Structural fire design of square tubed-reinforced-concrete columns with connection to RC beams in composite frames

The tubed-reinforced-concrete (TRC) column is an innovative type of steel-concrete composite column that is gaining increasing engineering usage, and structural fire performance should be an important consideration of its design. As affected by the adjacent members and beam-to-column joints, the fire behaviour of TRC columns integrated into a frame may largely differ from that of isolated columns with idealised end conditions. However, the fire performance of TRC columns in composite frames has not been studied, except for some work done on the fire performance of isolated TRC columns with certain end restraints. This paper presents finite element analysis on single-storey and single-span non-sway frames with square TRC columns, employing the software package ABAQUS. The thermal and deformation behaviour, development of axial stress, axial force and bending moment during heating, working mechanism and failure mode of the frame exposed to fire are numerically investigated. The difference between the fire performance of TRC columns in a frame and those of equivalent isolated columns is quantified. The fire resistance of the framed column is found to lie between those of the columns with rotational end restraints and columns with idealised pinned ends. The complex effects of the connected RC beam on the fire behaviour of TRC columns, e.g. the beneficial restraining effect and the negative effect due to restrained differential thermal expansion, are clarified. Finally, a practical fire resistance design approach is proposed for TRC columns considering the continuity and interactions between structural elements within a composite frame.