Failure Mode Scaling Transitions in RC Beams in Flexure: Tensile, Shearing, Crushing

Reinforced concrete beams in flexure exhibit three different collapse mechanisms by varying the mechanical and geometrical parameters. The limit cases are: tensile failure for low steel percentages and/or small and slender beams, and crushing failure for high steel percentages and/or large and stocky beams. The intermediate collapse mechanism, and, therefore, the most frequent, is represented by diagonal tension failure, in which the collapse is dominated by unstable propagation of one or more shear cracks [1]. In this paper, a study of the transitions between these mechanisms is proposed inside the theoretical framework of nonlinear fracture mechanics. More in details, fracture nonlinearity as well as strain and dissipated energy localizations, both in tension and compression, are taken into account by means of the cohesive crack model and the dual overlapping crack model [2]. Due to the presence of different nonlinear contributions and to the strong interaction between the different collapse mechanisms, the problem is dealt with by a numerical procedure. Relevant results concern the prediction of the predominant collapse mechanisms, the failure load as well as the analysis of the mutual transition between the different failure modes by varying the scale, the slenderness and the reinforcing steel amount. Then, other specific aspects are also investigated, such as the problem of minimum reinforcement necessary to prevent the phenomenon of hyperstrength at low steel percentages, and the rotational capacity of plastic hinges. Both these aspects, also affected by size-scale effects, have practical implications in defining structural elements with ductile response, as required by current design codes [3]. A wide validation of the proposed mechanical model is finally presented on the basis of the results of experimental research programmes available in the literature, as well as recently carried out by the authors [4].