Summary: | The concrete-filled double skin steel tubular (CFDST) column is becoming more
popular nowadays due to its superior performance compared to conventional
composite column and concrete-filled steel tubular (CFST) column. However, the
use of this type of column is still limited to outdoor construction such as bridge
piers and transmission towers where fire is not the main concern. Moreover, existing
research studies on the CFDST column only focused on fire performance, and limited
research studies can be found on the residual strength of theCFDSTcolumn. Residual
strength can be used to determine the most suitable repair method needed in order to
retrofit the column. Therefore, this study aims to study the effect of different parameters
on the residual strength of the CFDST column. Among discussed parameters
are the thickness of the outer steel tube (t0) and fire exposure time. In addition, this
study also aims to determine the effectiveness of the repair method using Single and
Hybrid fiber-reinforced polymer (FRP) of fire-damaged CFDST columns. CFDST
columns were heated in accordance with ASTM E119-11: Standard Test Methods
for Fire Tests of Building Construction and Materials until the temperature reached
600 °C. Afterwards, the temperature was kept constant for two different durations,
i.e., 60 and 90 mins. The specimen was then left to cool down to room temperature
inside the furnace before itwas taken out and repaired by Single and Hybrid FRP. The
specimens were categorized into the following three groups: (1) unheated or control
specimens, (2) heated and unrepaired specimens and (3) heated and repaired specimens.
All specimens were subjected to axial compression loading until failure. The
first and second category specimens failed by local outward buckling of outer steel
tube, crushing of concrete and local buckling of inner steel tube, whereas specimens
in the third category failed by rupture of FRP followed by similar local buckling and
concrete crushing as those observed in first and second category specimens. Ultimate
strength, secant stiffness and Ductility Index (DI) decreased as the temperature of
the specimen increased. The loss in secant stiffness of thinner CFDST specimens
exposed to 60 mins of fire exposure time is similar to thicker CFDST specimens
exposed to 90 mins of fire exposure time regardless of their diameter. In addition,
CFDST specimens exposed to 90 mins of fire exposure time were more ductile than
control specimen. RSI and secant stiffness increased with the increase in fire exposure time. Interestingly, the highest RSI achieved is only 22% whichmeans the specimens
were still able to carry more than 70% of their initial load after being exposed to 90
mins of fire exposure timewith only 3mmthickness of outer steel tube. Repairing the
fire-damaged CFDST columns with Single and Hybrid FRP is proven to improve the
ultimate compressive strength significantly. The increment in ultimate compressive
strength is more pronounced in the specimen with Hybrid FRP and thinner outer
steel tube. The secant stiffness and Ductility Index (DI) of repaired specimens were,
however, not able to be restored to those of the control specimen.
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