Abstract:
This research investigates the axial compression behavior of fire-damaged circular concrete-filled steel tube (CFST) stub columns externally confined with fiber-reinforced polymer (FRP) sheets. A total of 38 CFST specimens were tested under axial compression to investigate the load capacity, ductility, stiffness and failure mode of fire-damaged CFST columns before and after strengthening with FRP sheets. The test variables include the level of fire damage (no damage and 2-hour ISO standard fire), compressive strength of infill concrete (24MPa and 55MPa), type of FRP sheets (carbon and glass), and the number of FRP layers (0, 1, and 2 layers). The test results showed that 55-MPa CFST columns are more damaged by fire than 24-MPa CFST columns. However, confinement effectiveness due to FRP wrapping is similar. The level of strength enhancement due to FRP for fire-damaged CFST columns is lower than undamaged ones. In contrast to undamaged CFST columns, FRP cannot improve the stiffness of fire-damaged ones. Furthermore, an increase in the number of FRP layers improves the load capacity of specimens but deteriorates the ductility. The glass fiber sheets provide competitive strength enhancement and superior in ductility enhancement than carbon fiber sheets. An analysis of compression behavior of FRP-confined fire-damaged CFST columns is also proposed. Three main components include (1) two-dimensional finite element heat transfer analysis, (2) Post-fire mechanical properties of steel tube and concrete, and (3) analytical model that incorporates the confinement effect. The validity of the proposed model is shown by comparing the predicted load-strain relationships with experimental results.
