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Recent Research

Behavior of Encased and Filled Composite Steel Box Columns in Fire


Fire may cause structural damage or collapse, and seriously endanger the human
safety. Columns are main supporting members in structural systems, thus columns
play an important role in the fire safety. In recent years, steel reinforced concrete
structures are frequently used for medium- and high-rise buildings in Taiwan.
Concrete filled steel box columns are usually used in engineering practice. When
filled composite columns are subjected to axial compression at elevated temperatures,
different material properties will cause discrepancy in strength degradation and
thermal expansion of the materials. This discrepancey may influence fire resistance of
the filled composite columns. This work investigates effects of design parameters on
the fire resistance of the filled composite columns.
This study elucidates experimentally and numerically to explore the behavior of
axially loaded filled composite columns in fire. Four specimens were designed and
tested at elevated temperatures to study the effects of shear studs and axial
compression on fire behavior. The size and spacing of the shear studs are referred to
engineering practice. Axial compression, corresponding to either 0.3 or 0.6 times
column axial strength, was applied to the specimens. Finite element analyses were
performed for both heat transfer and nonlinear static analyses, and, consequently,
sequentially coupled thermal stress analysis was conducted to combine the effects
from temperature and loading. The analysis models were used to simulate the test
results and, furthermore, used to carry out parametric study.
During the test, axial expansion of the column and strength provided by
materials resisted the applied constant axial compressive force. Therefore, less axial
compressive force resulted in greater axial elongation of the specimen and longer fire
resistance time. Applied axial compression highly influenced the axial deformation
and fire resistance of the specimens. However, the shear studs increased slightly the
fire resistance of the filled composite columns subjected to axial compression. The
modes of failure of the specimens included shortening of the specimens, bulge of the
column steel plate, and crushing of concrete inside the steel box. To simulate the test
condition, finite element analysis reasonably calculated temperature distributions on
the cross section in time history, and accurately predicted the tendency of the axial
deformation of the specimens.
According to these research results, the following suggestions are proposed.
For immediate strategy:
The design philosophy of domestic design codes for steel reinforced concrete
members is based on the concept of superposition. This concept implies no need of
shear studs although shear studs are used for shear transfer between concrete and
structural steel. The findings of this work indicate that shear studs can increase
slightly the fire resistance for axially loaded filled composite columns. The research is
needed that whether the shear studs affect the fire resistance of filled composite
columns subjected to additional flexural moment.
For long-term strategy:
The mechanical and thermal properties of concrete at elevated temperatures
depend on concrete mix proportions and admixtures. These properties are lack for
concrete used in domestic. It is needed that symmetrically study and collection of
mechanical and thermal properties of materials used in steel reinforced concrete
structures in order to be referenced in the research and practice.