Influence of Long-Term Loads on Concrete Column Strength

Two long-term loads concrete stress/strain models are introduced. One model, the typical long-term model, uses modified parabolic and linear equations to include creep and shrinkage effects. The other, the realistic long-term model, incorporates a realistic load path for concrete in compression as well as the creep and shrinkage phenomena. The proposed models were used to investigate the influence of their constituents on the ultimate strength of reinforced concrete-column cross sections under long-term loading. An analytic procedure was used to generate the strength-interaction relationships of the column cross sections. A series of calculations was performed for two selected column cross sections, and the short-term and long-term results are compared. The short-term model strengths computed using the same procedure compare well to the strengths computed according to ACI 318. Small differences are discernible between the strengths obtained using the typical long-term model and the short-term model at low reinforcement ratios. The differences increase at large reinforcement ratios. A noticeable reduction in strength can be observed above the balance point when using the realistic long-term model.