How is the bearing pressure under a base plate calculated?

The allowable bearing pressure on concrete is 0.6 × fcu, where fcu is the characteristic cube strength of the concrete (e.g., C30 gives 0.6 × 30 = 18 N/mm²). The required plate area = axial load ÷ allowable bearing pressure. The plate must be large enough to keep the actual bearing stress below this limit.

How thick should a steel base plate be?

Base plate thickness depends on the cantilever projection beyond the column flange. The formula is t = c × √(3w/py), where c is the maximum overhang, w is the bearing pressure, and py is the steel design strength (275 N/mm² for S275). Typical base plates are 15-40mm thick. The minimum practical thickness is 12mm for bolt hole drilling.

What size holding-down bolts do I need?

For nominally pinned bases (most standard columns), 4 × M20 Grade 8.8 bolts are standard. They primarily resist shear and provide location during erection. For moment-resisting bases, larger bolts (M24 or M30) in groups of 4-8 may be needed. The bolt embedment length in concrete should be at least 12 × bolt diameter with a hook or plate washer.

Do I need grout under a base plate?

Yes, grout is always required under a base plate to ensure full contact bearing. The standard grout thickness is 25-50mm. Use non-shrink cementitious grout (e.g., Fosroc Conbextra GP). The plate is levelled on shims or levelling nuts, then grouted. The grout must be at least as strong as the foundation concrete.

What concrete grade should I specify for base plate foundations?

C30/37 (30 N/mm² cube strength) is the standard minimum for column pad foundations in the UK. For heavily loaded columns, C35/45 or C40/50 may be needed. The concrete grade directly affects the allowable bearing pressure and therefore the required base plate size. Higher-grade concrete means smaller plates.

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Base Plate Calculator — Steel Column Base Plate Design

Design steel column base plates per BS EN 1993-1-8. Calculates plate area from bearing pressure, thickness from bending, and bolt layout for holding-down bolts.

Axial Compressive Load

Total factored load on column

Concrete Grade

Foundation concrete cube strength

Column Serial Size

Column section serial size

Steel Grade

Base plate steel grade

How We Calculate This

This calculator designs a steel column base plate per BS EN 1993-1-8, determining the plate dimensions from the bearing capacity of the concrete and the plate thickness from the bending of the plate overhang.

Design steps

Bearing pressure: Allowable = 0.6 × fcu (N/mm²)
Plate area: A = Load (N) ÷ Bearing pressure (N/mm²)
Plate size: Must extend at least 50mm beyond column in each direction
Plate thickness: t = c × √(3w/py) from cantilever bending

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.