ACI 318-19 Chapter 17 | Anchoring to Concrete (Breakout & Shear)
Anchor bolts embedded in concrete can fail either by the steel rod snapping or by pulling a 35-degree pyramid-shaped cone of concrete out of the foundation (Concrete Breakout). Edge distances, group overlapping cones, and concrete cracking heavily influence breakout capacity.
Governing Formula
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Notation & Variables
Worked Example
Compute basic breakout strength Nb
For a cast-in headed anchor (kc = 24) with hef = 10 in and f'c = 4,000 psi: Nb = 24(1.0)√4000 (10^1.5) = 48,000 lb (48.0 kips).
Apply projected area & edge factors
Compute ANc / ANco and edge modification factor ψed,N when edge distance ca,min < 1.5 hef.
Check tension-shear interaction
Verify (Nu / φNn) + (Vu / φVn) ≤ 1.2 per Section 17.8 when both tension and shear exceed 20% of capacity.
When does this govern?
Concrete breakout almost always governs over steel bolt tensile rupture for anchors near foundation edges or in unreinforced pedestals.
Common Mistake
Measuring effective embedment hef to the bottom of the nut instead of the bearing face of the anchor head.