Abstract This paper presents planar joint enlargement for increasing the shear strength of sub-standard beam–column joint. Five beam–column specimens were tested under quasi-static cyclic loads. Two control specimens were tested. One was the beam–column joint without joint enlargement and the other was the beam–column joint strengthened by monolithically cast square enlargement. The other three specimens were strengthened by joint enlargements with various sizes. Test results indicated a brittle joint shear failure in the first control specimen and a beam flexural failure in the second control specimen. Depending on the size of enlargement, the failure types of strengthened specimens ranged from flexural failure in the beam to crushing failure in joint panel and enlarged areas. The joint enlargement can increase the strength, stiffness and energy dissipation. The experimental results indicated the decrease in joint shear stress in strengthened specimens. However, the distribution of horizontal shear stress is not uniform, with larger value in the joint panel than in the beam sections within the enlarged areas. Finite element analysis has been conducted to examine the flow of stresses in the joint and enlarged areas. The finite element analysis illustrates the principal diagonal strut in the joint panel and additional struts along the edge of enlargements and in beam sections within the enlargement zone. Based on principal stress pattern obtained from finite element analysis, strut-and-tie model is constructed to analyze the strengthened specimens. The proposed strut-and-tie model can predict the failure mode, column shear force and tension forces in longitudinal steel bars and dowels. It can also predict the distribution of horizontal shear stress in the joint panel and beam sections within the enlarged areas.