ACI 318-19 Chapter 17 distinguishes between anchors installed in cracked and uncracked concrete. The same anchor installed at the same embedment can have a published capacity 30–50% higher in uncracked concrete than in cracked. The design assumption is binary, the implications are large, and plan reviewers consistently catch engineers misapplying it.

Why concrete is "cracked" by default

ACI 318-19 §17.5.2.1 requires anchors to be designed assuming the concrete is cracked unless analysis demonstrates that the concrete remains uncracked under service loads throughout the life of the anchor. In practice this is almost never satisfied at:

  • Tension faces of flexural members.
  • Locations subject to environmental thermal/shrinkage cracking.
  • Floors above grade in normal-weight concrete-on-metal-deck (the deck creates restraint cracking).
  • Anywhere a Design Earthquake is the controlling load case — by definition the structure is in the inelastic range.

For seismic anchorage of nonstructural components, assume cracked concrete. ICC-ES evaluation reports tabulate cracked capacities specifically for this reason — that is the ASCE 7-22 design case.

Where the cracked/uncracked switch shows up in the calculations

  • Concrete breakout in tension (Ncb) — the basic capacity Nb uses kc = 17 (uncracked) or kc = 24 (cast-in-place uncracked) vs kc = 24 for some post-installed; ACI requires multiplying by Ψc,N= 1.0 (cracked) or 1.25 (uncracked, cast-in-place) — but post-installed anchors often have product-specific values listed in the ESR.
  • Pullout (Npn) — multiplied by Ψc,P = 1.0 (cracked) or 1.4 (uncracked) per §17.6.3.
  • Concrete breakout in shear (Vcb) — Ψc,V = 1.0 (cracked + no edge reinforcement) or as high as 1.4 (uncracked).

Net effect: switching from cracked to uncracked can boost the controlling capacity by 25–40%. That is exactly why a careless "use the higher number" assumption gets calculations rejected.

Where uncracked is legitimately permissible

ACI 318-19 §17.5.2.1 lets you use uncracked capacities only when analysis demonstrates that flexural cracks do not occur at service load levels. Practical situations:

  • Mass concrete or thick footings where stresses are well below the modulus of rupture.
  • Anchors in compression-only regions with no significant tensile flexure.
  • Some manufacturer ESRs allow uncracked capacities only when explicitly verified by the engineer.

In a hospital, equipment is rarely in those locations. Floor slabs, composite decks, and walls almost always require the cracked assumption.

How cracked vs uncracked interacts with seismic Ω0

ASCE 7-22 §13.4.2 imposes the over-strength factor Ω0p on anchor design when the concrete-controlled limit state governs. The cracked assumption already lowered the breakout capacity; Ω0pon top of that is what often turns a "comfortable" hand check into a rejected calc. See our Ω0 article for the mechanics.

What the ESR tells you

Every ICC-ES Evaluation Report for a post-installed anchor includes separate capacity tables for cracked and uncracked concrete. Pick the cracked column in seismic design unless you can defend the uncracked assumption per ACI 318-19 §17.5.2.1. Some ESRs explicitly state that seismic design must use cracked values regardless of the supporting member's analysis state — read the ESR.

Checklist for plan-check readiness

  1. State the assumption ("Cracked concrete per ACI 318-19 §17.5.2.1") on the cover page.
  2. Reference the ESR table for the cracked column you are using.
  3. Confirm Ψc,N, Ψc,P, and Ψc,V are applied at the correct (cracked) values.
  4. Apply Ω0p to the concrete-controlled limit states.
  5. Show the §17.8 interaction with cracked capacities.

Rules of thumb

  • Above-grade slab → cracked. Always.
  • Mass concrete pad on grade with no superimposed flexure → potentially uncracked, with documentation.
  • Wall-mounted equipment → cracked.
  • Cast-in-place headed bolt in a thick foundation pour → potentially uncracked, but check ESR/ACI commentary.
  • Adhesive anchor in seismic application → cracked, with seismic-qualified epoxy per ACI 318-19 §17.10.5.

How PANACHE ENGINEERING handles this

Our calculator defaults to cracked concrete and explicitly flags any capacity input that assumes uncracked. See the methodology behind it in Equipment Anchorage Design or talk to an engineer for project-specific review.