Parametric investigation on behaviors of eccentric RC exterior and interior beam-column subassemblies

Beam-column joints' behavior is a key research focus due to their critical role in maintaining structural integrity and safety during earthquakes. While concentric joints have been extensively studied, eccentric joints, where beams' centerlines deviate from columns' centerlines, remain relatively unexplored. Beam-column joints exhibit more complex behavior, experiencing a significantly higher shear force (approximately six-fold that of a column), along with flexural moments and normal forces. The joint's eccentricity introduces a torsional moment that amplifies shear stress, and the beam is subjected to an out-of-plane moment (around one-sixth of the primary moment). The out-of-plane moment reduces the joint's resistance mechanism by reducing the confinement provided by beams. To examine the influence of beam eccentricity on the ductility, strength, and failure mechanism of beam-column joints, comprehensive nonlinear finite element (FE) analyses were conducted utilizing experimental eccentric beam-column joints, followed by meticulous validation of the developed model. The models employed incompatible mode mesh element and energy-based models for the concrete material behavior, utilizing an explicit solver. The models were used to examine the impact of various geometric parameters, boundary conditions, and axial load levels on behaviors (global and local) of the beam-column joint subassemblies. The examined geometric parameters encompass aspect ratios of joint and column for both joint types (exterior and interior). Joint behaviors were observed to be significantly influenced by the boundary conditions of the column. Furthermore, the aspect ratio of the joint, representing the beam-to-column depth ratio, played a predominant role in determining the joint failure mechanism by limiting their ability to develop beam hinges. Increasing the aspect ratio of the column had a modest effect on the joint's shear strength but improved the joint subassemblies' ductile behavior, despite increasing the joint's torsional moment. Higher column axial load levels increased the joint's shear strength and led to a shift in cracking patterns from diagonal to vertical. Interestingly, the study found that interior joints experienced higher out-of-plane moments. Exterior and interior joints maintained a level of approximately 62.5% as a ratio between their shear strength across all models. Moreover, it was observed that yielding of joint stirrups correlated with a shear distortion angle of approximately 0.5x10-2rad. Restrictions of ACI provisions on the aspect ratio of the joint seem to be conservative for eccentric interior joints. On the other side, the ACI provisions might overestimate eccentric exterior joint shear strengths as the aspect ratio exceeds unity.