Structural behaviour and design of end-restrained square tubed-reinforced-concrete columns exposed to fire

Tubed-reinforced-concrete columns are gaining increasing usage in engineering practices, whereas research on their fire behaviour is still limited and mainly concentrated on unrestrained columns. Since end-restrained columns might behave differently in fire as compared to the unrestrained columns, the fire behaviour of square tubed-reinforced-concrete columns with end restraints was investigated via numerical modelling in this study. The axial and rotational restraints were modelled using linear springs and then validated against fire test results. The effects of end restraints on the failure mode, axial force and buckling length at failure and fire resistance, and on the evolution of deformations, internal force and bending moment during heating, of end-restrained columns were systematically analysed through parametric studies. The axially-restrained columns mostly undergo axial contraction only in fire; their typical failure mode is uncontrolled overall axial deformation, except for those columns of relatively high axial restraints. An increase in axial restraint leads to a nearly linear decrease of axial force at failure and a linear increase of fire resistance. For rotationally-restrained columns, the typical failure mode is also uncontrolled overall axial deformation. As the rotational restraint increases, the bucking length at failure decreases and the fire resistance increases, which is particularly significant when the rotational restraint is relatively low. A practical design method is proposed for the fire resistance design of square tubed-reinforced-concrete columns with either axial, or rotational, or combined axial and rotational restraints. Both constant and temperature-dependent end restraints have been adopted and the outcome is that they provide very similar results.