Why are spacing, edge distance, and embedment important for concrete anchors?
Anchors transfer loads into concrete, making them vital for safe design.
Their strength depends not only on the anchor itself but on how it interacts with the concrete.
Three factors are key: spacing, edge distance, and embedment depth. If these are not respected, the anchor may lose capacity and the concrete can crack, spall, or fail.
Understanding these details is essential for designing safe, durable, and code-compliant connections.
Spacing Between Anchors
Proper spacing between anchors is essential because concrete doesn’t behave like steel: it is strong in compression but weak in tension. When an anchor is loaded, it spreads stress into the concrete in the shape of a cone. If anchors are too close together, these cones overlap, which means the stresses combine and the concrete between them is forced to carry more tension than it can handle. This often results in splitting or premature cracking, and both anchors lose much of their capacity.
By providing adequate spacing, each anchor can develop its full pullout and shear strength, and the slab or foundation is able to distribute loads safely into the surrounding mass of concrete. This not only increases the reliability of the connection, but also prevents progressive failures, where the failure of one anchor weakens nearby ones.
A good rule of thumb is to keep at least 12 times the anchor diameter as spacing to the next anchor or to a concrete edge. Always check the manufacturer’s specifications for the exact values to use.
Edge Distance
The edge distance is the required spacing between an anchor and the closest free edge of the concrete. It plays a crucial role because the concrete around an anchor transfers the applied loads through a cone-shaped zone of stress.
If the anchor is installed too close to an edge, this cone cannot fully develop within the concrete mass. Instead, the stress zone is cut off, which makes the edge more vulnerable to cracking or spalling. In this situation, the anchor may fail prematurely, not because of the steel itself, but because the surrounding concrete cannot resist the concentrated forces.
Ensuring sufficient edge distance provides the anchor with enough material to distribute loads evenly, improving both the strength of the connection and the durability of the concrete element.
Watch a Demonstration of Edge Breakout !
Embedment Depth
The embedment depth is the distance an anchor is installed into the concrete, and it has a direct impact on its performance. As the anchor’s embedment decreases, the cone of influence becomes smaller, which reduces the amount of concrete engaged. This results in a noticeable decrease in both tensile and shear capacities compared to the maximum tested values.
To ensure proper performance, the concrete thickness must also be considered. A general guideline is that the slab or member should have a thickness of at least 1.5 times the anchor embedment depth. This prevents the breakout cone from extending beyond the bottom surface of the concrete and ensures that the anchor can fully develop its strength.
Providing sufficient embedment depth therefore allows the anchor to transfer forces into a larger volume of concrete, increasing both load resistance and long-term reliability of the connection.
Further Guidance
For safe and reliable design, it is important to go beyond general rules of thumb. Always consult the technical manuals provided by the manufacturer of the anchors you are using, or rely on specialized software that performs these calculations accurately. Most importantly, always consult a qualified design professional before installing any anchors that could affect the life and safety of the public or property.
Here are a few helpful resources:
• DeWalt Anchor Software – Technical Manual
• Hilti PROFIS Engineering – Technical Guides
• Simpson Strong – Technical Guides
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When an anchor is installed into concrete, there is an area surrounding the anchor called a cone of influence in which the anchor is affecting and it is affected by.
When two anchors are spaced too closely to one another and/or too closely to an edge, the anchor’s cone of influence reduces or becomes interfered with.
When this occurs, the anchor’s maximum tension & shear capacities, which are obtained from test data, are significantly reduced.
Most manufacturers provide reduction tables for tension & shear capacities for these limitations as these are common occurrences in the field.
As the anchor’s embedment into the concrete decreases, the anchor’s ‘cone of influence’ also decreases and there will be also be a decrease in tension & shear capacities from maximum tested values.
However, embedding an anchor too close to the edge of the concrete can lead to spalling damage (like in the video below).
A rule of thumb is that an anchor should generally have a minimum of 12x the diameter of anchor spacing to an adjacent anchor or to any concrete edge
(check with manufacturer specifications for the actual values to be used, see some guides below).
Also, the concrete should have a minimum thickness of 1.5x the depth of embedment of the anchor.
Watch a video of this happening: Engineering Express Test Video of Anchor Into Concrete Edge Distance Failure
Consult the technical manuals of the anchor brand you are using or look for software that helps with these calculations. And always consult a design professional before installing any anchors that can affect the life and safety of the public or property.
Here are a few links to software & technical guides to help:
DeWalt Anchor Software – Technical Manual
Last Update: September 15, 2025