Abstract: Reinforced concrete (RC) beams tend to elongate after flexural cracking and yielding. The elongation is restrained by the surrounding structural components in the RC frame and therefore axial comprehensive force is formed. The seismic performance and failure modes of RC frame joints are affected by the unexpected axial compressive force. The influence of axial restraint force on the shear transfer mechanism of the joints was analyzed. Six 1/2-scale RC beam-column subassemblies, four of which were restrained by an equivalent device, were tested under cyclic loading to study the impacts of the beam elongation on the seismic shear demand, strength and failure modes. The results show that the beam elongation effects on the seismic shear demand were more obvious than those on the strength. The beam elongation caused large compressive axial force in beams with the increase of the beam bending deformation. Compared with the unrestrained specimens, the shear demand of the joints due to the axial restraint force was increased by 1.14~2.22 times. The widths of the inclined cracks in the joint area were larger, and the damage to the joints were relatively more serious in the axially restrained specimens.