Thus, the concrete presence should not be neglected. In fact ignoring the concrete expansion effect practically leads to only local buckling, that is, between 2 consecutive stirrups, given their high stiffness and strength. Thus, for stirrups within the buckling
length of the longitudinal reinforcement, two sources of deformation are considered: first, the lateral expansion undergone by the confined concrete, and second, the additional tensile strain by lateral displacement of the longitudinal bar once buckling starts.