Anchor Bolts Calculator

Steel anchor bolt design per ACI 318-19 Chapter 17. Tension, shear, and combined loading capacity including breakout and pullout limit states. Educational use only.

This page documents the scope, inputs, outputs, and computational approach of the Anchor Bolts Calculator on steelcalculator.app. The interactive calculator runs in your browser; this documentation ensures the page is useful even without JavaScript.

What this tool is for

Checking tension, shear, and combined loading capacity for cast-in or post-installed anchors per ACI 318 Chapter 17.
Evaluating all anchor failure modes: steel failure, concrete breakout, pullout, side-face blowout, and pryout.
Understanding the CCD method (Concrete Capacity Design) for breakout strength with edge and spacing effects.

What this tool is not for

It does not design the base plate or the connection between the base plate and the column.
It does not handle adhesive (chemical) anchor design in detail or seismic qualification per ACI 355.
It does not check the concrete pedestal or foundation for global stability.

Key concepts this page covers

steel strength of anchor in tension and shear
concrete breakout strength (CCD method)
pullout strength and bearing capacity
anchor group effects and edge distance modifications
phi factors: 0.75 tension, 0.65 shear (AISC/ACI)

Inputs and outputs

Typical inputs: anchor diameter, embedment depth, anchor type (headed, hooked, post-installed), concrete strength f'c, edge distances (ca1, ca2), anchor spacing, number of anchors, supplementary reinforcement, and factored tension/shear demands.

Typical outputs: capacity for each failure mode (steel, breakout, pullout, pryout, blowout), controlling failure mode, modification factors applied, combined tension-shear interaction check, and demand-to-capacity ratio.

Computation approach

The calculator evaluates each limit state per ACI 318-19 Section 17.6 (tension) and 17.7 (shear). Concrete breakout uses the CCD method: the basic breakout strength of a single anchor (based on embedment depth and concrete strength) is modified by factors for edge distance, spacing, eccentricity, cracking, and lightweight concrete. Combined tension and shear is checked using the tri-linear interaction equation from ACI 318-19 Section 17.8.3.

Frequently Asked Questions

What is the CCD method for concrete breakout? The Concrete Capacity Design (CCD) method assumes a 35-degree breakout cone projected from the anchor head to the concrete surface. The basic breakout strength of a single anchor in tension is Nb = kc sqrt(f'c) hef^1.5 (in US customary units). This basic strength is then modified for edge distance (the breakout cone is truncated), anchor spacing (overlapping cones reduce per-anchor capacity), eccentricity of the resultant tension force, and whether the concrete is cracked. The CCD method is the basis of ACI 318 Chapter 17.

Why are the phi factors different for anchor tension and shear? ACI 318 uses phi = 0.75 for anchor steel strength in tension and phi = 0.65 for anchor steel strength in shear (when the anchor is governed by a brittle failure mode in concrete). The lower shear phi reflects the greater uncertainty in concrete breakout and pryout failure modes, which can be sudden and brittle compared to ductile steel yielding. If supplementary reinforcement is provided to restrain the breakout cone, the phi factor may be increased.

What is the difference between cast-in and post-installed anchors? Cast-in anchors (headed bolts, J-bolts, headed studs) are placed before the concrete is poured and develop capacity through bearing on the anchor head. Post-installed anchors (expansion, undercut, adhesive) are installed in hardened concrete by drilling a hole and engaging the concrete through expansion, mechanical interlock, or adhesion. Post-installed anchors generally require product-specific qualification testing and may have different capacity equations than the generic ACI 318 provisions for cast-in anchors.

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.