INELASTIC BUCKLING OF CONCRETE COLUMN IN BRACED FRAME

The paper proposes an improved method of analysis of reinforced concre te columns in braced (no-sway) frames, which is suitable as a simple c omputer solution for design practice and is more realistic than the ex isting ACI and CEB methods. The elastic restraint provided by beams ad jacent to columns is described by rotational springs. The inelastic be havior of concrete is defined by a uniaxial stress-strain curve with p ostpeak softening in compression and a zero strength in tension. Plast icity of reinforcement is also considered. The deflection curve is ass umed to be a sine curve. The improvement consists in considering the w avelength as unknown and variable during loading. The problem is reduc ed to a system of seven nonlinear algebraic equations, which are easil y solved for small increments of axial displacement by a standard libr ary optimization algorithm. The convergence always occurs and is fast if the increments are small enough. The influence of various parameter s on the load-deflection curve, the path in the diagram of axial load P versus moment M, and the failure envelope are studied. Various pheno mena, such as the possibility of a concave P(M) path at constant load eccentricity, are explained. It is shown that the ACI approach is slig htly conservative in most cases, although situations exist in which th e ACI approach is either grossly overconservative or slightly unconser vative.