This thesis presents the procedure to develop a new analytical model to define the axialrncapacity of concrete encased composite column by considering confinement effect on therncore concrete from structural steel and transverse reinforcement. First a method to definernthe unconfined, partially confined, and highly confined concrete areas ware developed. Torndefine this, arching action was assumed to act in the form of second-degree parabola withrnan initial tangent slope of 450. The arching action is expected to occur horizontally betweenrnlongitudinal bars and the tips of structural steel flange and vertically between transversernreinforcement. Then a mathematical formulation was developed to define the lateralrnconfining pressure from both transverse reinforcement and structural steel on the partiallyrnand highly confined concrete. By using the lateral confining pressure, the partially andrnhighly confined compressive strength of concrete was determined. Finally by using therndefined area of concrete and by their corresponding confined compressive strength,rnanalytical model to define the axial compression resistance of concrete encased compositerncolumn was developed. And the analytical model is verified using eleven experimentalrnconcrete encased composite column results. It was showed that the analytical model haverna good approximation with the experimental result.rnParametric study was carried out to investigate how material and geometrical variablesrnaffect the confinement effectiveness in axial compression resistance of concrete encasedrncomposite columns. Compressive strength of concrete, yield strength of structural steel,rnand yield strength of transverse reinforcement ware selected as a material variables.rnMoreover, thickness of structural steel flange, width of structural steel flange, distributionrnof longitudinal reinforcement, and spacing of transverse reinforcement ware selected as arngeometrical variables. From the result, it was observed that partially confined concreternhighly affected by the transverse reinforcement spacing, compressive strength of concrete,rnand yield strength of transverse reinforcement. In addition, highly confined concreternhighly affected by the compressive strength of concrete, transverse reinforcement spacing,rnand yield strength of structural steel. Additionally by developing a finite element modelrnthe effect of structural steel yield strength on the confinement effect is showed.