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The total factored axial (compressive) load the column transfers to the baseplate.
The FACTORED load, matching the concrete design method the allowable bearing stress comes from — mixing a service load with a factored resistance is the commonest way this check passes when it should not. Where the column also carries moment, the bearing pressure is not uniform and the peak under one edge of the plate is higher than this arrangement reports.
The baseplate's footprint area in contact with the concrete.
The plate's plan area in contact, which is the plate itself and not the grout pad or the pier below — the concrete's confinement from a larger pier is a separate allowance and is not what this field is. Deduct nothing for bolt holes; they are small and the standard treatment ignores them. A plate that overhangs its pier bears on air at the overhang, and the area that counts is the overlap.
The specified compressive strength of the supporting concrete.
Commonly 25-30 MPa (3,600-4,350 psi) for a footing or pier.
The ratio of the supporting concrete area (A2) to the loaded baseplate area (A1).
ACI 318 caps the effective √(A2/A1) term at 2, so a ratio above 4 does not raise the allowable bearing stress any further. The calculator applies that cap itself — the bearing stress is the lesser of 0.85·f'c·√(A2/A1) and 1.7·f'c, and the second term governs everywhere above a ratio of 4 — so entering your real ratio is correct and safe. THIS HELP USED TO SAY THE OPPOSITE: that the calculator used the ratio directly and you had to cap it at 4 yourself. It never did, and following that instruction changed nothing, because at f'c = 25 MPa a ratio of 9 and a ratio of 4 both return 42.5 MPa nominal. The text described behaviour the code did not have, and implied an inflated figure the code never produced.
Actual bearing pressure
453 psi
The bearing pressure under this plate is below the design bearing strength, with phi = 0.65 applied to the ACI nominal shown with it — ACI 318 gives it. The load compared here has to be a factored one, not a service load. Being under one limit is not a design. Nothing else is checked here — not the other limit states, not the connections, not the member the load arrives from.
- Nominal bearing strength Bn/A1
- 6,164.1 psi
- Design bearing strength phi*Bn/A1 (phi = 0.65)
- 4,006.67 psi
They open the calculator with your figures already in it
Steel Column Baseplate Concrete Bearing Pressure Calculator: 453 psi — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ACI 318 §10.17 (bearing on concrete): allowable bearing stress = min(0.85×f'c×√(A2/A1), 1.7×f'c), where A2/A1 is the ratio of the supporting concrete area to the loaded (baseplate) area
Inputs used
- Axial Load
- 112.4 kip
- Baseplate Area
- 1.72 sq ft
- Concrete Strength f'c
- 3625.94 psi
- Concrete-to-Plate Area Ratio (A2/A1)
- 4
Intermediate steps
- Nominal bearing strength Bn/A1
- 6,164.1 psi
- Design bearing strength phi*Bn/A1 (phi = 0.65)
- 4,006.67 psi
Confidence note: The bearing pressure under this plate is below the design bearing strength, with phi = 0.65 applied to the ACI nominal shown with it — ACI 318 gives it. The load compared here has to be a factored one, not a service load. Being under one limit is not a design. Nothing else is checked here — not the other limit states, not the connections, not the member the load arrives from.
What this calculation does not cover
- The pressure reported is the axial load spread evenly over the whole plate, which is only true for a concentric load with no base moment. Once a column moment gives the resultant an eccentricity greater than one sixth of the plate length, the plate bears on part of its area only and the peak pressure at the compression edge rises well above the uniform figure shown here.
- The A2/A1 ratio is taken as typed and is never checked against real geometry. ACI permits the square-root term only where A2 is geometrically similar to and concentric with the loaded area, lies wholly within the support, and can be reached on a load spread no flatter than two horizontal to one vertical, so a shallow pier or a column set near a pier edge has a smaller usable A2 than its plan dimensions suggest.
- The comparison is made in LRFD terms: a factored load against a design strength with phi = 0.65. An allowable stress design check instead compares an unfactored service load against the nominal strength divided by AISC 360 J8's omega of 2.31, which this page does not calculate, so entering a service load against the phi-reduced strength shown here overstates the margin by roughly the load factor.
- The stress is checked against the concrete only, but where the plate is set on grout or shims it is the material at the plate face that governs first. Grout weaker than f'c, an incompletely filled pocket, or setting shims carrying load before the grout cures all reduce the contact area that the uniform pressure assumes.
- Bearing is a local crushing check on the concrete directly beneath the plate. It says nothing about the soil pressure under the footing, punching or one-way shear through the pier or footing, or the bursting reinforcement needed where the concentrated load spreads out, and a pier can satisfy this check while failing any of those.
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This result is a specification — 453 psi — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- ACI 318 §10.17 (bearing on concrete): allowable bearing stress = min(0.85×f'c×√(A2/A1), 1.7×f'c), where A2/A1 is the ratio of the supporting concrete area to the loaded (baseplate) area
Which documents these citations point at
Standards referenced: ACI 318 (American Concrete Institute, United States).
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