Cracking and failure mode in HSC hollow beams under torsion

•Cracking development and failure mode of HSC hollow beams under torsion are studied.•Sixteen HSC hollow beams were tested under torsion.•The variables study were the concrete’s compressive strength and the total torsional reinforcement ratio.•The influence of the variables study on the cracking development and failure mode is analysed.•A new type of fragile failure mode was indentified. This paper presents the failure mode and the development of cracking analysis in high strength hollow beams under pure torsion up to failure. Sixteen hollow beams were tested. The beams had a constant cross-section and were symmetrically reinforced along the four membranes. The variable parameters were the concrete’s compressive strength (46.2–96.7MPa) and the total torsional reinforcement ratio (0.30–2.68%). The strength, ductility and the formation of plastic hinges were already presented by the authors in previous articles [1,2]. The beams presented different types of behaviour, failure mode, cracking development and showed the damages provoked by the torque loading. Such results complement the cited articles. The authors found an unexpected new type of failure (corner break off) that takes place for a certain interval of reinforcement ratio and that does not allow the beams to reach the maximum strength and ductility that would be expected if this type of failure did not exist.