In order to reveal the P-δ effect of sliding columns in reinforced concrete structures with partial columns sliding at upper ends,the validity of a simplified model with upper end joints is verified.Then,the characteristics of the second-order moment of the sliding column without sidesway caused by the P-δ effect are investigated in different cases,the boundary condition considering the second-order moment is presented,and a method to determine the increment coefficient of the bending moment for the sliding column without sidesway is proposed.Moreover,the concrete nonlinearity is taken into account primarily by amending the parameters in the proposed method,and some examples are finally analyzed.It is found that the friction support located at the upper end of the sliding column under minor earthquakes can be approximately treated as a joint; and that,in some cases,the column's maximum bending moment increases owing to the second-order moment of the sliding column without sidesway caused by the P-δ effect and that it can be easily determined by using the first-order moment of the sliding column without sidesway and the aforementioned increment coefficient.