Concrete Anchor Bolt Pullout & Shear Breakout Calculator
Calculate structural anchor bolt tensile capacity, 35° concrete breakout cone strength, head pullout resistance, concrete shear breakout, pryout, and combined tension-shear interaction per ACI 318-19 Chapter 17 and AISC Design Guide 1.
ACI 318-19 Chapter 17 Anchor Capacity Diagnostics
ACI 318 Concrete Breakout Cone Geometry & Tension/Shear Vector Field
Schematic illustrating anchor embedment depth hef, 35° concrete breakout failure prism (pyramid), free edge boundary distance ca1, and anchor head bearing zone.
Mathematical Derivations: ACI 318-19 Chapter 17 Governing Limit States
5 Fatal Pitfalls in Structural Anchor Bolt Sizing & ACI 318 Compliance
1. Edge Breakout Neglect on Near-Edge Foundation Piers
When shear load acts towards a free concrete edge, breakout strength is governed by concrete edge distance ca1 rather than steel shear strength. If ca1 < 1.5 × hef, the concrete breakout prism cannot fully develop. A 1-inch anchor bolt capable of 30 kips in pure steel shear violently spalls off the corner of a narrow pedestal at just 8 kips!
2. Assuming Uncracked Concrete in Flexural Pier Zones
Designers often claim the 25% strength bonus for "uncracked concrete" (ψc,N = 1.25). But ACI 318-19 Section 17.6.2.5 mandates that concrete must be assumed cracked unless rigorous analysis proves tensile stresses under service loads do not exceed flexural cracking stress fr = 7.5λ√f'c. In wind, seismic, and moment-resisting baseplates, concrete is virtually always cracked!
3. Group Cone Overlap Overcounting (Spacing s < 3 × h_ef)
The failure cone of an anchor extends radially outward by 1.5 × hef in all directions, defining an ideal single-anchor projected area ANco = 9 hef². In a 4-bolt group with 6-inch spacing and 12-inch embedment, the 4 breakout cones completely overlap. The group does NOT have 4× the capacity of a single bolt—it provides barely 1.8× capacity!
4. Hooked "J" and "L" Bolts in High-Tension Foundations
Traditional smooth bent-bar J-bolts and L-bolts rely entirely on mechanical hook bend bearing. Under sustained tension, the concrete inside the bend crushes prematurely, allowing the hook to straighten and pull out like a hot knife through butter at 25% of steel yield strength. Modern codes and AISC Design Guide 1 strictly mandate headed anchor bolts or heavy hex nuts with anchor plates for structural moment frames.
5. Levelling Nut Baseplate Grout Gap Bending Arm
When columns are installed with levelling nuts and a 1.5" to 2.0" grout pad gap, shear forces cannot transfer via pure bearing friction until non-shrink grout is fully cured. During erection, the anchor bolts act as unbraced cantilever beams. The shear force induces severe local bending moments in the threaded shank, reducing tensile capacity by over 50%!
Frequently Asked Questions: ACI 318-19 Anchor Bolt Design
What are the 6 mandatory failure modes evaluated in ACI 318 Chapter 17?
ACI 318-19 evaluates: 1) Steel strength in tension (φNsa), 2) Concrete breakout strength in tension (φNcbg), 3) Pullout strength of anchor head (φNpn), 4) Steel strength in shear (φVsa), 5) Concrete breakout strength in shear towards free edge (φVcbg), and 6) Concrete pryout strength in shear (φVcpg).
What is the 35-degree breakout cone model?
Extensive empirical testing at the University of Stuttgart and University of Texas demonstrated that concrete tensile breakout fractures follow an approximate 35° angle from the anchor head to the concrete surface. This corresponds to an idealized pyramid extending a horizontal distance of 1.5 × hef in each direction, producing a base dimension of 3 × hef on each side (ANco = 9 hef²).
How is combined tension and shear interaction calculated?
Per ACI 318-19 Section 17.8, if Nua > 0.2 φNn and Vua > 0.2 φVn, the combined interaction must satisfy the trilinear or 5/3 power relationship: (Nua / φNn)5/3 + (Vua / φVn)5/3 ≤ 1.0. This elliptic curve accurately matches biaxial fracture test envelopes.
What is concrete pryout strength in shear?
Pryout occurs when short, stiff anchors subjected to shear force tilt in the concrete hole, prying out a chunk of concrete behind the anchor rather than shearing the steel shank. ACI 318 defines pryout strength as: Vcpg = kcp × Ncbg, where kcp = 1.0 for hef < 2.5", and 2.0 for hef ≥ 2.5".
Why is ASTM F1554 the premier structural anchor bolt standard?
ASTM F1554 was created specifically for structural foundation anchor bolts and is the only anchor specification endorsed by AISC. It provides three distinct yield strength grades: Grade 36 (weldable mild carbon steel), Grade 55 (high-strength alloy with mandatory Charpy V-notch toughness option), and Grade 105 (heat-treated alloy steel for heavy industrial columns).