It is similar like a group pile where the stress regime will overlap. This will cause an efficiency that will developed within the regime. Normally it will reduced the capacity to a certain percentage. The actual value of reduction is yet to be known by reserachers who are interested in this area.

It is similar like a group pile where the stress regime will overlap. This will cause an efficiency that will developed within the regime. Normally it will reduced the capacity to a certain percentage. The actual value of reduction is yet to be known by reserachers who are interested in this area.

The problem is a minimum energy failure surface problem.  This problem occurs in soil mechanics, rock mechanics and concrete.  Yous should look at some fundamental research papers in these areas.

The basic approach is to consider failure surfaces that treat the anchors as individual failures and also with degrees of overlap.  The shape is usually considered as a truncated cone, although the correct shape is more like a truncated hyperboloid, with the precise surface depending on the relationship between tensile strength and shear strength.  For unreinforced concrete, the approximate failure surface is usually taken as a truncated cone with an internal angle of 60 degrees. 

The anchors pull out analysis for concrete is given in detail in ACI 318-14. This analysis is based on the Concrete Capacity Design (CCD) Method by Fuchs et al. The internal angle for failure cone is taken as 55 degrees. The projected concrete failure area for a single anchor is given by :

Anf = (2x1.5hef)x(2x1.5hef) =9 hef^2.................(Eq1)

where hef= anchor impediment depth in concrete.

In case the center-to-center (c/c) distance between anchors is greater than 1.5 hef, the anchors act individually. If the c/c distance is less than 1.5 hef, the anchors are considered to act as a group and the group capacity is always less than the sum of the individual capacities. Also, if an anchor is located near the edge of concrete and its distance from the concrete edge is shorter than 1.5 hef, the anchor capacity is reduced. The group capacity is calculated by replacing, in Eq. 1 above, 1.5hef with the c/c distance between the anchors and the actual distance from the edge of concrete.

There is no failure pattern as such.Wedges overlap & the one which has more lapping is likely to fail first although there is safety factor of 1.5 as illustrated above.

Sometimes assumed wedge pattern do not occur, then it could be any eventuality.