Influenza del tension stiffening sulla duttilità degli elementi pressoinflessi in c.a.

The ductility of the reinforced concrete elements depends on the local behaviour of the cracked section and on the spreading of the plastic strain. After cracking in the concrete, slip at the concrete-steel interface occurs and the local behaviour of the cracked section loses its importance. The bond between the two constitutive materials put in tension the concrete and, for this reason, in the un-cracked section the tensile stress in the concrete must be considered in the equilibrium equations. This phenomenon is known in literature with the name of tension stiffening effect and it limits the deformation capability of the element. In this paper a formulation of the mean curvature of a reinforced concrete element is proposed and adopted to evaluate the influence of the tension stiffening effects on the ductility. The equation of the mean curvature is function of the mechanical and geometrical properties of the reinforced concrete element, such as the dimension of the cross section, the amount of the longitudinal steel in tension and in compression and of the stirrups, the compressive strength of the concrete and the yielding stress and the hardening moduli of the steel. A parameter, able to highlight the influence of the tension stiffening effect on the ductility, is introduced. The obtain results show that the tension stiffening effect cannot be neglected when it is necessary to evaluate ductility characteristics. For example, the estimation of the ultimate deformation capacity with reference to the local section can cause an error of about 50%, if the hardening modulus of the steel is low.