Anchor Bolt Calculator: ACI 318 Chapter 17 Breakout and Interaction

How It Works

1. Enter Anchor Properties

   Input anchor diameter, effective embedment depth, and steel grade. Select anchor type: cast-in-place headed bolt, post-installed expansion, or adhesive (chemical). Each type has different capacity factors.

2. Enter Concrete Properties

   Input concrete compressive strength (f'c) in PSI, edge distances, anchor spacing (if grouped), and whether concrete is cracked or uncracked. Cracked concrete reduces breakout capacity by about 25%.

3. Calculate Failure Modes

   ACI 318 Chapter 17 checks five tension modes (steel, breakout, pullout, side-face blowout, bond) and three shear modes (steel, breakout, pryout). The governing capacity is the lowest value.

4. Check Interaction

   For combined tension and shear, the tri-linear interaction equation applies. If N/phi-N + V/phi-V is 1.2 or less (with individual ratios at or below 1.0), the anchor passes.

Built For

• Structural engineers designing base plate connections for steel columns on concrete foundations
• Contractors selecting anchor bolt sizes for equipment mounting on concrete slabs
• Inspectors verifying anchor bolt installations meet the specified embedment and edge distance
• Maintenance engineers evaluating capacity of existing anchors before hanging new overhead loads
• Seismic retrofit designers checking anchor capacity for hold-down connections in wood-frame structures
• Fabricators verifying embed plate anchor patterns in precast concrete panels

Features & Capabilities

ACI 318 Chapter 17 Compliance

Checks all required tension and shear failure modes per the current ACI 318 standard. Returns the governing mode and capacity.

Tension Breakout Cone

Models the concrete breakout cone at 35 degrees from the anchor axis. Adjusts projected area for edge distance and anchor spacing.

Cracked/Uncracked Concrete

Applies the cracking modification factor for concrete in tension zones. Only permanently compressed zones qualify as uncracked.

Tension-Shear Interaction

Runs the tri-linear interaction check for combined loading. Shows the interaction ratio and whether it passes the 1.2 limit.

Cast-In and Post-Installed Types

Supports headed bolts, expansion anchors, and adhesive anchors with appropriate capacity reduction factors for each type.

PDF Export

Export anchor bolt analysis as a branded PDF for engineering submittals or inspection records.

Assumptions

• Concrete breakout cone modeled at 35 degrees from anchor axis per ACI 318 Chapter 17 (CCD method)
• Projected breakout area for single anchor: A_Nc = 9 x h_ef\u00b2 when unaffected by edges or adjacent anchors
• Concrete compressive strength f'c assumed uniform throughout the breakout zone — no localized weak areas or voids
• Cracked concrete modification factor applied by default unless user specifies permanently compressed (uncracked) zone
• Strength reduction factors (\u03c6) per ACI 318-19: \u03c6 = 0.75 for tension breakout, \u03c6 = 0.70 for shear breakout (Condition B, no supplementary reinforcement)
• Tri-linear interaction for combined tension and shear: full capacity if N/\u03c6N_n \u2264 0.2 or V/\u03c6V_n \u2264 0.2, otherwise N/\u03c6N_n + V/\u03c6V_n \u2264 1.2
• Steel tensile and shear strength based on ASTM anchor material specifications (A307, F1554 Grade 36/55/105)

Limitations

• Does not design anchor reinforcement per ACI 318 Section 17.5.2.1 — anchor reinforcement can significantly increase breakout capacity
• Does not evaluate adhesive anchor bond strength in detail — requires manufacturer-specific bond stress values per ACI 355.4 qualification testing
• Seismic design category modifications (Section 17.10) for anchor ductility classification are not applied automatically
• Group anchor analysis assumes uniform load distribution — eccentric loading and flexible base plates redistribute loads unevenly
• Does not check concrete side-face blowout for headed anchors with shallow edge distances in thin members
• Post-installed anchor pullout capacity depends on installation quality (hole cleaning, torque, cure time) not modeled here
• Does not evaluate through-bolt or anchor plate bearing on the back face of concrete members

References

• ACI 318-19 Chapter 17 — Anchoring to Concrete (Concrete Capacity Design method)
• ACI 355.2 — Qualification of Post-Installed Mechanical Anchors in Concrete
• ACI 355.4 — Qualification of Post-Installed Adhesive Anchors in Concrete
• ASTM F1554 — Standard Specification for Anchor Bolts, Steel, 36, 55, and 105-ksi Yield Strength
• IBC Section 1705.1.1 — Special Inspection for Anchors in Concrete (cast-in and post-installed)
• PCI Design Handbook — Anchorage Design Examples Using ACI 318 Appendix D / Chapter 17

Frequently Asked Questions