You are probably here because you designed an anchorage using engineering software, one or more checks failed, and you were not sure what to change next.
This tutorial is written for new engineers and engineering students who want to understand anchor failure modes under ACI 318-19 and how to adjust a design logically. This is not a replacement for the code. For full provisions and requirements, always refer to ACI 318-19 Capítulo 17.
The goal here is to help you recognize what is failing, why it is failing, y which design parameters actually increase capacity, instead of randomly changing inputs.
If you want to see how these checks are applied step by step in a design workflow, you may also refer to the Software de diseño de placa base SkyCiv, which reports all ACI anchor checks with complete calculations.
What Is an Anchor?
An anchor is typically a steel rod embedded into concrete to connect another structural element, most commonly a steel base plate. Anchors transfer tension, el cortante, or combined forces from steel into the concrete support.
Anchors are commonly classified by installation method.
Cast-In Anchors
Cast-in anchors are placed before concrete is poured and become embedded as the concrete hardens.
Post-Installed Anchors
Post-installed anchors are installed into hardened concrete by drilling holes and fastening the anchor using:
- Mechanical expansion
- Adhesive or chemical bonding
Which One Is Better?
Neither anchor type is inherently better. The choice depends on constructability, project constraints, and availability. Por ejemplo, if a steel column is added to an existing slab or footing, cast-in anchors are no longer an option, and post-installed anchors are typically used.
Availability also matters, as anchor types, tamaños, and installation limits depend on manufacturer supply. Common anchor manufacturers include Hilti, DeWalt, y Fischer, each offering different mechanical and adhesive anchoring systems with product-specific design data and installation requirements.
Single Anchors vs Anchor Groups
When anchor checks fail, the failure does not always occur at just one anchor. Depending on the layout, failure may occur at a single anchor or across a group of anchors acting together. ACI 318 makes this distinction because the governing failure mode and capacity can be very different.
Whether a failure is evaluated as a single anchor failure or an anchor group failure depends primarily on the overlap of projected failure surfaces. This overlap is typically controlled by anchor spacing, profundidad de empotramiento, and edge distance.
To visualize this behavior during design, tools such as the Software de diseño de placa base SkyCiv display projected failure areas and automatically determine whether anchors are evaluated individually or as a group based on geometry.
Single Anchors
If anchors are widely spaced or have shallow embedment depth, their projected failure areas do not overlap. En este caso, failure is evaluated at the individual anchor level. One anchor may reach its limit without significant contribution from adjacent anchors.
Grupos de anclaje
When anchors are placed closer together with sufficient embedment depth, their projected failure surfaces overlap. En este caso, the concrete limits the capacity of the entire group, and failure occurs when the combined projected failure area reaches its limit. The group capacity is not equal to the sum of individual anchor capacities.
This distinction is critical because several ACI tension and shear checks explicitly change depending on whether failure is governed by a single anchor or an anchor group. Misidentifying the governing failure type can lead to unconservative or overly conservative designs.
Ejemplos de diseño
Design examples illustrating single-anchor and anchor-group failures can be found in the SkyCiv base plate design resources. Here is a sample set of design checks performed by the Software de diseño de placa base SkyCiv.
Anchor Tension Checks per ACI 318-19
When anchors are subjected to tension, ACI 318-19 requires several checks. Each check corresponds to a different physical failure mechanism. Once you understand the mechanism, it becomes much easier to adjust the design.
Steel Strength in Tension
Anchor steel check considers rupture of the anchor steel.
Its capacity depends on:
- Diámetro de anclaje
- Anchor material strength
Larger diameters provide higher tensile capacity. Higher material grades increase capacity but also increase cost.
Common anchor materials include ASTM F1554. A practical design approach is to start with lower grades such as Grade 36, then increase to Grade 55 or Grade 105 only if required by demand.
For diameter selection, many engineers begin in the range of 1/2 inch to 3/4 pulgada. If demand is higher than expected, increase the diameter. If demand is much lower, the size can be reduced later. This judgment improves with experience.
If the anchor diameter and material grade are already maximized and the steel tension check still governs, adding more anchors may be an option. This typically requires adjusting spacing, distancias al borde, or base plate dimensions. Adding additional rows is permitted, but it changes load distribution and should be evaluated carefully.
Capacity Equation:
\( NORTE_{a} = A_{se,norte} F_{uta} \)
Concrete Breakout Strength in Tension
Concrete breakout occurs when a cone-shaped portion of concrete separates from the support. En este caso, the anchor steel remains intact, but the surrounding concrete fails.
This failure mode applies to headed anchors, expansion anchors, screw anchors, and undercut anchors.
How to Increase Concrete Breakout Capacity
Increase embedment depth
The breakout cone is idealized as extending from the embedded end of the anchor to the concrete surface. Increasing embedment depth enlarges the cone and significantly increases capacity. Embedment depth also directly increases the basic breakout strength defined by ACI.
Increase anchor spacing
Closely spaced anchors restrict the width of the projected failure area. Increasing spacing allows a larger effective breakout area, particularly for anchor groups.
Increase edge distance
Anchors placed near edges cannot develop a full breakout cone. Increasing edge distance often results in a noticeable capacity increase.
Use higher-strength concrete
Upgrading from a lower grade concrete to a higher grade concrete increases the basic breakout strength and is often effective when geometry is constrained.
Assume non-cracked concrete when appropriate
Uncracked concrete provides slightly higher capacity. This assumption should only be used when justified, as it changes design assumptions.
Provide reinforcement designed to carry tension
When reinforcement is explicitly designed and detailed to carry the anchor tension force, concrete breakout checks may be waived. This must be an intentional design decision, not an assumption.
Capacity Equation for Single Anchors:
\( NORTE_{cb} = frac{UNA_{Carolina del Norte}}{UNA_{Recuerda}} \Psi_{ed,norte} \Psi_{c,norte} \Psi_{cp,norte} Nótese bien \)
Capacity Equation for Anchor Groups:
\( NORTE_{cbg} = frac{UNA_{Carolina del Norte}}{UNA_{Recuerda}} \Psi_{CE,norte} \Psi_{ed,norte} \Psi_{c,norte} \Psi_{cp,norte} Nótese bien \)
Anchor Pullout Strength
Pullout failure occurs when the anchor is pulled out of the concrete without forming a full breakout cone. This check applies to cast-in anchors and certain post-installed mechanical anchors and is evaluated for individual anchors only.
Pullout failure typically indicates insufficient bearing at the embedded end of the anchor.
How to Fix Pullout Failure
- Increase embedment depth
- Increase anchor diameter
- Increase hook length for hooked anchors
- Use larger bearing plates or bolt heads
- Consider non-cracked concrete where justified
Pullout failures are usually addressed by improving bearing conditions rather than changing spacing or edge distances.
Capacity Equation:
\( NORTE_{pn} = Psi_{c,pag} N_p \)
Concrete Side-Face Blowout Strength
Side-face blowout occurs when an anchor with relatively deep embedment is placed too close to a free edge. Instead of forming a breakout cone, the side face of the concrete fractures and blows out.
This failure mode is governed by the relationship between:
- Embedment depth
- Edge distance
Both single anchors and anchor groups must be checked.
How to Fix Side-Face Blowout
- Increase edge distance
- Increase spacing between anchors
- Reduce embedment depth where possible, while maintaining other capacities
Side-face blowout is a clear indication that embedment depth and edge distance are not well balanced.
Capacity Equation for Single Anchors:
\( NORTE_{sb} = 160c_{a1}\sqrt{UNA_{brg}}\lambda_a\sqrt{f’_c} \)
Capacity Equation for Anchor Groups:
\( NORTE_{como} = left(1 + \frac{s}{6C_{a1}}\verdad) NORTE_{sb} \)
Bond Strength of Adhesive Anchors
For post-installed adhesive anchors, tension capacity is governed by bond strength.
Bond failure considers:
- The bond between anchor and adhesive
- The bond between adhesive and concrete
Both single anchors and anchor groups are evaluated.
How to Increase Bond Capacity
- Increase embedment depth
- Increase anchor diameter
- Increase spacing and edge distances for anchor groups
- Use an adhesive with higher characteristic bond stress
Characteristic bond stress values are based on experimental testing. When test data is unavailable, conservative values from ACI 318-19 should be used. Assuming non-cracked concrete is permitted but should be done with sound engineering judgment.
Capacity Equation for Single Anchors:
\( N_a = \frac{UNA_{Na}}{UNA_{Nao}} \Psi_{ed,Na} \Psi_{cp,Na} NORTE_{licenciado en Letras} \)
Capacity Equation for Anchor Groups:
\( NORTE_{ag} = frac{UNA_{Na}}{UNA_{Nao}} \Psi_{CE,Na} \Psi_{ed,Na} \Psi_{cp,Na} NORTE_{licenciado en Letras} \)
Anchor Shear Checks per ACI 318-19
This section will be published soon.
Anchor Tension and Shear Interaction Checks per ACI 318-19
This section will be published soon.
