Experimental behaviour of restrained reinforced concrete columns subjected to equal biaxial bending at elevated temperatures

This paper experimentally investigates the effects of equal biaxial bending, restraint ratio, and concrete strength on the structural behaviour of reinforced concrete columns at elevated temperatures. Nine full-scale column specimens equally grouped into three series were designed and tested with three levels of equal biaxial eccentricities. Three specimens of the first series were loaded to failure at room temperature to verify the EC2 Pt.1.1 simplified criterion for axial resistance of columns subjected to biaxial bending. The remaining six specimens in two fire-test series of different concrete compressive strengths (55 and 29 MPa, respectively) were first loaded to 55% of their EC2 eccentricity-dependent axial resistances and were then exposed to fire conditions with different restraint ratios (3.5% and 6.0%, respectively) until failure. Experimental results were compared with analytical and numerical predictions. It was consistently shown that (i) EC2 predictions were reasonable for the tested columns; (ii) lateral deflections of the heated columns were proportional to equal biaxial eccentricities; (iii) the development of thermal-induced restraint forces increased with equal biaxial eccentricities, restraint ratio, as well as concrete strength, and was overpredicted by numerical analyses that neglected concrete spalling; and (iv) the column failure times were adversely affected by equal biaxial eccentricities.