Steel base plate
The plate has two jobs and each sets a dimension. The plan area has to spread the column load onto the concrete without exceeding the bearing strength of §J8 — which is allowed to be higher than f’c, because the concrete around the plate confines it. The thickness has to carry the cantilever bending between the column face and the free edge.
| Step | Value | Note |
|---|---|---|
| Factored load | 300 kip | as entered |
| A2/A1 available | 4.0 | capped at 4 by §J8 |
| Confinement √(A2/A1) | 2.00 | capped at 2 |
| Bearing capacity | 4420 psi | 0.65 × 0.85 f’c × √(A2/A1) |
| Required area | 67.9 in² | load ÷ capacity |
| Plate B × N | 12.0 × 15.0 in | rounded to half inches |
| Bearing pressure | 1667 psi | 38% utilised |
| Projection m | 0.85 in | (N − 0.95d)/2 |
| Projection n | 2.80 in | (B − 0.8bf)/2 |
| Governing projection | 2.80 in | n governs |
| Thickness required | 0.76 in | l √(2Pu / 0.9 Fy B N) |
| Thickness used | 1.00 in | next quarter inch |
The bearing pressure is 1667 psi against a capacity of 4420 psi, and the n projection governs the thickness.
φc 0.85 f’c A1 √(A2/A1) · φc = 0.65, root capped at 2.
Pu / capacity per area · Plan size.
m = (N − 0.95d)/2 · n = (B − 0.8bf)/2.
tp = l √(2Pu / 0.9 Fy B N) · l is the larger of m and n.
A clear path from inputs to results.
The plate has two jobs and each sets a dimension. The plan area has to spread the column load onto the concrete without exceeding the bearing strength of §J8 — which is allowed to be higher than f’c, because the concrete around the plate confines it. The thickness has to carry the cantilever bending between the column face and the free edge.
How to use
- Enter the factored axial load and the column dimensions.
- Set the concrete and plate strengths.
- Choose the A2/A1 ratio the pier can provide.
- Read the plate size, the bearing pressure and the required thickness.
The formula
Reference: AISC 360 §J8 for bearing; AISC Design Guide 1 for the plate thickness.
Worked example
300 kip on a W14×8 column
- 300 kip factored load, d = 14 in, bf = 8 in.
- 4,000 psi concrete, A572 Gr 50 plate.
- A2/A1 = 4, the maximum §J8 allows.
- 1
The confinement factor is √4 = 2, so the bearing capacity is 0.65 × 0.85 × 4 × 2 = 4.42 ksi.
- 2
The required area is 300 / 4.42 = 67.9 in².
- 3
A 12 × 15 in plate provides 180 in², comfortably above the requirement.
- 4
The cantilever projections are m = 0.85 in and n = 2.80 in, so n governs.
- 5
The required thickness is 2.80 × √(2 × 300 / (0.9 × 50 × 12 × 15)) = 0.76 in, rounded to 1 in.
12 × 15 in · 0.76 in required · 1 in plate.
Interpreting the result
The confinement credit is the part most often missed. A plate set back from the pier edges can bear at twice the pressure of one that covers the whole pier, which halves the plate area — and the plate is usually the expensive item, not the concrete.
Assumptions & limitations
- A concentrically loaded column on a grouted plate.
- A2/A1 capped at 4 by §J8 and the root capped at 2.
- Plate thickness from the DG1 cantilever model with l = max(m, n).
- Anchor bolt tension and moment are separate checks.
Frequently asked questions
What is A2/A1?
The ratio of the concrete area that can confine the plate to the plate area itself. It is capped at 4 because beyond that the surrounding concrete stops helping.
How thick should a base plate be?
Usually 0.75 to 2 inches for building columns. Thicker plates are a sign the load or the concrete is unusual.
Does grout count?
The grout is assumed to spread the load but not to add bearing capacity; the plate bears on the concrete through it.
Why does n govern more often than m?
Because n uses 0.8 times the flange width while m uses 0.95 times the depth. A wide-flange column is deeper than it is wide, so the transverse cantilever is usually longer.
Do I need stiffeners?
Only when the plate becomes impractically thick, typically above 2.5 inches. Stiffeners change the yield-line pattern and are a different calculation.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.