Concrete breakout assumes a failure forming a concrete cone based on a prism angle of 35 degrees. This method predicts the strength of a group of anchors by using a basic equation for a single anchor Nb, and multiplied by factors that account for the number of anchors, edge distance, spacing, eccentricity, etc.
The denominator is the projected breakout area of a single anchor, and the numerator is the group breakout area. The former can be easily calculated as 9 hef 2, but the latter may be quite difficult, since it depends on the location of the anchors and the geometric conditions of the support.
When the anchor group is located away from the concrete edges, the concrete cone will develop fully in all directions. However, if the anchors are located closer than 1.5 hef from one or two edges, the concrete cone cannot develop fully and it will be truncated, as shown above.
Furthermore, where anchors are located less than 1.5 hef from three or more edges, the value of hef used in the calculations shall be reduced as the larger of Ca/1.5 and s/3, where s is the spacing of anchors within the group. This is to account for the edge effects and to correct the otherwise unconservative calculation.
To illustrate the point, consider the same base plate in the two images below, generated by ASDIP STEEL. The left image shows a narrow support, but long enough to develop the full extent of the cone. In this case only two edges are closer than 1.5 hef from the tension anchors, therefore the actual rod embedment hef remains, and the concrete cone gets truncated at the short sides.
On the other hand, the right image below shows the same support but now the right edge distance is shorter, so we have now a narrow support at three sides from the tension anchors, which forces the embedment hef to be reduced accordingly. Note that the breakout area is now much smaller.
The breakout failure mode can be prevented by adding anchor reinforcement, designed and detailed to take the full tension at the failure surface of the concrete cone. If for any reason this anchor reinforcement cannot be added, then the breakout capacity should be carefully calculated.