Anchor Embedment Design

Complete reference for anchor bolt embedment depth in concrete per ACI 318-19 Chapter 17. Covers tension breakout, pullout, side-face blowout, shear pryout, and a worked example for headed anchors in normal-weight concrete.

PRELIMINARY — NOT FOR CONSTRUCTION. All results are for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.

Overview

Anchor embedment depth is the single most important geometric parameter in cast-in-place anchor design. The embedment depth (hef) directly controls the concrete breakout capacity, which is often the governing limit state for anchors in tension. Per ACI 318-19 Chapter 17, the nominal concrete breakout strength in tension is proportional to hef raised to the 1.5 power -- doubling the embedment depth increases breakout capacity by a factor of approximately 2.83.

This reference covers the minimum embedment requirements per ACI 318-19, the failure modes affected by embedment depth, the calculation methodology for concrete breakout, and practical guidance for selecting embedment depths in common base plate configurations.

Minimum Embedment Depth Requirements

ACI 318-19 Section 17.6 — Cast-In Headed Anchors

Headed anchors (J-bolts, headed studs, headed bolts) must satisfy the following minimum embedment requirements:

• Minimum embedment (hef): The greater of 6db and 2 in. (50 mm), where db is the anchor diameter.
• Minimum edge distance: 1.5hef for anchors governed by concrete breakout (to ensure the breakout cone can fully develop).
• Minimum anchor spacing: 3.0hef for anchors in a group to avoid overlapping breakout cones.

Practical note: The 6db minimum is rarely the controlling depth in design. Concrete breakout capacity almost always requires substantially deeper embedment than the 6db minimum. The "recommended hef" column above reflects depths that typically satisfy breakout requirements for medium-to-heavy loads in normal-weight concrete (f'c = 4,000 psi).

Failure Modes Affected by Embedment Depth

Embedment depth hef influences all five primary anchor failure modes in tension:

1. Concrete Breakout (ACI 17.6.2)

Concrete breakout is a conical failure surface propagating from the anchor head to the concrete surface at approximately 35 degrees from the vertical. The nominal breakout strength is:

Ncb = (Anc / Anco) * psi_ed,N * psi_c,N * psi_cp,N * Nb

Where Nb (basic concrete breakout strength) = kc _ lambda_a _ sqrt(f'c) * hef^1.5

• kc = 24 for cast-in headed anchors (ACI 17.6.2.2)
• kc = 17 for post-installed anchors (ACI 17.6.2.2)
• lambda_a = 1.0 for normal-weight concrete
• hef = embedment depth (in.)

Key insight: The hef^1.5 relationship means breakout capacity increases faster than linearly with depth. An anchor at hef = 12 in. has 2.83 times the breakout capacity of an anchor at hef = 6 in. for the same concrete and anchor type.

2. Pullout Strength (ACI 17.6.3)

For headed anchors, pullout strength depends on the bearing area of the anchor head:

Npn = 8 * Abrg * f'c    (for headed studs and headed bolts)

Where Abrg = net bearing area of the head = (pi/4) * (d_head^2 - db^2).

Pullout strength is independent of embedment depth. However, the anchor must have sufficient embedment for the head to develop its full bearing capacity without interaction with the concrete surface.

3. Side-Face Blowout (ACI 17.6.4)

For anchors with deep embedment near an edge (hef > 2.5ca1, where ca1 is the edge distance), a side-face blowout failure can occur. This is a lateral bursting of the concrete on the side face near the anchor head:

Nsb = 160 * ca1 * sqrt(Abrg) * sqrt(f'c)    (single anchor near edge)

This failure mode is most common with deep embedment and small edge distances -- think of a deeply embedded anchor in a narrow column or thin wall.

4. Concrete Pryout in Shear (ACI 17.7.3)

For anchors in shear, the concrete pryout strength is directly related to the embedment depth:

Vcp = kcp * Ncp

Where Ncp = nominal breakout strength calculated as if the anchor were in tension, and kcp = 1.0 for hef < 2.5 in., or 2.0 for hef >= 2.5 in.

5. Anchor Steel Strength (ACI 17.6.1)

Independent of embedment depth:

Nsa = Ase,N * futa    (tension)
Vsa = 0.60 * Ase,V * futa    (shear, cast-in headed)

Concrete Breakout Calculation — Step by Step

Concrete breakout in tension is the most common governing limit state for headed anchors. The calculation follows ACI 318-19 Section 17.6.2:

Step 1: Determine basic breakout strength Nb

Nb = kc * lambda_a * sqrt(f'c) * hef^1.5

Where kc = 24 for cast-in headed anchors and lambda_a = 1.0 for normal-weight concrete.

Step 2: Determine projected area Anc

For a single anchor (no group effects, no edge effects):

Anc = 9 * hef^2     (full cone, no edge distance limitations)
Anco = 9 * hef^2    (reference projected area)

When edge distance or anchor spacing limits the cone:

Anc = min(1.5hef, ca1) * 2 * 1.5hef    (one edge limits)

Step 3: Apply modification factors

psi_ed,N = 0.7 + 0.3 * (ca,min / 1.5hef)   <= 1.0    (edge effect)
psi_c,N = 1.0 for cast-in anchors at service temperature    (cracking factor)
psi_cp,N = 1.0 for cast-in headed anchors if ca,min >= cac    (splitting factor)

Step 4: Compute design strength

phi * Ncb = 0.75 * (Anc / Anco) * psi_ed,N * psi_c,N * psi_cp,N * Nb

Strength reduction factor phi = 0.75 for anchors in tension governed by concrete breakout (Condition B, supplementary reinforcement not provided).

Worked Example — Column Base Plate Anchor

Given: A W12x65 column base plate with (4) 3/4 in. diameter headed anchors (F1554 Gr 36, futa = 58 ksi) embedded in normal-weight concrete (f'c = 4,000 psi). The base plate is 16 in. x 16 in. with anchors at 12 in. centers. Edge distance ca1 = 2 in. to the concrete edge. Applied factored tension Tu = 35 kips per anchor (uplift load case).

Find: Required embedment depth hef to resist the applied tension.

Step 1: Steel Strength Check

Ase = 0.334 in^2 (tensile stress area for 3/4" UNC)
phi * Nsa = 0.75 * 0.334 * 58 = 14.5 kips    (governs steel!)

Steel strength per anchor = 14.5 kips. Applied Tu = 35 kips >> 14.5 kips. Steel strength is inadequate. We need higher-strength steel or larger diameter anchors.

Let us revise: Use 1 in. diameter headed anchors, F1554 Gr 105 (futa = 125 ksi).

Ase = 0.606 in^2 (tensile stress area for 1" UNC)
phi * Nsa = 0.75 * 0.606 * 125 = 56.8 kips > 35 kips    OK

Step 2: Concrete Breakout Check

Edge distance ca1 = 2 in. This is very small. Because ca1 < 1.5hef, the breakout cone is truncated.

Let us target hef = 10 in. and verify:

1.5hef = 1.5 * 10 = 15 in. > ca1 = 2 in.    (cone is significantly restricted)

Anco = 9 * hef^2 = 9 * 10^2 = 900 in^2
Anc = (ca1 + 1.5hef) * 2 * 1.5hef = (2 + 15) * 2 * 15 = 510 in^2
    (assuming edge on one side only; the other side = 2 * 1.5hef = 30 in. for interior anchors)

psi_ed,N = 0.7 + 0.3 * (2 / 15) = 0.7 + 0.04 = 0.74

Nb = 24 * 1.0 * sqrt(4000) * 10^1.5 = 24 * 63.25 * 31.62 = 48,000 lb = 48.0 kips

phi * Ncb = 0.75 * (510/900) * 0.74 * 1.0 * 1.0 * 48.0
          = 0.75 * 0.567 * 0.74 * 48.0
          = 15.1 kips < 35 kips    NOT OK

The small edge distance (ca1 = 2 in.) severely restricts the breakout cone. The breakout capacity of 15.1 kips is less than the applied 35 kips.

Step 3: Increase Edge Distance

Revise: Move anchors to 4 in. edge distance. Maintain hef = 10 in.

1.5hef = 15 in. > ca1 = 4 in.    (still restricted)

Anc = (4 + 15) * (2 * 15) = 19 * 30 = 570 in^2
psi_ed,N = 0.7 + 0.3 * (4 / 15) = 0.7 + 0.08 = 0.78

phi * Ncb = 0.75 * (570/900) * 0.78 * 48.0 = 0.75 * 0.633 * 0.78 * 48.0
          = 17.8 kips < 35 kips    Still NOT OK

Step 4: Increase Embedment to hef = 16 in.

1.5hef = 24 in. > ca1 = 4 in.
Anc = (4 + 24) * (2 * 24) = 28 * 48 = 1,344 in^2
Anco = 9 * 16^2 = 2,304 in^2
psi_ed,N = 0.7 + 0.3 * (4 / 24) = 0.7 + 0.05 = 0.75

Nb = 24 * 1.0 * sqrt(4000) * 16^1.5 = 24 * 63.25 * 64.0 = 97,200 lb = 97.2 kips

phi * Ncb = 0.75 * (1344/2304) * 0.75 * 97.2
          = 0.75 * 0.583 * 0.75 * 97.2
          = 31.9 kips < 35 kips    Still marginal

Step 5: Final Design

Use hef = 18 in. with ca1 = 4 in.:

1.5hef = 27 in. > ca1 = 4 in.
Anc = (4 + 27) * (2 * 27) = 31 * 54 = 1,674 in^2
Anco = 9 * 18^2 = 2,916 in^2
psi_ed,N = 0.7 + 0.3 * (4 / 27) = 0.744

Nb = 24 * 1.0 * 63.25 * 18^1.5 = 24 * 63.25 * 76.37 = 116,000 lb = 116 kips

phi * Ncb = 0.75 * (1674/2916) * 0.744 * 116
          = 0.75 * 0.574 * 0.744 * 116
          = 37.2 kips > 35 kips    OK

Final design: (4) 1 in. diameter F1554 Gr 105 headed anchors, hef = 18 in., edge distance >= 4 in.

Key takeaway: Small edge distances are the enemy of concrete breakout capacity. At ca1 = 2 in., even with hef = 18 in., breakout is inadequate. At ca1 = 4 in. with hef = 18 in., we achieve adequate capacity. Edge distance and embedment depth work together — increasing only one while neglecting the other does not solve the problem.

Practical Embedment Depth Selection Guide

*Assumes normal-weight concrete f'c = 4,000 psi, edge distance >= 1.5hef, no supplementary reinforcement, phi = 0.75. Values are approximate for preliminary sizing.

Frequently Asked Questions

How deep should I embed anchor bolts in concrete? The minimum embedment per ACI 318 is the greater of 6db (6 times the bolt diameter) and 2 in., but the required embedment for adequate concrete breakout capacity is almost always deeper than this minimum. For column base plates, typical embedment depths range from 8 to 18 in. for 3/4 in. to 1-1/4 in. diameter anchors. The required depth depends on the applied tension, concrete strength, edge distance, and anchor spacing. For a preliminary estimate, use hef = 10db to 15db for tension anchors in normal-weight concrete.

What failure modes does embedment depth control? Embedment depth directly controls concrete breakout capacity (proportional to hef^1.5), side-face blowout (triggers when hef > 2.5ca1), and concrete pryout in shear (proportional to the breakout strength). It does NOT control anchor steel strength, pullout strength (which depends on head bearing area), or bond strength for adhesive anchors. Increasing embedment depth is the most effective way to increase overall anchor capacity because breakout often governs.

Does increasing embedment depth always increase anchor capacity? Not always. If steel strength governs, deeper embedment provides no additional capacity — you need a larger diameter or higher-strength anchor instead. If pullout governs, a larger anchor head is needed, not deeper embedment. Additionally, for anchors close to an edge, increasing embedment depth while keeping edge distance constant can reduce the psi_ed,N modification factor (because the ratio ca,min / 1.5hef decreases), partially offsetting the gain from increased hef. The net effect is still usually positive, but the interaction must be checked.

How does edge distance interact with embedment depth? Per ACI 318, the breakout cone projects at approximately 35 degrees from vertical, reaching a radius of 1.5hef at the concrete surface. If the edge distance ca1 is less than 1.5hef, the breakout cone is truncated and the capacity is reduced by the factor psi_ed,N = 0.7 + 0.3(ca,min / 1.5hef). The ratio ca1 / hef is therefore critical: at ca1 = hef (close edge), psi_ed,N = 0.7 + 0.3/1.5 = 0.90; at ca1 = 0.5hef, psi_ed,N = 0.7 + 0.3*0.5/1.5 = 0.80. This 10-20% reduction, combined with the reduced projected area Anc, can significantly reduce capacity.

What is the difference between cast-in-place and post-installed anchor embedment requirements? Cast-in-place headed anchors use kc = 24 in the breakout equation, while post-installed anchors use kc = 17 (unless qualified by testing per ACI 355.2). This means a post-installed anchor requires approximately (24/17)^(2/3) = 1.26 times the embedment depth to achieve the same breakout capacity as a cast-in headed anchor — about 25% deeper. Post-installed anchors also have additional requirements for hole cleaning, installation torque, and bond strength verification that cast-in anchors do not.

Key References

• ACI 318-19, Chapter 17 — Anchoring to Concrete
• ACI 318-19, Section 17.6 — Cast-In Headed Anchors
• ACI 318-19, Section 17.6.2 — Concrete Breakout Strength in Tension
• ASTM F1554 — Standard Specification for Anchor Bolts
• AISC Design Guide 1 — Base Plate and Anchor Rod Design (3rd Edition)

Related Resources

• Base Plate Design
• Anchor Bolts Reference
• Anchor Bolt Embedment Reference
• Column Base Plate Design
• Steel Connection Design
• All Reference Tables
• How to Verify Calculations

Disclaimer: For educational use only. All results must be independently verified by a licensed Professional Engineer.