Anchor bolts transfer loads from steel columns, equipment bases, and structural attachments into concrete foundations. A failed anchor bolt can drop a column, release pressurized equipment, or let a handrail pull out under a person's weight. The design is governed by ACI 318 Appendix D (now Chapter 17 in ACI 318-19), which evaluates five distinct failure modes: steel rupture, concrete breakout in tension, pullout, side-face blowout, and concrete breakout in shear.
Each failure mode has its own capacity calculation, and the anchor design is only as strong as the weakest mode. This guide covers the failure modes, the breakout cone geometry that drives most designs, and the practical installation details that determine whether the anchor performs as calculated.
Five Failure Modes
Steel failure is the simplest: the bolt shank yields or fractures. This capacity depends on bolt grade and area. For a 3/4-inch F1554 Grade 36 anchor, the nominal tensile strength is about 15,500 lbs. Steel failure is ductile and predictable. It is actually the preferred failure mode because it gives warning before collapse.
Concrete breakout is the most common governing failure mode. In tension, the concrete fractures in a cone radiating outward from the anchor head at approximately 35 degrees. The breakout cone has a radius of 1.5 times the embedment depth (h_ef). In shear, the breakout is a half-cone radiating from the anchor toward the free edge. Pullout occurs when the anchor head or expansion mechanism cannot develop enough bearing pressure against the concrete. Side-face blowout applies to deep anchors near an edge. The designer must check all applicable modes and use the lowest capacity.
Anchor Bolt Pull-Out Calculator
Calculate anchor bolt capacity per ACI 318 Chapter 17 with tension and shear breakout checks.
Combined Tension and Shear
Most anchor bolts see both tension (uplift or overturning) and shear (horizontal force from wind, seismic, or equipment thrust) simultaneously. ACI 318 uses a tri-linear interaction equation: if the tension demand is less than 20 % of tension capacity, full shear capacity is available, and vice versa. When both exceed 20 %, the interaction check is N_ua/φN_n + V_ua/φV_n ≤ 1.2.
For equipment bases with known forces, resolve the applied moment into a tension-compression couple in the anchor bolt group. The outermost anchors take the highest tension. Add any direct uplift to the moment-induced tension. Shear is typically distributed equally among all anchors in the group unless the base plate analysis shows otherwise. Use the combined interaction check on the most heavily loaded anchor.
Installation Best Practices
Cast-in-place anchors (J-bolts, L-bolts, headed studs) are embedded before the concrete pour. They must be set to the correct embedment depth, location tolerance (typically ±1/8 inch for equipment, ±1/4 inch for structural columns), and held plumb during the pour. Use a template (a steel or plywood plate with holes matching the bolt pattern) to maintain bolt spacing and alignment. Wire the template to rebar or form bracing so it does not shift during vibration or concrete placement.
Post-installed anchors (mechanical expansion, adhesive, or undercut types) are drilled into cured concrete. Hole diameter, depth, and cleanliness are critical. A hole drilled 1/16 inch oversize or not cleaned of dust can reduce anchor capacity by 30–50 %. Follow the manufacturer's installation instructions exactly. ACI 318 requires this as a condition of using the published design values. Adhesive anchors require specific hole preparation (blow, brush, blow, brush, blow) and cure times that vary with temperature.
Anchor Bolt Pull-Out Calculator
Calculate anchor bolt capacity per ACI 318 Chapter 17 with tension and shear breakout checks.