Performance of nodal regions of reinforced concrete frame corners subjected to opening bending moments

Design of frame corners in presence of opening moments is a task that has concentrated many research and engineering efforts in the past. It deals with the response of discontinuity regions, where beam theory does not hold valid and the use of equilibrium-based models (such as strut-and-tie models or stress fields) could be a suitable manner to understand the transfer of forces and to lead to consistent reinforcement detailing. However, the assessment of efficiency factors for equilibrium-based models is not straightforward for such details as their response has been observed to be governed in many cases by the tensile resistance of concrete and crack propagation. Furthermore, most efforts so far have been devoted to determining only the resistance of the region as a function of its reinforcement detailing, but limited works have been dedicated to their post-peak response. The post-peak response is nevertheless instrumental for redundant structures, as it governs the level of potential redistributions of internal forces in order to develop the full structural resistance of the system. Such aspect has become even more significant in present days, where the assessment of existing structures with potentially poor nodal detailing has become a need to avoid unnecessary retrofitting or replacement, in an effort to reduce carbon footprint of construction. In this paper, comprehensive research on this topic is presented. It is based on the results of a large database compiled by the authors comprising 126 tests. After filtering of the database, conclusions on the expected response of several details are obtained. Also, a general procedure grounded on an equilibrium-based model is proposed, providing tailored efficiency factors as a function of the mechanical reinforcement ratio and detailing type, but also providing a complete description of the post-peak response. This latter model is based on a kinematically-compatible displacement field for the post-peak phase, allowing to evaluate the opening of the cracks in the nodal region and the associated softening of resistance. • Database with tests on nodal regions of RC frames subjected to opening bending moments consisting on 126 tests. • Failures of nodal regions depend on the detail, mechanical reinforcement ratio and the presence of secondary reinforcement. • In brittle failures, toughness of nodal regions growths with secondary reinforcement; in ductile ones, it is always achieved. • Models for calculation of the resistance and the post-peak response of nodal regions are proposed.