Anchor Bolt Calculator — ACI 318-19 Tension & Shear Capacity
Check anchor bolt steel strength, concrete breakout, and pullout capacity for ASTM F1554 anchor bolts in cast-in-place concrete. Results follow ACI 318-19 Chapter 17 with phi-factored design strengths and governing failure mode identification.
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View Chart →How This Anchor Bolt Calculator Works
Select Head Type and Grade
Choose headed, hooked, or headless anchor style, then pick an ASTM F1554 grade. Grade sets the yield and tensile strength used in the steel strength check.
Enter Embedment and Edge Distance
Embedment depth (hef) and edge distance (ca1) drive the concrete breakout calculation. Shallow embedment or a close edge reduces capacity.
Run Three Limit-State Checks
The calculator computes steel strength, concrete breakout, and pullout strength (for headed anchors), then applies ACI 318-19 phi factors to each.
Compare Demand to Governing Capacity
The lowest phi-factored capacity governs. The calculator reports pass or fail against your applied tension and shear loads.
ASTM F1554 Anchor Bolt Grade Properties
Anchor bolt material properties come from ASTM F1554-20, the governing specification for straight, bent, and headed carbon and alloy steel anchor bolts. For base plate design that pairs with these anchors, see the base plate calculator.
| Grade | Yield Strength, Fy | Tensile Strength, Fu | Diameter Range | Typical Use |
|---|---|---|---|---|
| Grade 36 | 36 ksi | 58-80 ksi | 1/4 in - 4 in | Standard structural connections |
| Grade 55 | 55 ksi | 75-95 ksi | 1/4 in - 4 in | Higher-capacity structural anchors |
| Grade 105 | 105 ksi | 125-150 ksi | 1/4 in - 3 in | Heavy structural, seismic applications |
Source: ASTM F1554-20 Standard Specification for Anchor Bolts, Steel, 36, 55, and 105-ksi Yield Strength. Effective tensile stress area (Ase) is calculated per thread series, not gross bolt area.
What Determines Anchor Bolt Capacity in Concrete
An anchor bolt in tension can fail in three distinct ways, and ACI 318-19 Chapter 17 requires checking all three. The lowest phi-factored strength governs the design.
Steel failure happens when the bolt itself yields and fractures, calculated as Nsa = Ase x futa per ACI 318-19 §17.6.1.2. Concrete breakout happens when a cone of concrete pulls out around the embedded anchor, following the ANc/ANco projected area method in §17.6.2. Pullout happens when the anchor head slips through the concrete without a full breakout cone, per §17.6.3.
Edge distance and embedment depth control which failure mode governs. Shallow anchors near a free edge almost always fail by concrete breakout before the steel yields. For related bearing checks, see the concrete bearing pressure calculator and the concrete PSI strength calculator.
ACI 318-19 Strength Reduction Factors (phi)
| Limit State | Phi Factor | ACI 318-19 Reference |
|---|---|---|
| Steel strength in tension | 0.75 | Table 17.5.3 |
| Concrete breakout in tension (Condition B) | 0.65 | Table 17.5.3 |
| Pullout strength | 0.70 | Table 17.5.3 |
| Steel strength in shear | 0.65 | Table 17.5.3 |
| Concrete breakout in shear (Condition B) | 0.70 | Table 17.5.3 |
Condition B applies when no supplementary reinforcement is present, which is the typical case for standard base plate anchoring. Condition A phi factors are higher when supplementary reinforcement controls splitting.
Sample Anchor Bolt Calculations
The two scenarios below walk through the governing math this calculator performs, using verified inputs and formulas from ACI 318-19 Chapter 17.
Scenario 1: 3/4 in Grade 36 Headed Anchor
| Diameter | 0.75 in |
| Ase (threaded) | 0.334 in² |
| Embedment, hef | 6 in |
| Edge distance, ca1 | 6 in |
| f'c | 4,000 psi |
Steel strength: Nsa = Ase x futa = 0.334 in² x 58,000 psi = 19,372 lb. Applying phi = 0.75 per Table 17.5.3 gives 14,530 lb. Compare against breakout and pullout to find the governing value.
Scenario 2: Edge Distance Reduction Check
| Embedment, hef | 8 in |
| 1.5 x hef (full capacity threshold) | 12 in |
| Actual edge distance, ca1 | 5 in |
| ca1 / (1.5 hef) | 0.417 |
Because ca1 (5 in) is less than 1.5 x hef (12 in) per §17.6.2.1, the edge distance factor psi_ed,N is reduced below 1.0, cutting concrete breakout capacity. Moving the anchor away from the edge restores full capacity.
Common Anchor Bolt Design Errors
Designers often calculate Nb without applying psi_ed,N when ca1 is less than 1.5 x hef. This overstates capacity. ACI 318-19 §17.6.2.1 requires the reduction whenever the anchor sits close to a free edge.
Nsa = Ase x futa requires the effective area at the threaded section (Ase), not the nominal shank area. Using shank area overstates steel tension capacity by 15-25% depending on thread series.
Pullout (Npn) can govern over breakout in high-strength concrete with a small anchor head. Skipping this check per §17.6.3 risks an undersized head bearing area.
Uncracked-section factors increase capacity, but ACI 318-19 §17.6.2.5 requires analysis confirming the concrete remains uncracked at service loads. Default to cracked-section values unless verified.
Torqued cast-in anchors need at least 6 bolt diameters of spacing and edge distance. Anchors placed closer together trigger group breakout effects that this calculator does not model beyond a spacing flag.
Where Anchor Bolt Design Applies On Site
Anchor bolts connect steel columns, light poles, equipment pads, and handrail posts to concrete foundations. Setting them correctly on the first pour avoids costly post-installed repairs. The anchor bolt embedment calculator and anchor bolt spacing calculator focus on the two inputs that matter most here: depth and clearance.
IBC 2024 §1901 references ACI 318 by reference for all concrete anchor design in permitted construction. Inspectors typically check anchor projection, plumbness, and embedment depth before concrete placement, since embedment cannot be corrected after the pour cures. Coordinate anchor layout with the base plate calculator to confirm bolt pattern fits the column base plate before setting templates.
Grade 105 anchors require special handling because their higher hardness makes them less ductile. Some jurisdictions restrict Grade 105 in seismic design categories per AISC 341 unless specifically detailed for ductile yielding elsewhere in the connection.
Anchor Bolt Calculator FAQ
Per ACI 318-19 Table 17.9.2(a), a torqued cast-in anchor needs a minimum edge distance of 6 times the anchor diameter (6da). Anchors that will not be torqued only need to satisfy the concrete cover requirements in §20.5.1.3, which is typically less restrictive.
Calculate steel strength as Nsa = Ase x futa per §17.6.1.2, then calculate concrete breakout strength Ncb per §17.6.2 and pullout strength Npn per §17.6.3 if the anchor is headed. Apply the phi factor from Table 17.5.3 to each and use the lowest phi-factored value as the design capacity.
Concrete breakout strength Ncb models the concrete cone that can separate from the base material around a loaded anchor. It equals (ANc/ANco) x psi_ed,N x psi_c,N x Nb per §17.6.2, where Nb = kc x lambda_a x sqrt(f'c) x hef^1.5 is the basic breakout strength of a single anchor away from any edge.
ASTM F1554 Grade 36 (36 ksi yield, 58-80 ksi tensile) covers most standard structural connections. Grade 55 (55 ksi yield) provides more capacity in the same diameter for tighter layouts. Grade 105 (105 ksi yield) is a high-strength weldable option for heavy structural work, though some jurisdictions restrict it in seismic applications.
Table 17.5.3 sets phi at 0.75 for steel tension, 0.65 for concrete breakout in tension (Condition B, no supplementary reinforcement), 0.70 for pullout, 0.65 for steel shear, and 0.70 for concrete breakout in shear. Condition A, with supplementary reinforcement, uses higher factors for breakout.
Pullout strength Npn applies only to headed and some hooked anchors. It equals psi_c,P x 8 x Abrg x f'c per §17.6.3.2.1, where Abrg is the net bearing area under the anchor head. This limit state governs when the head is undersized relative to the surrounding breakout cone strength.
A common field rule of thumb is 12 times the bolt diameter for standard embedment, though the actual required depth depends on load demand. For full, unreduced concrete breakout capacity, the edge distance should also be at least 1.5 times the embedment depth per §17.6.2.1.
Sources & Methodology
- ACI 318-19, Chapter 17: Anchoring to Concrete, American Concrete Institute. concrete.org
- ASTM F1554-20, Standard Specification for Anchor Bolts, Steel, 36, 55, and 105-ksi Yield Strength. astm.org
- International Building Code (IBC) 2024, Section 1901: Concrete, incorporates ACI 318 by reference.
- PCI Design Handbook, 8th Edition, anchor bolt spacing and edge distance guidance.
- AISC 341, Seismic Provisions for Structural Steel Buildings, anchor rod ductility guidance for Grade 105.
Last reviewed: July 2026. Reviewed by site author.
⚠ Engineering Disclaimer
This calculator provides estimates for planning purposes. For permitted structural work, foundations, multi-story construction, retaining walls over 4 feet, and commercial projects, calculations must be verified by a licensed structural engineer per IBC 2024 §1604. ConcreteCalculate.com is not liable for structural decisions made from these estimates.
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