Anchor rods are elements designed to resist mostly tension forces, sometimes in combination with shear. Out of all the tension limit states required by ACI 318, concrete breakout is particularly important because a concrete failure would be non-ductile, and therefore it should be avoided. This blog post discusses how to calculate the concrete breakout capacity of anchor rods. Our software ASDIP STEEL will be used to support the discussion.
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What is the tension concrete breakout?
Tension 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. per ACI 17.6.2.
One of the most important factors in this equation is ratio of the two areas. The denominator is the projected breakout area of a single anchor, and the numerator is the projected breakout area of the anchor group. The former can be easily calculated as 9 hef 2, but the group breakout area may be quite difficult to calculate, since it depends on the location of the anchors in tension and the geometric conditions of the concrete support.
How do you calculate the concrete breakout area?
When the anchor group is located away from the concrete edges, the concrete cone will develop fully in all directions, and the breakout area will be relatively easy to calculate. 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 the tension 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 non-conservative 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. The breakout area is then 1080 in2, as shown below.
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 effective anchor embedment is 9", instead of 12", and the concrete breakout area is now 810 in2, much smaller than the original 1080 in2.
Is the calculation different for biaxial base plates?
For base plates subject to biaxial bending, the location and shape of the tension anchor group may be irregular and hard to calculate, particularly for narrow supports. In these cases the calculation can be time-consuming. ASDIP STEEL calculates the tension breakout area accurately for tension anchors in biaxial base plates as well.
The concrete breakout failure mode can be prevented by adding anchor reinforcement. This type of reinforcement must be designed and detailed to resist the full tension at both sides of the failure surface of the concrete cone. The development length of the rebars must be thoroughly checked for this purpose. If for any reason this anchor reinforcement cannot be added, then the breakout capacity should be carefully calculated.
The calculation of the tension concrete breakout area is affected by the location of the anchors and the geometry of the support. ASDIP STEEL accurately calculates this area and shows graphically the dimensions. This is useful when you have narrow and irregular supports, and the area calculation is not straightforward.
For engineering background on tension anchor rod design please see my blog post Anchor Rod Design – The Complex ACI Provisions. For our collection of blog posts about base plate and anchorage design please visit Anchor Rods Design.
Javier Encinas, PE
ASDIP Structural Software