Expressions, which can evaluate the bending ultimate strength of reinforced concrete columns of rectangular, cross section about the principal and the diagonal axes continuously under the all range of axial load, were already proposed (see reference 3). And using them, we show the interaction curved surface of column strength (see reference 2). Continuously, this paper propose expressions for calculating bending ultimate strength of reinforced concrete columns of circular cross section, in connection with the expressions of rectangular cross section. By a series of these expressions, bending ultimate strength of almost all real reinforced concrete columns can be evaluated.
