Anchor Bolt Embedment Depth — ACI 318 Chapter 17 Design Guide

Anchor bolt embedment depth governs tensile breakout capacity in concrete. ACI 318-19 Chapter 17 (Anchoring to Concrete) provides the design method for headed and hooked anchor bolts subject to tension, shear, and combined loading.

Anchor Types

Type	Description	Typical Use
Cast-in headed bolt	J-bolt or L-bolt with hex head	Column base plates, equipment
Cast-in headed stud	Welded stud	Composite beams, precast
Post-installed mechanical	Expansion or undercut	Concrete that is already placed
Post-installed adhesive	Epoxy/chemical	Structural retrofits

Tensile Breakout Capacity (ACI 318-19 Cl 17.6.2)

The concrete breakout strength in tension for a single anchor:

Ncb = (ANc/ANco) × Ψed,N × Ψc,N × Ψcp,N × Nb

Basic breakout strength of a single anchor (Nb):

Nb = kc × λa × √f'c × hef^1.5   [lb, psi, in units]

Where:

kc = 24 for cast-in anchors, 17 for post-installed
λa = lightweight concrete factor (= 1.0 for normal weight)
f'c = concrete compressive strength (psi)
hef = effective embedment depth (in)

The projected failure cone area for a single anchor:

ANco = 9 × hef²

Required Embedment Depth — Design Tables

For cast-in headed bolts, Grade 55 (fy = 55 ksi), normal-weight concrete:

f'c = 3,000 psi (20.7 MPa)

Bolt Dia	Bolt Area	φTn,steel	hef req'd	Min spacing	Min edge
1/2"	0.196 in²	6.5 kips	4.5 in	6.8 in	4.5 in
5/8"	0.307 in²	10.1 kips	6.0 in	9.0 in	6.0 in
3/4"	0.442 in²	14.6 kips	7.5 in	11.3 in	7.5 in
7/8"	0.601 in²	19.8 kips	9.0 in	13.5 in	9.0 in
1"	0.785 in²	25.9 kips	10.5 in	15.8 in	10.5 in
1-1/4"	1.227 in²	40.5 kips	13.5 in	20.3 in	13.5 in

f'c = 4,000 psi (27.6 MPa)

Bolt Dia	φTn,steel	hef req'd	Min spacing	Min edge
1/2"	6.5 kips	4.0 in	6.0 in	4.0 in
5/8"	10.1 kips	5.5 in	8.3 in	5.5 in
3/4"	14.6 kips	6.5 in	9.8 in	6.5 in
7/8"	19.8 kips	8.0 in	12.0 in	8.0 in
1"	25.9 kips	9.5 in	14.3 in	9.5 in
1-1/4"	40.5 kips	12.0 in	18.0 in	12.0 in

Note: Minimum embedment depth = 6 × bolt diameter per ACI 318-19 Table 17.2.3.

Shear Breakout (ACI 318-19 Cl 17.7.2)

The concrete breakout capacity in shear for a single anchor near an edge:

Vcb = (AVc/AVco) × Ψed,V × Ψc,V × Ψh,V × Vb

Basic shear breakout strength:

Vb = 7(le/da)^0.2 × √da × λa × √f'c × ca1^1.5

Where:

le = load-bearing length of anchor (≤ 8da for headed anchors)
da = outside diameter of anchor
ca1 = distance from anchor to free edge in shear direction

Critical edge distance for shear: ca1 must be ≥ 1.5 × hef to avoid reduction

Combined Tension and Shear (ACI 318-19 Cl 17.8.3)

ACI 318-19 uses a trilinear (linear) interaction approach:

If Vua ≤ 0.2φVn → tension-only check governs; shear interaction term neglected
If Nua ≤ 0.2φNn → shear-only check governs; tension interaction term neglected
Otherwise, use the linear interaction equation:

Nua/(0.85φNn) + Vua/(0.85φVn) ≤ 1.0   [ACI 318-19 Eq. 17.8.3]

Code edition note: The power-law formula (Nua/φNn)^(5/3) + (Vua/φVn)^(5/3) ≤ 1.0 appeared in ACI 318-14 (and is compatible with AISC Design Guide 1). ACI 318-19 replaced this with the linear form above. Both the 0.2 cutoffs and the 0.85 reduction factor are ACI 318-19 provisions. If working to ACI 318-14, substitute the 5/3 power formula; if working to ACI 318-19, use the linear form.

Grout Pad Consideration

When base plates are set on grout, ACI 318-19 permits the anchor bolt tension to be developed from the top of the grout pad. The grout height (typically 1–3 in) adds to the effective anchor length but does not count toward hef.

Rules of Thumb for Preliminary Design

Minimum embedment: hef ≥ 6 × bolt diameter
Minimum edge distance: ca ≥ 6 × bolt diameter (to avoid edge effects)
Minimum bolt spacing: s ≥ 6 × bolt diameter
Preferred: design anchors so steel controls, not concrete breakout

Frequently Asked Questions

What controls anchor design — steel yielding or concrete breakout? For well-designed anchors, steel yielding should govern. When steel controls, the failure is ductile and the load-deflection curve has a yield plateau before rupture. Concrete breakout is a brittle failure with no warning. ACI 318-19 Chapter 17 achieves steel-controlled behavior by requiring that the concrete breakout capacity (φNcb) exceed the steel tension capacity (φNsa) by a margin, or by using supplemental reinforcement to bypass the breakout limit state.

Why does ACI 318 specify a minimum 6 × bolt diameter embedment depth? The 6d minimum reflects the practical lower bound for developing enough projected failure cone area to resist even the lightest steel capacity. Shorter embedments produce very shallow cones with small ANc/ANco ratios and low Nb values. The 6d rule is a simplified floor that prevents under-embedded anchors being specified before any actual capacity calculation is done.

How do the phi factors for anchor tension differ from structural steel? ACI 318-19 Chapter 17 uses φ = 0.70 for concrete breakout tension (cast-in anchors, supplemental reinforcement present) or φ = 0.65 without supplemental reinforcement. Compare this to φ = 0.75 for bolt tensile rupture on the steel side. The lower phi for concrete reflects greater variability in concrete tensile behavior and the brittle nature of breakout failure.

How does edge distance affect anchor tensile capacity? When the edge distance ca,min is less than 1.5 × hef, the projected failure cone is truncated by the free edge. The ANc/ANco ratio drops below 1.0, reducing Ncb proportionally. Additionally, the edge distance modification factor Ψed,N < 1.0 applies when ca,min < 1.5 × hef: Ψed,N = 0.7 + 0.3 × ca,min/(1.5 × hef). In extreme cases (anchor very close to an edge), the shear cone is also truncated and both tension and shear capacities are reduced simultaneously.

Can grout pad thickness count toward the effective embedment depth? No. ACI 318-19 defines hef as measured from the concrete surface (top of slab or footing), not the top of a grout pad. The grout contributes to the physical anchor length but is not included in the breakout cone depth calculation. Grout is typically softer than structural concrete and does not contribute to the tensile failure cone. This is a common detailing oversight — the anchor must extend deep enough into the concrete itself.

→ Design base plate anchor bolts → → Reference: Bolt hole sizes →

Run This Calculation

→ Base Plate & Anchors Calculator — design base plate bearing, bending, and anchor bolt embedment per ACI 318 Chapter 17.

→ Concrete Spread Footing Calculator — spread footing design with ACI 318 flexure and shear checks, including anchor development.

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