Overview of Factors Affecting the Behavior of Reinforced Concrete Columns with Imperfections at High Temperature

In this paper, the behavior of initially imperfect reinforced concrete columns with different eccentricity and end restraint conditions at elevated temperatures is studied. By constructing column curves for the heated columns, the combined and interrelated effects of initial imperfection patterns, rotational end restraint level, self-equilibrating thermal stresses, and end eccentricity condition for columns with different slenderness ratios are investigated. The study shows that for each column curve, there exists a characteristic fire resistance that is a maximum and not affected by end restraint characteristic. It was found that for all columns, there is a critical value of slenderness ratio after which column responses to fire change. It was also found that under certain conditions, an antisymmetrically loaded column with an initial out-of-straightness experiences a double-phase unwinding phenomenon and may fail in a single-curvature buckling mode, but they cannot buckle in an antisymmetrical double-curvature mode. This is quite different from symmetrically loaded columns with initial out-of-straightness of opposite sense of gravest buckling mode, which show a single-unwinding phenomenon and can fail in either a single- or a double-curvature buckling mode. The investigation revealed that the values of effective length factor at high temperatures are different from those at ambient temperature, and these values are slightly affected by the end eccentricity, column slenderness, and load ratio.