Anchorage of shear reinforcement in beams and slabs

Anchorage of shear reinforcement, such as links or stirrups, can be performed by providing hooks, bends or heads at their ends, by welding transverse reinforcement or by closing open stirrups with pins. Hooks and bends, also used to enhance the anchorage of flexural reinforcement at the end regions of beams and slabs, have often been preferred because of their simple and cost-effective production. Such anchorages present nevertheless several peculiarities that shall be accounted for. They are relatively sensitive to their detailing (mandrel diameter, length of the tail segment between bar end and bent region) as well as to the cracking state of the surrounding concrete. Also, brittle failures can occur due to spalling of the concrete cover in case of bars near to a free surface. The anchorage with hooks and bends is currently still widely designed according to old detailing rules, based on the results of tests performed in many cases with materials whose properties are significantly different than those of nowadays. Also, no mechanical models are available for a consistent verification and detailing, acknowledging the different potential failure modes and their interaction with the surrounding concrete. In an effort to provide a consistent approach to its design and verification, this paper presents an investigation addressed at understanding the mechanical response and performance of bend and hook anchorages. To that aim, the results of an experimental programme performed with state-of-the-art instrumentation are introduced. By means of analysis of Digital Image Correlation and Fibre-Optic Measurements, the complete transfer of forces is analysed under different circumstances. On that basis, a consistent mechanical model is developed and validated, also reproducing a large variety of tests found in the literature. The paper introduces eventually several practical considerations on the activation of shear reinforcement in beams, and the level of performance required at its anchorages.