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Concrete Anchor Breakout Capacity (ACI 318)

Calculate pure mechanical ACI 318 tensile concrete cone breakout strength limits for cast-in-place and post-installed wedge anchors.

Concrete Slab Kinetics

Anchor Specifications

⚠️ ACI SAFETY WARNING: This calculates the absolute ultimate point of catastrophic concrete fracture. A structural engineer must rigorously apply $\phi$ reduction factors, edge distance limits ($c_{a1}$), and spacing overlap deductions ($A_{Nc}$) before formally specifying safe operating limits.

Ultimate Breakout Capacity

7,449 lbs
Absolute tensile cone fracture threshold.
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What is ACI 318 Concrete Cone Breakout: The Physics of Anchor Pullout?

When a tension load is applied to an anchor embedded in concrete, the concrete fails by forming a cone-shaped fracture surface radiating outward from the anchor head at approximately 35 degrees. The ACI 318 Appendix D method calculates the ultimate load at which this cone separates from the slab.

Mathematical Foundation

ACI 318 Concrete Breakout Strength
Nb=kc×λ×fc′×hef1.5N_b = k_c \times \lambda \times \sqrt{f'_c} \times h_{ef}^{1.5}
NbN_b= Basic concrete breakout strength in tension (lbs). This is the ultimate capacity of the concrete cone — NOT the allowable design load.
kck_c= Breakout coefficient: 24 for cast-in-place headed anchors, 17 for post-installed mechanical anchors (wedge, sleeve, drop-in).
λ\lambda= Lightweight concrete factor: 1.0 for normal-weight concrete (145 pcf), 0.75 for lightweight concrete (110 pcf).
fc′f'_c= Specified 28-day compressive strength of the concrete in PSI. Standard values: 3,000 PSI (residential), 4,000 PSI (commercial), 5,000+ PSI (structural).
hefh_{ef}= Effective embedment depth in inches — measured from the concrete surface to the bearing point of the anchor head or expansion mechanism.

Laws & Principles

  • Embedment depth (h_ef) is raised to the 1.5 power, making it the dominant variable. Doubling the embedment depth increases breakout capacity by 2.83x (2^1.5). By comparison, doubling the concrete strength only increases capacity by 1.41x (square root of 2). Deeper anchors are far more effective than stronger concrete.
  • The k_c coefficient reflects installation reliability: cast-in-place anchors (k_c=24) develop 41% more capacity than post-installed anchors (k_c=17) at the same embedment depth because they bond directly to the concrete during the pour, with no drilling disturbance.
  • This calculator gives the ULTIMATE breakout capacity — the load at which the concrete cone fractures. The allowable design load must include a strength reduction factor: phi = 0.75 for ductile failure modes (steel yielding) or phi = 0.65 for brittle failure modes (concrete breakout). Always apply the appropriate phi factor before establishing working loads.
  • Edge distance and spacing effects: if the anchor is closer than 1.5 x h_ef to a free edge, or closer than 3 x h_ef to another anchor, the breakout cones overlap and capacity is reduced. This calculator gives the single-anchor, unconfined capacity — edge and group effects require additional ACI 318 Appendix D calculations.

Step-by-Step Example Walkthrough

" A structural engineer evaluates a post-installed wedge anchor (k_c=17) set 4 inches deep into a 3,000 PSI garage slab, normal weight concrete. "

  1. 1. Identify variables: k_c = 17, lambda = 1.0, f'c = 3,000 PSI, h_ef = 4 inches.
  2. 2. Compute square root of f'c: sqrt(3,000) = 54.77.
  3. 3. Compute h_ef^1.5: 4^1.5 = 8.0.
  4. 4. Multiply: N_b = 17 x 1.0 x 54.77 x 8.0 = 7,449 lbs ultimate capacity.
  5. 5. Apply phi factor for brittle failure: 0.65 x 7,449 = 4,842 lbs design capacity.
  6. 6. Apply safety factor of 4:1 for working load: 7,449 / 4 = 1,862 lbs safe working load.
Final Result: Ultimate capacity = 7,449 lbs. Design capacity (phi=0.65) = 4,842 lbs. A structural engineer would typically specify a working load of 1,800-2,000 lbs for this anchor configuration.

Quick Answer: How do I calculate concrete anchor breakout capacity?

Use the ACI 318 formula: Nb = kc × λ × √f'c × hef1.5. Enter the anchor type (cast-in or post-installed), concrete strength (PSI), and embedment depth (inches). A 1/2" wedge anchor set 4 inches deep in 3,000 PSI concrete has an ultimate breakout capacity of about 7,450 lbs. The allowable working load is typically 25% of that value after safety factors.

ACI 318 Breakout Formula

Nb = kc × λ × √f'c × hef1.5

The 1.5 exponent on embedment depth (hef) makes depth the most powerful variable — doubling depth increases capacity by 2.83x. The square root on concrete strength (f'c) means doubling the concrete strength only increases capacity by 41%. For higher pullout resistance, go deeper before specifying stronger concrete.

Breakout Capacity by Embedment Depth

hef (inches) Cast-in (kc=24) Post-installed (kc=17) Working Load (4:1 SF)
2"3,720 lbs2,636 lbs659 lbs
3"6,836 lbs4,842 lbs1,211 lbs
4"10,517 lbs7,449 lbs1,862 lbs
6"19,316 lbs13,682 lbs3,420 lbs
8"29,760 lbs21,080 lbs5,270 lbs

All values assume 3,000 PSI normal-weight concrete (λ=1.0). Working load uses a 4:1 safety factor on post-installed ultimate capacity. Actual design loads require a licensed engineer's review per ACI 318 Appendix D.

Common Applications

Structural Steel Base Plate

A steel column base plate anchored to a 4,000 PSI foundation with four 3/4" cast-in-place headed anchors at 6" embedment. Per anchor: Nb = 24 × 1.0 × 63.2 × 14.7 = 22,297 lbs. Four anchors provide 89,188 lbs total ultimate capacity. With phi = 0.75 and group/edge reductions, the engineer designs for approximately 40,000 lbs of safe uplift resistance.

Equipment Mounting on Garage Slab

A drill press bolted to a 3,000 PSI garage slab with 3/8" post-installed wedge anchors at 3" embedment. Per anchor: Nb = 17 × 1.0 × 54.77 × 5.20 = 4,842 lbs. With a 4:1 safety factor, each anchor safely resists about 1,200 lbs of pullout — more than sufficient for shop equipment that may vibrate but produces minimal uplift.

Pro Tips

Do This

  • ✓Prioritize deeper embedment over stronger concrete. Because hef is raised to the 1.5 power, increasing depth by 50% boosts capacity by 84%. Increasing concrete strength by 50% only boosts capacity by 22%. Depth is 3-4x more effective per dollar than stronger concrete.
  • ✓Check edge distance before calculating capacity. If the anchor is closer than 1.5 × hef to a free edge, the breakout cone is truncated and capacity drops sharply. A 4-inch-deep anchor needs at least 6 inches of edge distance for full cone development.
  • ✓Always apply the phi reduction factor. This calculator gives the ultimate breakout capacity. For design, multiply by phi = 0.65 (brittle concrete breakout) or phi = 0.75 (ductile steel yielding). Using ultimate capacity as the design load provides zero margin for concrete variability.

Avoid This

  • ✗Don't install anchors in cracked concrete zones. Concrete near edges, joints, and high-stress areas is assumed cracked per ACI 318. Cracked concrete reduces anchor capacity by 20-40%. If cracks are visible near the anchor location, use a cracked-concrete-rated anchor and apply the cracking reduction factor.
  • ✗Don't ignore anchor spacing effects. When anchors are closer than 3 × hef, their breakout cones overlap. Two anchors at 4" depth spaced only 8" apart do NOT provide 2x the capacity — the overlapping cone means they share the same failure surface. Minimum spacing is typically 6 × hef for full independent capacity.
  • ✗Don't drill into rebar or post-tension tendons. When installing post-installed anchors, use a rebar locator (pachometer) to scan the drill location. Hitting a tendon in a post-tensioned slab releases stored energy catastrophically. Hitting rebar weakens the structural reinforcement at that location.

Frequently Asked Questions

What is the difference between cast-in-place and post-installed anchors?

Cast-in-place anchors (headed studs, J-bolts, L-bolts) are set into wet concrete before it cures. They develop a direct mechanical bond with the surrounding concrete and use kc = 24. Post-installed anchors (wedge, sleeve, drop-in, epoxy) are drilled into hardened concrete after curing. The drilling process disturbs the surrounding concrete matrix, so they use kc = 17 — producing 29% less breakout capacity at the same embedment depth.

Why is embedment depth more important than concrete strength?

Because the formula uses hef1.5 but only √f'c. A 50% increase in depth gives an 84% capacity increase, while a 50% increase in concrete strength gives only a 22% increase. The exponents reflect physics: a deeper anchor creates a larger breakout cone with more concrete surface area resisting the load. Going from 3" to 6" embedment nearly triples the breakout capacity.

What is a concrete breakout cone?

When an anchor is pulled out of concrete, the concrete fractures along a cone-shaped surface that radiates outward from the anchor head at approximately 35 degrees from vertical. The top diameter of this cone at the concrete surface is roughly 3 × hef. A 4-inch-deep anchor creates a breakout cone about 12 inches in diameter. This cone geometry is why edge distance and anchor spacing matter — if the cone extends past an edge or overlaps another anchor's cone, the capacity is reduced.

What safety factor should I use for anchor design?

ACI 318 uses strength reduction factors (phi): phi = 0.65 for concrete breakout (brittle failure) and phi = 0.75 for steel yielding (ductile failure). For non-engineered applications, a working load of 25% of ultimate capacity (4:1 safety factor) is a common industry practice. Overhead and life-safety anchors typically require a 5:1 or higher safety factor per OSHA and ANSI standards. Always consult the anchor manufacturer's published load tables for your specific anchor and application.

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