الفهرس 
Title : Behavior of fully encased-high strength concrete composite columns subjected to axial and cyclic loads
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Abstract
Composite column is a structural member that uses a combination of structural steel
shapes and reinforced concrete to provide adequate load carrying capacity to sustain
either axial compressive loads alone or a combination of axial loads and bending
moments. The most usual types of composite columns are the concrete-filled steel tubes
and the partially or fully encased steel profiles. Fully encased composite column provides
compressive strength, stability, stiffness, improved fire proofing, and better corrosion
protection.
There is limited information on the performance of encased steel-concrete (ESC)
columns constructed of high-strength concrete. For instance, the analytical approach of
Eurocode 4 applies only to columns made of normal weight concrete in the strength
classes C20/25 to C50/60. As such, this thesis presents an experimental and numerical
investigation of the mechanical behavior of ESC columns made from normal strength
(NS) concrete class C24/30 and high strength (HS) concrete class C56/70. The
experiments comprise twelve ESC columns: four of which are axially loaded, while the
other eight are subjected to axial load combined with lateral cyclic loads. The main design
parameters investigated in the conducted experiments are concrete compressive strength,
encased steel ratio, and axial compression load ratio (percentage of applied load to the
nominal compressive strength of the composite column).
Relative effects of these parameters on strength enhancement, stiffness degradation,
displacement ductility, and mode of failure are examined. Measured experimental results
showed that ESC columns built of HS concrete failed abruptly with cracks passing
through aggregates, whereas ESC columns made of NS concrete failed more gradually.
For columns with encased steel ratios of 4% and 6%, the ESC columns made of HS
concrete had ductility indices that are less than those made of NS concrete by up to 25%
and 22%, respectively. Nevertheless, ESC columns built of HS concrete were able to
withstand up to 24% more lateral loads than corresponding ESC columns made of NS
concrete. Furthermore, increasing the axial load ratio resulted in a loss in ductility and
stiffness degeneration beyond the peak point. The finite element models are developed
using the finite element program, Diana v.10.5. First, the results of the created FE model
are compared to those of experimental research. It is found that the FE predictions
reasonably agree with the corresponding experimental behavior. Subsequently, the
validated FE model is utilized in an extensive parametric study to investigate the
performance of encased steel-high strength concrete columns under different design and
loading situations. The analyzed cases comprised columns with encased steel ratios of
over 10% carrying various levels of axial compression. These cases are investigated with
respect to column crack patterns, modes of failure, lateral strength, and ductility. The
numerical results showed that increasing the encased steel ratio results in increased lateral
strength and ductility. Moreover, in all cases, increasing the axial compression load level
adversely affected the cyclic behavior of encased steel-high strength concrete columns
by accelerating the degradation of column strength and stiffness.