Flexural Stiffness of Reinforced Concrete Columns and Beams: Analytical Approach

Abstract

The present ACI code 318-02 provisions on effective stiffnesses of beams and columns are reviewed. Factors influencing moments of inertia of beams and columns are discussed. The primary variables considered are: the reinforcement ratio, the axial load ratio, the eccentricity ratio, and the compressive strength of concrete. On the basis of a parametric study, simple formulas are proposed to determine the effective stiffnesses of reinforced concrete columns and beams. The proposed stiffness expressions are applicable for all levels of applied loading, including both service and ultimate loads. The analytical results show that the flexural stiffness assumption in the current ACI code procedure for design of slender columns using the moment magnifier method (Eq. 10-11 and 10-12) is extremely conservative. Recommendations are made concerning stiffness assumptions in the analysis of reinforced concrete frames under lateral loads.