The design of members with shear reinforcement is based on a truss model (see figure below).

where

α is the angle between shear reinforcement and the beam axis, 

θ is the angle between the concrete compression strut and the beam axis perpendicular to the shear force, 

F_td is the design value of the tensile force in the longitudinal reinforcement, 

F_cd is the design value of the compression force in the direction of the longitudinal member axis, 

b_w is the minimum width between tension and compression chords, 

z is the inner lever arm. In shear analysis without axial force, the approximate value z = 0.9 d may normally be used.