# Base Plate Design Calculator

Thicker is not the answer — check the bearing area first, then the cantilever.

- Page: https://jianzhujisuan.com/en/base-plate
- Category: Steel Design (https://jianzhujisuan.com/en/c/steel-design)
- Chinese version: https://jianzhujisuan.com/zh/base-plate.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Column axial load P 1500 kN, Concrete f'c 30 MPa, Steel yield Fy 250 MPa, Column depth d 300 mm, Column flange width bf 300 mm, Plate width B 350 mm, Plate length N 350 mm): **Required plate thickness 24.7 mm**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Column axial load P | number | 1500 | kN |  |
| Concrete f'c | number | 30 | MPa |  |
| Steel yield Fy | number | 250 | MPa |  |
| Column depth d | number | 300 | mm |  |
| Column flange width bf | number | 300 | mm |  |
| Plate width B | number | 350 | mm |  |
| Plate length N | number | 350 | mm |  |

## Formula

- **Bearing strength**: fp,max = φc·0.85·f'c, φc = 0.65
- **Required area**: Areq = P/fp,max
- **Bearing pressure**: fp = P/(B·N)
- **Cantilever**: m = (N − 0.95d)/2, n = (B − 0.8bf)/2, n′ = √(d·bf)/4
- **Governing**: l = max(m, n, n′)
- **Thickness**: t = l·√(2fp/(0.9Fy))

## Worked example

1500 kN on C30 with Fy 250 MPa, a 300×300 column and a 350×350 plate: bearing strength 16.58 MPa, 90.5 cm² required against 122.5 cm² provided, pressure 12.24 MPa, cantilevers 32.5, 55.0 and n′ 75.0 mm, l 75.0 mm, 24.7 mm required and 26 mm suggested.

## Questions

### How thick should a base plate be?

Treat it as a cantilever. The projection beyond the column carries the uniform bearing pressure, so a unit-width strip sees M = fp l²/2 and needs t = sqrt(4M/(0.9 Fy)), which reduces to t = l sqrt(2 fp / (0.9 Fy)). Take l as the largest of m = (N − 0.95d)/2, n = (B − 0.8bf)/2 and n′ = sqrt(d·bf)/4. At 1500 kN on C30 with a 350×350 plate and a 300×300 column: 12.24 MPa, m 32.5, n 55 and n′ 75 which governs, so 24.7 mm required and 26 mm specified.

### What if the plate area is too small?

Add area, not thickness. Pressure is P/(B N), so doubling the area halves it and cuts thickness to 71%, whereas doubling thickness costs money squared. In practice you enlarge the plate to the required area first and then check that bolts and stiffeners still fit. Where the layout forbids a larger plate, raise the concrete strength or use local reinforcement such as a grout pad or spreader.

### Why is grout needed under a base plate?

Because plate and concrete never meet perfectly. With no grout, contact is limited to a few high points and the local pressure multiplies many times over; the concrete crushes, the plate settles and the column goes out of plumb. Non-shrink grout spreads the load evenly, typically 25 to 50 mm thick. Watch the strength — grout is usually weaker than the foundation concrete, so where the bed is thicker than 50 mm the bearing check must use the grout strength, not the base strength.

### How do base plate and anchor bolts work together?

The bolts take uplift and shear, the plate takes bearing — different jobs. Edge distance must be at least 1.5 bolt diameters and no less than 40 mm, or the plate tears out. Under moment one side of the column goes into tension and the bolt force has to travel through the plate cantilever, so thickness is often governed by bolt tension rather than bearing; check both cases.

## Related tools

- [Steel Beam Span Calculator](https://jianzhujisuan.com/en/steel-beam-span): The depth is not looked up in a table — it falls out of the deflection limit.
- [Section Properties Calculator](https://jianzhujisuan.com/en/section-modulus): Inertia governs deflection, section modulus governs strength — they are not interchangeable.
- [Fillet Weld Strength Calculator](https://jianzhujisuan.com/en/fillet-weld): A 6 mm leg is not a 6 mm throat — capacity follows the 4.2 mm throat.
- [Column Buckling Calculator](https://jianzhujisuan.com/en/column-buckling): Half the effective length means four times the Euler load — but the design resistance differs by less, and how much depends on slenderness.

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