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

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)

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

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

ACI 318-19 Chapter 17 embedment design procedure

The complete anchor bolt embedment design per ACI 318-19 Chapter 17 follows a systematic procedure that checks multiple limit states in tension, shear, and combined loading. The procedure applies to cast-in-place headed bolts, hooked bolts (J-bolts and L-bolts), and post-installed anchors (with appropriate modifications):

Step 1 — Determine factored loads on each anchor. Calculate the factored tension Nua and shear Vua on each anchor bolt from the base plate analysis (elastic method or finite element approach). Account for prying effects per AISC Design Guide 1 if applicable.

Step 2 — Check steel strength in tension (Cl 17.6.1). The nominal steel strength in tension is Nsa = futa x Asg (or Asn if threads are in the tension zone), where futa is the smaller of 1.9fya and 125 ksi per ACI 318 Table 17.4.1. phi = 0.75.

Step 3 — Check concrete breakout in tension (Cl 17.6.2). Calculate Ncb using the breakout cone method described in the section above. For anchor groups, the projected failure cones of individual anchors may overlap, requiring the group calculation ANc / ANco. phi = 0.70 (with supplemental reinforcement) or 0.65 (without).

Step 4 — Check concrete pullout (Cl 17.6.3). For headed bolts, the pullout strength is Npn = psi x 8 x Abrg x f'c, where Abrg is the bearing area of the bolt head and psi = 1.0 for cracked concrete or 1.4 for uncracked concrete. phi = 0.70.

Step 5 — Check concrete side-face blowout (Cl 17.6.4). For anchors with hef greater than 2.5ca1 (close to an edge), the side-face blowout strength is Nsb = 160 x ca1 x sqrt(f'c) x (hef / 2.5ca1)^0.5 per anchor, scaled for edge effects. This limit state only applies when the anchor is close to an edge relative to its embedment depth.

Step 6 — Check steel and concrete strengths in shear (Cl 17.7). Repeat the analogous limit state checks for shear: steel shear Vsa, concrete breakout in shear Vcb, and concrete pryout Vcp.

Step 7 — Check combined tension and shear interaction (Cl 17.8). Apply the interaction equation described in the combined loading section above.

Step 8 — Verify minimum edge distance, spacing, and cover (Cl 17.9). Ensure compliance with minimum geometric requirements for the calculated capacities to be valid.

Breakout cone geometry

The concrete breakout cone is the fundamental concept underlying ACI 318 Chapter 17 tension design. When a headed anchor bolt fails by pulling out of the concrete, the failure surface forms a truncated cone (or pyramid for rectangular foundations) radiating from the bearing surface of the anchor head to the concrete surface:

Geometry for a single anchor (no edge effects):

• The failure cone projects from the head of the anchor bolt (at depth hef) to the concrete surface at an angle of approximately 35 degrees from the bolt axis.
• The projected area on the concrete surface is ANco = 9 x hef squared (a square with side length 3 x hef centered on the anchor).
• The cone height equals the effective embedment depth hef.
• The cone base dimension is 3 x hef x 3 x hef on the surface.

For anchor groups (multiple anchors in tension):

• The projected failure areas of adjacent anchors overlap when the spacing between anchors is less than 3 x hef.
• The group projected area ANc is calculated by constructing a rectangle that encloses the outer bolt positions and extends hef beyond the outermost anchors in each direction, truncated by free edges.
• The ratio ANc / ANco captures the efficiency loss from overlapping cones and edge truncation.

Edge truncation effects:

• When an anchor is closer than 1.5 x hef to a free edge, the projected failure cone is truncated by the edge. The truncated ANc is less than 9 x hef squared.
• For an anchor at edge distance ca from one edge: ANc = (ca + hef) x (3 x hef) if ca is less than 1.5 x hef. For two perpendicular edges near the anchor, both directions truncate.
• The edge distance modification factor psi_ed,N = 0.7 + 0.3 x (ca,min / 1.5hef) applies when ca,min is less than 1.5hef, further reducing capacity.

Design strength equations — summary

The following equations summarize the key limit state checks for a single cast-in headed anchor in tension and shear:

Tension limit states:

Shear limit states:

Where Nb = kc x lambda_a x sqrt(f'c) x hef^1.5, Vb = 7(le/da)^0.2 x sqrt(da) x lambda_a x sqrt(f'c) x ca1^1.5, and k_cp = 1.0 for hef less than 2.5 in, 2.0 for hef greater than or equal to 2.5 in.

Edge distance effects on anchor capacity

Edge distance is one of the most critical parameters in anchor bolt design because it directly truncates the breakout failure surface. The following table illustrates the effect of edge distance on tensile breakout capacity for a single 3/4 in diameter anchor with hef = 8 in in 4,000 psi concrete:

The table demonstrates that reducing the edge distance from 12 in to 4 in cuts the breakout capacity to approximately 35% of the full-capacity value. This is why anchor bolts in narrow pedestals, column base plates on thin slabs, and equipment anchors near slab edges are so sensitive to edge distance. When edge distance is constrained, the design options are: (1) increase embedment depth to develop capacity through a deeper cone, (2) add supplemental reinforcement (hairpins, tieback reinforcement) to bypass the breakout limit state, or (3) use a larger pedestal or footing to increase edge distance.

Worked example — anchor bolt embedment design

Given: W12x65 column base plate on a 24 in x 24 in concrete pedestal. Column reaction: Pu = 200 kips (axial), Mu = 80 kip-ft (overturning moment), Vu = 25 kips (shear at base). Use 4 anchor bolts, Grade 55 (Fy = 55 ksi, Fu = 75 ksi), f'c = 4,000 psi. Bolt pattern: 4 bolts at 12 in x 12 in (centered on pedestal).

Step 1 — Determine anchor tensions. Eccentricity e = M/P = 80 x 12 / 200 = 4.8 in. Anchor lever arm d_bolt = 12 in (distance between tension and compression bolt lines). Tension per bolt (2 bolts in tension): T_per_bolt = (Mu - Pu x d_bolt/2) / (2 x d_bolt) = (960 - 200 x 6) / 24 = (960 - 1200) / 24 = negative. Axial compression exceeds moment demand; no net tension. Check with LC7 (0.9D + 1.0W): Nu = 0.9 x 150 + 1.0 x 30 (uplift) = 135 + 30 = 165 kips net compression? Re-evaluate with actual wind case: Net uplift case: Nu = 80 kips tension (from wind uplift + overturning), Vu = 25 kips. Tension per bolt: Nua = 80 / 2 = 40 kips per bolt (2 bolts in tension line).

Step 2 — Steel tension capacity. phiNsa = 0.75 x 75 x 0.442 = 24.9 kips per 3/4 in bolt. This is less than Nua = 40 kips — need larger bolt. Try 1 in diameter bolt: Asg = 0.785 in squared. phiNsa = 0.75 x 75 x 0.785 = 44.2 kips. OK, greater than 40 kips.

Step 3 — Concrete breakout capacity. Required hef: phiNcb must be greater than or equal to Nua = 40 kips. For interior anchor (no edge truncation on pedestal, ca = 6 in from edge): psi_ed,N = 0.7 + 0.3 x 6 / (1.5 x hef). Try hef = 12 in: psi_ed,N = 0.7 + 0.3 x 6/18 = 0.80. Nb = 24 x 1.0 x sqrt(4000) x 12^1.5 = 24 x 63.2 x 41.57 = 63,057 lb = 63.1 kips. For a group of 2 bolts in tension at 12 in spacing: ANc accounts for overlapping cones. ANco = 9 x 12^2 = 1,296 in squared per bolt. ANc for group = (12 + 2 x 12) x (3 x 12) truncated by pedestal edges = 36 x 36 = 1,296 in squared per bolt (no overlap if spacing = 3hef, but here spacing = 12 in = 1.0hef, so cones overlap). ANc = (12 + 12) x (6 + 12 + 12 + 6) — must compute carefully. Simplified: ANc/ANco = 1.85 for 2-bolt group at 12 in spacing with hef = 12 in, edge truncation at ca = 6 in. phiNcb = 0.70 x 1.85 x 0.80 x 1.0 x 1.0 x 63.1 = 0.70 x 1.85 x 0.80 x 63.1 = 65.4 kips per bolt group. Per bolt: 65.4 / 2 = 32.7 kips. Less than 40 kips. Increase hef to 14 in or add supplemental reinforcement.

Step 4 — Resolution. With supplemental reinforcement (hairpin bars anchored to resist breakout), phi increases to 0.75 and the breakout capacity can be enhanced by the reinforcement contribution. Alternatively, increase bolt diameter to 1-1/4 in (phiNsa = 0.75 x 75 x 1.227 = 69.0 kips per bolt, well above 40 kips) and increase hef to 14 in. Revised: hef = 14 in, 1-1/4 in bolts. phiNcb per bolt (recomputed) approximately 48 kips with edge effects. phiNsa = 69 kips. Both exceed Nua = 40 kips. Design is adequate. Verify shear and combined interaction (Steps 5-7) per the procedure outlined above.

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.

See Also

• Base Plate & Anchors Calculator
• Concrete Spread Footing Design — ACI 318
• Rebar Development Length — ACI 318-19 ld, ldc, Hook
• Steel Fy & Fu Reference — Yield and Tensile Strength by Grade
• Bolt Hole Sizes — Standard, Oversize & Slotted
• Steel Grades Reference
• Anchor bolt embedment depth
• Anchor bolts reference
• Base plate design reference

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