-1-8 Table 3.4
5. Welded connections (Clause 4) — column-to-base-plate full-strength welds

Concrete Bearing Strength

The design bearing strength of the grout/concrete joint per EN 1993-1-8 Clause 6.2.2(2):

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

Where:

The concrete bearing strength factor (f*{jd} \leq f*{cd}) where (f*{cd} = f*{ck} / \gamma_c) with (\gamma_c = 1.50) per EN 1992-1-1.

Concrete Bearing Capacity — Typical Values

T-Stub Model — Three Failure Modes

EN 1993-1-8 Clause 6.2.4 uses the equivalent T-stub flange in bending for the tension zone. Three failure modes govern the tension resistance (F_{T,Rd}):

Where (M*{pl,Rd}) is the plastic moment of the T-stub flange, (m) is the distance from bolt centreline to weld toe, (n) is the distance from bolt centreline to flange edge (limited to (1.25 \times m)), and (\Sigma F*{t,Rd}) is the sum of anchor bolt tension resistances.

Mode 1 is ductile (flange yielding before bolt failure) and is the preferred failure mode. Mode 3 is brittle (bolt fracture before significant plate deformation) and should be avoided by ensuring the flange is thick enough.

Anchor Bolt Tension and Shear

Per EN 1993-1-8 Clause 6.2.6, anchor bolts are checked for combined tension and shear:

[ \frac{F*{t,Ed}}{F*{t,Rd}} + \frac{F*{v,Ed}}{F*{v,Rd}} \leq 1.0 ]

Anchor bolt tension resistance: (F*{t,Rd} = k_2 \times f*{ub} \times As / \gamma{M2}) where (k_2 = 0.9) for standard nuts.

Worked Example — HEB 200 Column Base Plate

Problem: Design a pinned base plate for HEB 200 column (S355) under axial compression NEd = 850 kN. Concrete foundation C30/37. Base plate S355JR, 400 mm × 400 mm.

Step 1 — Concrete bearing check: Bearing area (A*{c0} = 400 \times 400 = 160,000) mm² Concrete design strength: (f*{cd} = 30 / 1.50 = 20.0) MPa Bearing resistance: (F_{Rdu} = 160,000 \times 20.0 / 1,000 = 3,200) kN

With (\betaj = 2/3) and assuming uniform bearing: (f{jd} = 2/3 \times 20.0 = 13.3) MPa Bearing pressure: (\sigma = 850,000 / 160,000 = 5.3) MPa (< 13.3) MPa — OK

Step 2 — Base plate thickness (compression zone): Cantilever projection beyond column flange: (c = (400 - 200)/2 = 100) mm Required thickness: (tp = c \times \sqrt{3 \times f{jd} / f_y} = 100 \times \sqrt{3 \times 13.3 / 355} = 100 \times 0.335 = 33.5) mm

Use 35 mm base plate (S355JR). With 2 mm corrosion allowance and fabrication tolerance, specify 40 mm nominal.

Step 4 — Anchor bolts (nominally pinned): 4 × M20 class 4.6 anchor bolts at 200 mm gauge. Tension from nominal uplift: (F*{t,Ed} = 15) kN per bolt. (F*{t,Rd} = 70.6) kN per bolt (> 15) kN — OK. Shear from nominal horizontal force: (F_{v,Ed} = 10) kN per bolt. Combined: (15/70.6 + 10/70.6 = 0.354 < 1.0) — OK.

Design Resources

• EN 1993 Steel Grades
• European Steel Properties
• EN 1993 Bolt Capacity
• IPE/HEA/HEB Beam Sizes
• EN 1993 Column Buckling
• All European References

Frequently Asked Questions

How is base plate bearing checked per EN 1993? Base plate bearing per EN 1993-1-8 Clause 6.2.2 uses concrete bearing strength (f*{jd} = \beta_j \times F*{Rdu} / (b*{eff} \times l*{eff})). The joint coefficient (\betaj = 2/3) applies when the grout quality is at least 20% of the concrete compressive strength. (F{Rdu}) is the concentrated bearing resistance per EN 1992-1-1 Clause 6.7, which accounts for the load spreading angle through the foundation. For a base plate covering the full foundation width, (f*{jd} = f*{cd}) (i.e., no confinement benefit).

What T-stub model is used for European base plates? EN 1993-1-8 Clause 6.2.4 uses the equivalent T-stub in bending for the tension zone of the base plate. Three failure modes are considered: Mode 1 (complete flange yielding — the preferred ductile mode), Mode 2 (bolt failure with flange yielding), and Mode 3 (pure bolt fracture — brittle, must be avoided). The T-stub effective length (l_{eff}) depends on the bolt layout and whether prying forces are developed. For anchor bolts in base plates, prying forces are generally smaller than in beam-to-end-plate connections because the base plate can lift off the grout.

What is the minimum base plate thickness per EN 1993-1-8? EN 1993-1-8 does not specify an absolute minimum, but practical European practice recommends a minimum nominal thickness of 15 mm for pinned base plates. Moment-resisting base plates typically require 25-50 mm depending on the column size and applied moment. The thickness is governed by the T-stub bending check (Clause 6.2.4 for tension) or the cantilever projection formula (Clause 6.2.2 for compression). For seismic moment-resisting frames, base plates are often 50-80 mm thick and stiffened with gussets or continuity plates.

How are anchor bolt tension and shear combined in base plate design? EN 1993-1-8 Clause 6.2.6 requires the linear interaction (F*{t,Ed}/F*{t,Rd} + F*{v,Ed}/F*{v,Rd} \leq 1.0). For pretensioned anchor bolts in moment-resisting base plates, shear can also be transferred through base plate / grout friction. If the friction coefficient is 0.30 (typical for steel-on-grout), a compression force of 200 kN provides 60 kN friction resistance. This friction can be subtracted from the shear demand on the anchor bolts, provided the compression is reliably present at the ULS.

What National Annex variations affect base plate design? The UK National Annex to EN 1993-1-8 specifies (\gamma_{M2} = 1.25) (same as recommended). The German NA (DIN EN 1993-1-8/NA) modifies the concrete joint coefficient and requires additional verification of the grout layer thickness. The French NA specifies modified T-stub effective lengths for base plates. Always verify the local NA for the specific jurisdiction. In the Netherlands, NEN-EN 1993-1-8 includes additional requirements for anchor bolt anchorages in the foundation.

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