An improved design formula for a biaxially loaded slender RC column

Nonlinear analyses are conducted to evaluate the ultimate resisting capacity of slender reinforced concrete (RC) columns subjected to an axial load with biaxial bending moments. Consideration is given to the geometric nonlinearities caused by the PP–ΔΔ effect and the long-term behavior of concrete and to the material nonlinearities caused by the cracking of concrete and the yielding of steel. In addition, the biaxial stress state in an RC section is simulated on the basis of a fiber model. Because of the complexity of Bresler’s load contour method, which was introduced in the ACI 318 code, this paper introduces a new design approach to the construction of the failure surface of a slender RC column subjected to biaxial bending. Through a parametric study of slender RC columns, where consideration is given to the PP–ΔΔ effect and the time-dependent deformation of concrete, two regression formulas are proposed on the basis of the slenderness ratio and the creep deformation of concrete. Furthermore, the direct multiplication of the proposed formulas on the PP–MM interaction diagram for a short RC column subjected to axial force and a uniaxial bending moment enables a PP–MM interaction diagram to be generated for a slender RC column subjected to long-term axial force and biaxial bending moments. Correlation studies between analytical and experimental results are conducted with the objective of establishing the validity of the introduced numerical model. In addition, the ultimate resisting capacities calculated from the regression formula are compared with those obtained from rigorous nonlinear analyses and from the ACI formula, with the objective of establishing the relative efficiency of the proposed regression formula.