Effect of concrete strength on biaxial bending moment

This paper evaluates the ultimate strength of high-strength concrete (HSC) columns subjected to both axial load and biaxial bending. Fifteen square section (200 × 200 mm) columns with concrete compressive strength of 34, 57 and 64 MPa, respectively, were axially loaded with bi-directional eccentricities. The loading point was at an angle of 0°, 22·5° and 45° and the distances of eccentricity were determined to induce tension, balanced and compression failures, respectively. Test results were compared with the results of nonlinear analysis on P–M interaction curves and load contours. It was observed that, as concrete strength increases from 34 to 57 MPa, the concrete strength has a minor effect on the biaxial moment strength of reinforced columns, but as concrete strength increases up to 64 MPa, the concrete strength has a significant influence on biaxial flexural strength. This observed result is complementary when compared with the load contour method of ACI, in which biaxial bending design constants of columns are limited to concrete compressive strength of 41 MPa and estimate the effect of concrete strength to be minor.