Shear and Punching 101: Distance in the codes

 I recently received a message with the following question:

Normally when we treat the problem of determination of shear stress in slabs, a critical section at a distance of half the effective depth of slab is taken from the support (like columns). Can you explain why this is recommended in many Codes?

Since I think this issue is not very clear in the background of the building codes, I’d like to share what I found while executing my literature review here.

There are two reasons for a distance of d/2 away from the column:
– for punching, you determine the punching perimeter at a certain distance (depending on the code). There is no physical explanation for the distance itself, although researchers like to relate this distance to the inclination of the shear crack that would result from the root of column to the top of the slab where the punching cone intersects (for the case of a flat slab floor for example).
If you look at the background of the codes, for example ACI 318, you find that the chosen distance is based on a better statistical result for the resulting punching perimeter in combination with the ACI formula as compared to test results (work done by Moe, 1961 [1]).
– for shear, we assume direct transfer of the load from its point of application to the support for loads that are at a distance d/2 to d from the column (also depending on the considered code). This direct transfer is by means of a compressive strut, which is of course much stronger than a section in shear.

[1] Moe, J. (1961). Shearing strength of reinforced concrete slabs and footings under concentrated loads, Portland Cement Association Research and Development laboratories, Skokie, IL.