Effects of boundary restraints on concrete-filled steel tubular columns with reinforcements exposed to fire

Abstract This study examines the fire responses of restrained concrete-filled steel tubular (CFST) columns subjected to axial and eccentric loads using high-temperature experiments. A new type of reaction steel frame was used to simulate the restraint stiffness of the boundary, and a set of adjustable knife-edge articulation systems provided the required end eccentricities and pin supports. Using this type of installation, the fire-resistances of CFST columns with finite restraint stiffnesses and different eccentricities were investigated and discussed. The experimental results indicate that low slenderness ratios can reduce the bending deformation and release the reaction forces induced by thermal actions. Owing to their high thermal capacities, large-scale specimens absorb more heat and are slow to heat up. When the eccentricity ratios exceed 0.33, the elongation increments are reduced. However, low eccentricity ratios cause the thermal reaction forces to quickly decay. The load is gradually redistributed to the surrounding structures through rigid connections between the columns and surrounding structures. Owing to the benefit of rotational restraints, the maximum reaction force ratios of fixed CFST columns accounted for 84%–86% of those of pinned CFST columns.