Haunch retrofit of RC beam–column joints: Linear stress field analysis and Strut-and-Tie method application

This paper addresses the stress field of reinforced concrete (RC) beam–column joints retrofitted with haunches. Design of such solution currently assumes internal forces evaluated by the so called β-factor approach, which was originally conceived targeting the enhancement of steel moment-resisting frames. Extension to RC is subsequent as it emerges from the literature survey. The analytical model is first critically rediscussed. Inconsistencies of the adopted structural scheme, with respect to the actual mechanical behavior, may lie on the compatibility conditions which are imposed between the haunch and concrete beam (or column). In this regard, two-dimensional finite element models (FEM), using linear-elastic materials, are employed to study the stress field of two benchmark specimens derived from literature. A partial validation is carried out against experimentally derived internal forces. Results show that, for haunches with extended flat plates and stiff diagonals, compressive diffusion affects the entire haunch region. Consequently, beam's kinematic hypothesis of linear strains is no longer valid. The predicted joint shear demand resulted underestimated by β-factor approach by 50%. Since 2D FEM may be not efficient for many practical circumstances, an application of Strut-and-Tie is alternatively proposed. Finally, both the limitations and possible extensions of the proposed approaches are stated transparently.