Numerical simulation and mechanical model of steel beam-to-column joints retrofitted by haunches

Abstract

Within the retrofitting of steel structures, welding haunches to retrofit weak panel zones in beam-to-column joints are deemed to provide reliable cyclic behavior based on previous tests. Also, the force transferring mechanism in haunched beam sections is substantially changed under the influences of the haunch and panel zone. For the investigation of the behavior of the haunched beam section and the haunch, finite element models (FEMs) were established with considerations of material and geometry nonlinearities and then verified against previous tests. The force distributions were obtained and also predicted by mechanical models. Results showed that the moment resistance and initial rotational stiffness increased by 34.3%–76.9% and 51.6%–83.3% through retrofitting. Moreover, a noticeable difference in force distributions was observed in the haunched beam section. The direction of the shear force within the shear plate was reversed and its value could be even greater than the beam load. After the yielding of panel zones, the force redistribution appeared and subsequently led to increased forces in the shear plate and haunch flange. Furthermore, the comparisons among mechanical models indicated that the AISC model might underestimate the contributions of both the haunch flange and the shear plate in the elastic stage, and the plane-section-assumption model showed better agreements. However, without considering the force redistribution in the post-yielding stage, the above two models underestimated forces by 8.8%–32.0% and 9.7%–64.2%, respectively, and the simplified model was proposed with better predictions concerning the impact of the force redistribution to provide a greater safety margin in design.