Creep Effects in Slender Reinforced and Prestressed Concrete Columns
R. I. Gilbert and N. C. Mickleborough
Appears on pages(s):
buckling; columns (supports); cracking (fracturing); creep properties; prestressed concrete; reinforced concrete; serviceability; shrinkage; slenderness ratio; Structural Research
A nonlinear procedure is described for the time-dependent analysis of reinforced and prestressed concrete columns under sustained eccentric compression. Both material and geometric nonlinearities are take into account in an iterative computer-based solution procedure. Individual cross sections are analyzed using the age-adjusted effective modulus method to include the effects of creep and shrinkage. By dividing the time scale into several increments, the gradual development of time-dependent cracking can be traced as the lateral deflection of the column and the internal secondary moments increase with time due to creep. Analytical predictions are shown to agree with laboratory measurements and numerical results from a parametric study of the behavior of slender columns are presented. The analytical model is also used to make a critical examination of the design provisions in ACI 318-83. Results indicate that the moment-magnifier method contained in the code for the design of slender columns becomes very conservative for very long columns.