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. In this paper, a study of the transitions between these mechanisms is proposed inside the theoretical framework of fracture mechanics. 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 hyper-strength at low steel percentages, and the rotational capacity of plastic hinges. Both these aspects, also affected by sizescale effects, have practical implications in defining structural elements with ductile response, as required by current design codes.