Effects of Biaxial Bending and Axial Restraint on Hybrid Fiber Reinforced High-Performance Concrete Columns at Elevated Temperatures

The presence of axial restraint and biaxial load eccentricity plays a vital role in the behavior and design of columns under fire conditions. Because there is limited research on these two parameters, an experimental study on hybrid fiber reinforced high-performance concrete (HPC) columns subject to standard fire curve is performed to explore the effect of biaxial bending and axial restraint. A total of four full-scale HPC columns are tested to explore the structural and spalling behavior at elevated temperatures. Test results including thermal and structural responses are elaborated, and temperature distribution, structural deformation, thermally induced restraint force, crack pattern, and fire endurance are analyzed and discussed. Explosive spalling does not occur in all the columns even under biaxial load eccentricity and axial restraint, which is attributed to the presence of steel and polypropylene fibers. It is shown that thermally induced restraint force accelerates the deformations of restrained columns. Consequently, the fire endurance of restrained columns is reduced compared with unrestrained columns. Besides, a numerical model to trace the development of axial deformation and restraint force is proposed. The proposed model is validated with test results with respect to temperature profiles, axial deformation, and restraint force. It shows that the proposed model has reasonable accuracy.