Anchoring to Concrete: ACI 318 Chapter 17 Design Provisions
Anchors are everywhere in a finished building beyond the obvious column base plates: equipment pads, pipe supports, curtain wall brackets, ledger connections for wood-framed additions to concrete walls, and countless retrofit attachments bolted into existing concrete years after the original pour. ACI 318 Chapter 17 provides the unified design method for all of these, covering both cast-in anchors placed before the concrete pour and post-installed anchors, mechanical or adhesive, drilled into hardened concrete, and the method's central concern is making sure the anchor's steel yields or the adhesive bond engages predictably before the surrounding concrete fractures suddenly and without warning.
The Failure Modes Chapter 17 Checks
An anchor loaded in tension can fail four different ways, and Chapter 17 requires checking all of them and using whichever governs. Steel failure is the anchor rod or bolt itself yielding and rupturing, a ductile mode that gives visible warning before complete failure. Concrete breakout is a cone-shaped fracture surface pulling free around the anchor, radiating outward from the embedded end toward the surface at roughly a 35-degree angle, a brittle failure mode with little to no warning. Pullout is the anchor head or expansion mechanism sliding through the concrete without a full breakout cone forming, relevant mainly to mechanical expansion anchors. Side-face blowout applies to anchors embedded close to a free edge, where the concrete cover in front of the anchor head splits off toward the nearest edge instead of a full cone forming.
Concrete breakout capacity is the check that most often governs anchor design in practice, and it depends heavily on embedment depth, since breakout capacity scales with the embedment depth raised to a power between 1.5 and 5/3 depending on whether a single anchor or a group is being checked. Edge distance and anchor spacing both reduce breakout capacity below the single, isolated-anchor value: an anchor placed near a free edge or close to a neighboring anchor cannot develop the full breakout cone the base equation assumes, and Chapter 17 applies specific reduction factors for both conditions.
Chapter 17's ductility requirement is the section's most consequential design philosophy: where practical, anchors are sized so that steel failure governs rather than concrete breakout, because a ductile steel failure yields visibly and redistributes load before final rupture, while a concrete breakout failure can occur suddenly with comparatively little warning. Where a brittle failure mode cannot be avoided, the code compensates with a larger strength reduction factor, effectively demanding more reserve capacity from that anchor.
Cast-In vs. Post-Installed Anchors
Cast-in anchors, headed bolts or J/L-hooks placed in the formwork before the pour, develop breakout capacity based on straightforward geometric assumptions since their embedment and edge distance are fixed and known at design time. Post-installed anchors are drilled into hardened concrete after the fact and split into two families: mechanical anchors, which develop capacity through friction or bearing from an expanding sleeve or wedge, and adhesive anchors, which bond a threaded rod into a drilled hole using an epoxy, vinylester, or polyester-based adhesive.
Every post-installed anchor product has to be qualified through standardized testing, cast-in anchors under ACI 355.2 for mechanical anchors or ACI 355.4 for adhesive anchors, before a manufacturer can publish design values usable in a Chapter 17 calculation. This testing establishes not just baseline capacity but sensitivity to installation variables: hole cleaning procedure, concrete temperature at installation, moisture in the hole, and installer training all measurably affect adhesive anchor capacity in ways that don't apply to a cast-in bolt, which is why adhesive anchor installation in structural, life-safety applications typically requires special inspection and installer certification that a cast-in anchor does not.
Seismic Provisions for Anchors
In structures assigned to higher seismic design categories, Chapter 17 imposes additional requirements on anchors resisting seismic load, reflecting that cyclic, reversing load is harder on a concrete anchor connection than a single monotonic pull to failure. Ductile steel elements are required more strictly, and post-installed anchors used in a seismic tension application generally have to be qualified under supplemental testing for cracked concrete and simulated seismic loading, since ordinary product qualification testing under static load in uncracked concrete does not represent the cyclic, cracked-concrete condition an anchor may actually experience during an earthquake.
This is a direct extension of the same seismic force amplification logic that governs anchor rod tension on moment-resisting base plates, discussed in more depth in base plate and anchor bolt design, and it feeds into the same overall building load path reliability discussed in seismic design principles.
The complete anchor design methodology, including the breakout, pullout, and side-face blowout equations summarized here, is published in Chapter 17 of ACI 318, maintained by the American Concrete Institute, with the post-installed anchor qualification testing standards published as ACI 355.2 and ACI 355.4 through the same organization.