Sulla duttilitá di elementi inflessi in conglomerato armato

The ductility of reinforced concrete (R/C) beams in bending is affected by numerous physical aspects, such as confinement by stirrups or loading plates, shape of the bending moment diagram, longitudinal reinforcement ratio, type of reinforcing steel, contribution of longitudinal reinforcement in compression, etc. These factors should be identified and always taken into account while studying flexural ductility both experimentally and theoretically. Therefore when looking for a general approach able to predict the ductility of R/C beams it seems necessary to specify the mechanical conditions of validity of a theoretical model. Starting from this necessary premise, in this paper it is proposed a mechanical model to study the ductility of four point bending moment R/C beams. In the constant bending moment zone stirrups and reinforcement in compression are absent while the collapse is mainly due by crushing of compressed concrete. In particular, the crushing is modeled by means of sliding planes, which allow the expulsion of concrete scraps. The sliding behavior is ruled by a parameter K that can be obtained in several ways, including uniaxial compressions tests on cylinders. The mathematical description of the mechanical behavior of this sliding phenomenon leads to a stress-strain relationship for concrete in compression with a particular softening branch.