Abstract: To investigate the effect of axial force variations induced by horizontal and vertical earthquakes on the seismic performance of reinforced concrete (RC) columns,three quasi-static tests were conducted on piers under constant axial force,variable amplitude and variable frequency of axial forces.Mechanical indicators,such as failure mode,hysteresis curve,skeleton curve,stiffness degradation,ductility and energy dissipation were studied.The numerical models were established based on the physical model,and the difference between the positive and negative peak loads and ultimate displacements were selected as quantitative indicators to assess the hysteresis asymmetry of the piers under different amplitude,frequency,and phase of variable axial force loading.The research indicates that variable axial loading leads to earlier cracking,yielding,and ultimate failure of the piers,accelerates the stiffness degradation rate after reaching the peak load,and results in significant differences in ductility levels on both sides.The smaller the initial axial compression ratio and the larger the amplitude of the variable axial force,the more pronounced the hysteresis asymmetry becomes.Changes in the loading frequency and phase between axial force and displacement can result in completely different hysteresis behaviors.The hysteresis behavior of the piers is primarily influenced by the axial compression ratio corresponding to the peak displacement,rather than the axial loading mode or the maximum axial force.Different variable axial loading modes essentially reflect changes in the relationship between peak displacement and axial force.