ABSTRACT

The use of steel-concrete composite structures, has had a steady increase in 
recent decades, because they allow a better use of the materials characteristics, 
compared to reinforced concrete structures and steel ones, working 
independently.  
 
This research focuses on the study and analysis of concrete filled steel tube, CFT, 
rectangular or square, filled with high strength concrete. This type of columns 
has advantages over other steel concrete composite columns, that during 
construction the steel provides permanent formwork to the concrete. The steel 
tube can also support a considerable amount of construction loads prior to 
pumping the wet concrete, which results in fast, efficient and cheaper 
construction. Other advantages of CFT are that the steel tube provides 
confinement to the concrete core while the infill of concrete delays or eliminates 
local buckling of steel tubes. All these advantages increased load carrying 
capacity and ductility, important properties during earthquakes, and that is the 
main reason why CFT have been spread in countries like Japan, USA and China. 
Finally, it is worth nothing to highlight the increment in fire resistance of the 
system compared to non-filled steel tube.
         
After analyzing the current state of the art, an experimental campaign has been 
performed to cover not studied issues, and which are the most common in 
normal practice: slender or semi slender columns under axial load and non-
uniform moment diagram. The experimental results are compared with 
predictions of the most important standards for the calculation of composite 
columns: EC4, AISC 2010, BS5400: 2005, AS5100-6, DBJ13-51, ... checking their.
         
Finally, has been proposed a simplified calculation model, based on the 
amplification method adjusting the effective flexural stiffness for composite 
columns, and proposing an equivalent uniform moment diagram factor.