Anchor Bolts Reference

Reference guide to anchor bolt terminology and typical limit states. Not a substitute for code-based concrete anchorage design.

Where steel design meets concrete design

Anchor bolts sit at the boundary between two different design disciplines. The steel side (base plate bending, bolt tension, shear on the bolt shank) is governed by steel codes like AISC 360 or AS 4100. The concrete side (breakout cones, pullout, side-face blowout) is governed by concrete codes like ACI 318 Chapter 17, AS 3600, or EN 1992-4. Many engineers are comfortable with one side but less familiar with the other, and this is where errors concentrate. A base plate design that checks out perfectly for steel limit states can still fail if the anchor embedment is too shallow or the edge distance is too small for the concrete to develop the required breakout capacity.

There are two broad categories of anchors: cast-in-place and post-installed. Cast-in-place anchors (headed bolts, hooked bars, headed studs) are set into the concrete before it is poured, providing reliable mechanical interlock with the surrounding concrete. Post-installed anchors (expansion anchors, undercut anchors, adhesive anchors) are drilled and set after the concrete has cured, and their capacity depends heavily on installation quality, hole condition, and the specific product's tested performance data. Most building codes require post-installed anchors to be qualified through testing programs (e.g., ACI 355.2 or ETAG/EAD for European products).

The critical concept in anchorage design is that multiple failure modes must be checked independently, and the governing mode is the one with the lowest capacity. A single anchor loaded in tension may fail by steel rupture of the bolt, concrete breakout of a cone around the anchor, pullout of the anchor from the concrete, or side-face blowout if the anchor is near an edge. Under shear, the modes include steel shear, concrete breakout toward a free edge, and concrete pryout. When both tension and shear are present simultaneously, an interaction equation (typically per ACI 318 Section 17.6) must be satisfied.

Anchor design checklist

When reviewing or designing an anchor bolt connection, verify the following:

• Anchor type is identified. Cast-in headed bolt, hooked bar, post-installed expansion, post-installed adhesive -- each has different capacity equations and installation requirements.
• All tension limit states are checked: steel tensile rupture, concrete breakout in tension, pullout (for headed anchors: bearing on the head; for adhesive: bond strength), and side-face blowout (when the anchor is near an edge).
• All shear limit states are checked: steel shear, concrete breakout in shear (toward a free edge), and concrete pryout.
• Tension-shear interaction is evaluated. ACI 318 Section 17.6 uses a tri-linear interaction: if both the tension and shear utilization ratios exceed 0.2, then the sum of the ratios must not exceed 1.2.
• Edge distances and spacing are adequate. Concrete breakout capacity drops sharply as edge distance decreases. Minimum edge distances also apply for constructability (cover, reinforcement clearance).
• Embedment depth is sufficient. Shallow embedment shifts the governing failure mode from steel to concrete. Increasing embedment is often the simplest way to increase concrete breakout capacity.
• Group effects are accounted for. When multiple anchors are close together, their breakout cones overlap, reducing the group capacity below the sum of individual capacities.
• Strength reduction factors match the failure mode. Concrete anchorage phi factors are lower than steel phi factors. AISC uses phi = 0.75 for anchor steel in tension and phi = 0.65 for anchor steel in shear. ACI 318 uses different phi factors depending on the failure mode and whether supplementary reinforcement is present.

For the full verification and documentation workflow, see How to verify calculator results.

FAQ

What is the difference between cast-in and post-installed anchors? Cast-in anchors are embedded in the concrete before it is poured and rely on mechanical bearing (headed bolts) or bond (hooked bars) for load transfer. Post-installed anchors are placed after the concrete cures by drilling a hole and either expanding against the hole wall (mechanical expansion or undercut) or bonding with adhesive. Cast-in anchors generally provide more reliable capacity because the concrete around them is undisturbed. Post-installed anchors require strict quality control during installation and must be qualified through standardized testing.

What is the most common failure mode for anchor bolts? Concrete breakout in tension is the most frequently governing failure mode, especially when embedment depths are shallow or edge distances are small. Designers often focus on the steel strength of the bolt and overlook that the concrete around it may fail first. This is why anchorage design requires checking every failure mode independently -- steel and concrete -- and using the lowest capacity.

How does the tension-shear interaction equation work? Under combined loading, ACI 318 Section 17.6 checks whether the anchor group can resist the applied tension and shear simultaneously. If both the tension utilization (Nu/phiNn) and shear utilization (Vu/phiVn) are below 0.2, no interaction check is needed. If either exceeds 0.2, the code requires that the sum of the two ratios not exceed 1.2. Some other codes use a circular or elliptical interaction equation instead.

Why does edge distance matter so much? Concrete breakout capacity is directly related to the size of the breakout cone, which is truncated when the anchor is close to a free edge. Reducing edge distance from 1.5 times the embedment depth to 0.75 times can cut the breakout capacity roughly in half. Edge distance also affects side-face blowout, a failure mode specific to deep anchors near edges where a shallow cone of concrete spalls off the side face.

How do anchor group effects reduce capacity? When anchors in a group are spaced close together (typically less than 3 times the embedment depth), their individual breakout cones overlap. The concrete between them is shared, so the group breakout capacity is less than the number of anchors multiplied by the single-anchor capacity. The ACI 318 Concrete Capacity Design (CCD) method accounts for this through projected area ratios.

Related pages

• Base plate & anchors calculator
• Concrete footing calculator
• Column capacity calculator
• Tools directory
• Reference tables directory
• Guides and checklists
• How to verify calculator results
• Disclaimer (educational use only)

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.