Behavior of axially-and-rotationally restrained concrete columns with ‘+’-shaped cross section and subjected to fire

Most of the previous investigations studied the structural behavior of concrete columns with ‘+’-shaped cross section at room temperature, but the behavior of such columns during a fire and the effect of boundary conditions on the mechanical properties of the heated columns have seldom been examined. In this paper, the influence of axial-and-rotational restraint on the behavior of heated concrete columns with ‘+’-shaped cross section is investigated. A self-developed finite element program RCSSCF is applied in this study. The development of column internal forces as well as deflections at the mid-height of concrete columns are discussed. Simulation results show that: (1) axial restraint can induce significant additional axial forces in concrete columns with ‘+’-shaped cross section and subjected to fire, and the additional axial forces in strongly restrained concrete columns during a fire can reach approximately 65–70% of the axial forces in columns at room temperature; (2) the fire resistance of concrete columns without rotational restraint decreases significantly with an increase of load eccentricity ratio, but that with rotational restraint is influenced lightly by load eccentricity ratio; and (3) for columns with different non-zero rotational restraint ratios, the internal axial forces, and the internal moments and deflections at the mid-height of these columns appear to follow common trends.