Nonlinear behavior of slender RC columns (2). Introduction of design formula

In this paper, a simple design formula to predict the resisting capacity of slender reinforced concrete (RC) columns is introduced. Using the proposed numerical model in the companion paper, nonlinear time-dependent analyses of slender RC columns that consider the creep deformation of concrete and the P-Δ effect are conducted to determine their ultimate resisting capacity according to the variation of design variables. On the basis of the numerical analysis results obtained, the resisting capacity reduction factors, which represent the reduction rates of the resisting capacity in a long column corresponding to a short column on a P-M interaction diagram, are determined. In addition, the relationships between the resisting capacity reduction factors and design variables are established from regression and used to determine the ultimate resisting capacity of slender RC columns without any rigorous numerical analysis. The ultimate resisting capacities calculated from the regression formula are compared with those constructed from rigorous nonlinear time-dependent analyses and from the ACI formula with the objective of establishing the relative efficiencies of the proposed formula.