Shear capacity model for reinforced concrete joints

Studies on the behavior of reinforced concrete joints of frames in seismic areas have been conducted in recent decades especially, and principally following the most recent seismic events, during which the crisis of nodal panels has been recorded more frequently. For the same reason, technical guidelines have paid specific attention to the construction details of joints. The models for calculating the capacity of joints have been designed primarily for cases where beams do not converge on all sides. These joints are rightly considered to be more critical because they benefit less from confinement effects. On the other hand, experimental tests are conducted on nodal panels loaded in the plane and, also because of experimental difficulties, most full-scale tests are conducted on panels in which only one beam converges. These panels represent the least confined joints but, at the same time, they are less loaded than joints in which two beams converge. When the loads transmitted by the beams are particularly high (e.g., two strongly reinforced beams), design relationships provide large quantities of reinforcement that cause some difficulties in the construction phase. To overcome this issue partially, some guidelines suggest less severe checks if a lower dissipative capacity of the structure is needed. Nevertheless, joint failure is a fragile mechanism that should be avoided because it considerably affects the ductility of a structure. This paper presents a model of joint shear capacity based on a traditional mechanical approach (i.e., Mohr's circles), which explicitly considers the presence of single or double beams in the plane of the nodal panel. Using the average value of the material strengths, the model was validated with a database of almost 300 tests on nodal panels that had failed due to shear. The predictions have a reasonable scatter compared to the typical variability of the mechanical strength of concrete and the variability of the construction details with which the panels have been made, which reflects the variability with which the joints are made in common practice. When used with the design values of the material strengths, the model predicts the capacity of nodal panels subject to cyclic action with a level of caution sufficiently similar for cases with one beam and two beams. Therefore, its application allows a significant reduction of the amount of reinforcement for joints in which beams converge on all sides, compared to several models given in the literature.