Modelling of shear hinge mechanism in poorly detailed RC beam-column joints
A simplified analytical model for reinforced concrete beam-column joint behaviour is presented and proposed as a viable tool for extensive parametric studies of the seismic response of existing frame systems. Based on experimental results for gravity-load-designed beam-column subassemblies and on a frame system, the concept of a shear hinge associated with the joint damage mechanism is introduced as an alternative to flexural plastic hinging and the observed implications for local and global level response are described. According to a concentrated plasticity approach, an equivalent rotational spring, governing the relative rotation of the beams and columns, is adopted to represent the joint behaviour in the linear and nonlinear range. The monotonic moment-rotation characteristics of the spring can be directly derived from equilibrium considerations of the bending moments of the adjacent elements, corresponding to principal tensile stress levels in the mid-depth of the joint panel zone. An appropriate hysteretic rule with "pinching" behaviour to take into account both bar slipping mechanisms or shear cracking in the joint region should be adopted to model the cyclic behaviour. Preliminary numerical-experimental comparisons with cyclic tests on beam-column subassemblies designed for gravity loads only, as typical of the Italian construction practice between the 1950s and 1970s, are also provided.