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Minimum eccentricity (M2,min) in ACI 318-19: does it apply to short columns?

ACI 318-19 defines a floor on design moment, M2,min, to keep nearly-concentric columns from being designed for zero moment. The interpretation question every short-column checker eventually hits: the provision lives inside the slenderness chapter — so is it required when your column is short? This page walks the code text, the answer, and how calcnote handles it.

The provision

$$M_{2,min} = P_u\,(0.6 + 0.03\,h)$$

with h in inches and the result in kip-in when Pu is in kips — the units trip people up, because (0.6 + 0.03·h) is really a minimum eccentricity in inches: emin = 0.6 + 0.03·h, representing construction-tolerance offset that no real column avoids.

Where the code puts it — and why that matters

§6.6.4.5.4 sits inside §6.6.4 (titled "Slenderness Effects"), within the moment-magnification procedure. Its role there is specific: when magnifying moments for a slender column, the magnifier must not be applied to an unrealistically small M2, so the code substitutes M2,min as the magnification base. The commentary (R6.6.4.5.4) ties the provision to the case "when moment magnification is required."

For a short column — one below the §6.2.5 slenderness limit, where moment magnification is not performed — §6.6.4.5.4 is therefore not a code-required check. Our review of published documentation found no short-column application of §6.6.4.5.4. The provision's rationale (accidental eccentricity) still has engineering merit for any column; its mandate is scoped to the magnification procedure.

What calcnote does

calcnote computes M2,min on every run and compares it to the user-entered M2. When M2 falls below the bound, calcnote emits a non-blocking advisory and proceeds using the user-entered M2 — the check is surfaced as information, not enforced as a code requirement, and the engineer of record retains the substitution decision. (Until 2026-08-05 calcnote enforced this as a hard error citing §6.6.4.5.4 as code-required; the correction is documented in the methodology errata.)

Worked example: the units trap

Take the canonical 16×16 in demo section under Pu = 200 kip:

$$e_{min} = 0.6 + 0.03(16) = 1.08\ \text{in}$$
$$M_{2,min} = 200\,(1.08) = 216\ \text{kip-in} = 18.0\ \text{kip-ft}$$

The common mistake is reading 216 as kip-ft — the formula's output is kip-in because h enters in inches. At M2 = 60 kip-ft the live demo section clears this bound comfortably, so no advisory appears there.

What the advisory looks like

Load the same section heavily and nearly concentrically — Pu = 300 kip with M2 = 0.5 kip-ft:

$$M_{2,min} = 300\,(0.6 + 0.48) = 324\ \text{kip-in} = 27.0\ \text{kip-ft} \gg M_2 = 0.5\ \text{kip-ft}$$

calcnote proceeds with the user-entered M2 = 0.5 kip-ft and shows the advisory naming both values and the §6.6.4.5.4 scope note, with a link to the methodology rationale. Caveat: results in the M2 < M2,min regime are not part of calcnote's published cross-validation set — the advisory exists precisely so the engineer confirms that construction-tolerance eccentricity and unmodeled lateral effects are captured elsewhere in the load path.

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