Tension Field Action Theory: 
In this theory, we assume that the diagonal compressive force in the web is high enough to cause its buckling, and therefore, we neglect the ability of the web to resist this diagonal compression and we provide vertical stiffeners so that the vertical component of the diagonal compression is resisted by the stiffeners and the horizontal component of the diagonal compression is transmitted to the flanges and flange has to resist it. 
The vertical stiffeners and the top and bottom flanges, along with the web are taken to provide a truss action of N shapes truss; where a truss whose top and bottom chords are the top and bottom flanges, vertical stiffeners act as the compression members while the thin web providing a tension field acting as a diagonal tension member is formed.

Panel A is provided with vertical stiffeners due to which it is designed based on tension field action. These vertical stiffeners also called bearing stiffeners will prevent the buckling of web plate due to diagonal compression.