According to the image above there are 3 failure
categories related to the shear loads, which correspond to numbers (2), (3) and (5).
The next image is shown for a better understanding of the stresses that are produced due to the shear loads and how the vertical reinforcement bars help to bear them.

Shear failures:

Shear failures (2): They are produced because of the principal tension loads.
The next image shows its direction and it can be seen that the shear crack is produced by these loads.
If the reinforcement is insufficient, the compressed part of the beam shall resist the greatest percentage of the shear load causing the cracks go towards the top edge of the beam

Bending + shear failure (3): If the transversal reinforcement is not enough,
the cracks will develop in the compressed concrete area reducing its capacity and producing the failure with bending
forces lower than the maximum designed bending ones.

Failure due to an inadequate horizontal reinforcement anchoring (5):
Shear and bending forces in a beam are closely related. Tension loads in longitudinal reinforcement increase from
the support to the center of the beam. That variation is balanced through the adherence stresses between the concrete
and the steel. If they are not enough, the reinforcement bars may slip around the support area where the shear load is at its maximum

Concrete shear resistance, involved parameters and checks to be made:

Shear capacity of a beam reinforced with ties is given by the
sum of two terms: the shear capacity of the concrete and that of the transversal reinforcement.

The most common arrangement is that of the transversal reinforcement
made of stirrups. Besides, in those places where the horizontal reinforcement is no longer necessary, they may be bent 45º,
taken to the compressed part and tied there or they may cooperate with the available reinforcement in that area, helping to resist the shear loads.

Two initial checks shall be done. Firstly, it is needed to check the concrete strut in compression (VRdmax) and if it does,
it shall be solved changing the beam’s characteritics either increasing its section or increasing
the concrete compression capacity. In second place the section’s shear failure shall be checked. If this happens, shear
reinforcement shall be added. The Eurocodes as well as the EHE provide some guidance about the minimum amount of reinforcement that shall
be placed, the minimum and maximum distances between bars and the longitudinal distance between the stirrups.

Hereafter, with this tool, the section design can be done and checked if additional shear reinforcement is needed.
In each section (calculation done with and without shear reinforcement), the formulae may be found
unfolding the formulae panels. More detailed information about the different types of concrete shear failure, comparing between the different codes (EC-2, EHE-08 and ACI) might be found
here.