ACI 318-19: Plain concrete - Nominal bearing strength (14.5.6.1)

About this ACI 318-19: Plain Concrete – Nominal Bearing Strength (14.5.6.1) Calculator

This calculator checks the nominal bearing strength of plain concrete per ACI 318-19 Section 14.5.6.1 and Table 14.5.6.1. It determines whether the supporting surface is wider on all sides than the loaded area, selects the governing equation accordingly, applies the strength reduction factor, and compares the design bearing strength against a factored bearing demand.

• Structural engineer — verify plain concrete bearing capacity at column bases, pedestals, or bearing pads quickly without manually tracking which case from Table 14.5.6.1 governs.
• Civil/structural designer — run rapid checks during preliminary design to confirm concrete compressive strength and loaded area selections are adequate before full documentation.
• Plan checker or reviewer — audit the bearing check with full equation traceability, including the area ratio factor and the lesser-of comparison for the wider-support case.

This is an engineering-grade calculator built on CalcTree, with every intermediate value exposed for review, auditing, and integration into a broader project calculation package.

More Info on ACI 318-19: Plain Concrete – Nominal Bearing Strength (14.5.6.1)

Inputs

The calculator requires five inputs. The concrete compressive strength and loaded area are the core material and geometric parameters driving all bearing strength expressions. The factored bearing demand is the applied load against which the design bearing strength is checked. The supporting surface area is only active when the supporting surface is wider on all sides than the loaded area — a Yes/No selection controls which case is evaluated. Finally, the strength reduction factor is user-defined, giving flexibility to apply the value appropriate to the design context, with ACI 318-19 specifying a value for plain concrete bearing.

Nominal Bearing Strength Equations

The calculator implements Table 14.5.6.1 directly. When the supporting surface is wider on all sides than the loaded area, two expressions are evaluated: one scales the base bearing term by the square root of the ratio of supporting surface area to loaded area, and the other caps that amplification at twice the base term. The lesser of these two governs for that case. When the supporting surface is not wider on all sides, the nominal bearing strength equals the base term without amplification. The base term in all cases is the product of 0.85, the concrete compressive strength, and the loaded area — the standard plain concrete bearing expression from ACI 318-19.

Design Check

The design check compares the factored bearing demand against the design bearing strength, which is the product of the strength reduction factor and the nominal bearing strength. The utilization ratio is computed as the factored demand divided by the design bearing strength. A traffic light indicator displays Pass when utilization is at or below unity and Fail when it exceeds unity, giving an immediate read on whether the bearing condition is satisfied.

Outputs

The summary table presents the nominal bearing strength, the design bearing strength after applying the strength reduction factor, the utilization ratio, and the pass/fail result. All intermediate values — the area ratio, the amplification factor, and the governing case selection — are computed within the calculation block, so the logic path is fully traceable from inputs to the final check.

Common Calculation Errors to Avoid

• Applying the wider-support amplification when it does not apply — the square root area ratio factor only applies when the supporting surface is wider on all sides; if any side of the loaded area extends to or beyond the supporting surface edge, case (c) governs and no amplification is permitted.
• Confusing A1 and A2 geometry — A1 is strictly the loaded area at the bearing interface, and A2 is the full supporting surface area measured on a plane geometrically similar to A1; mixing these or using projected areas incorrectly distorts the area ratio.
• Ignoring the cap on the amplification factor — even when the wider-support condition is met, the nominal strength cannot exceed twice the base term; omitting the lesser-of comparison overstates capacity.
• Using the wrong strength reduction factor — ACI 318-19 specifies a phi value for plain concrete that differs from reinforced concrete bearing; substituting a value intended for reinforced concrete will produce an unconservative result.
• Entering f'c in inconsistent units — the base bearing term is sensitive to the units of concrete compressive strength; confirm f'c is entered in psi (or converted correctly) before reading bearing strength outputs in force units.
• Neglecting to update A2 when geometry changes — if the footing plan dimensions or support configuration are revised, the supporting surface area must be updated; leaving a stale A2 value in place silently changes the area ratio and the governing nominal strength.

FAQs

What does ACI 318-19 Section 14.5.6.1 cover for plain concrete bearing?

Section 14.5.6.1 gives the nominal bearing strength equations for plain (unreinforced) concrete members. The code distinguishes two cases: where the supporting surface is wider on all sides than the loaded area, allowing a confinement-based strength increase, and all other cases where no increase is permitted. The base expression in both cases is 0.85 f'c A1, which reflects the empirical upper limit on local compressive stress a plain concrete section can sustain.

Why does bearing strength increase when the supporting surface is wider than the loaded area?

When concrete surrounds or extends beyond the loaded footprint, confinement from the surrounding material resists lateral expansion under load, allowing higher local stress before crushing. ACI captures this with the sqrt(A2/A1) multiplier on the base term. The factor is capped at 2.0 by requiring the lesser of case (a) and case (b), so the nominal strength can never exceed twice the unconfined base value regardless of how large A2 is relative to A1.

What strength reduction factor phi should I use for plain concrete bearing?

ACI 318-19 assigns phi = 0.60 for plain concrete in most cases, but some practitioners use 0.65 depending on project-specific interpretations or governing load combinations. This calculation leaves phi as a user-defined input so you can match your project requirements or local code interpretation directly. Check ACI 318-19 Table 21.2.1 and your engineer-of-record's direction before finalizing the value.

How do I determine A1 and A2 correctly for this calculation?

A1 is the contact area between the applied load and the concrete surface, for example the base plate area or column footprint. A2 is the area of the supporting surface measured on a plane at the same level as A1, projected from the edges of the loaded area at a 2:1 slope if the support is a footing or pedestal. If the supporting surface is not wider on all sides, select "No" in the dropdown and A2 is ignored entirely.

What is the utilization ratio and how should I interpret it?

Utilization is Pu divided by phi Bn. A value at or below 1.0 means the factored demand does not exceed design bearing strength and the check passes. A value above 1.0 is a fail, meaning you need to increase the loaded area, increase concrete strength, or reduce the applied load. Keeping utilization noticeably below 1.0 also provides margin for load uncertainty or future load increases.

Can this calculation be used for reinforced concrete bearing checks?

No. This template applies specifically to plain concrete members per ACI 318-19 Chapter 14. For reinforced concrete columns, pedestals, or footings, bearing strength is checked under ACI 318-19 Section 22.8, which uses similar base expressions but applies reinforced concrete phi factors and interacts with development length and dowel requirements. Use the appropriate reinforced concrete template for those cases.