The results obtained from nonlinear fiber element analyses for high strength concrete filled steel tubes(CFT) are discussed. The flexural force-deformation behavior of high strength square concrete-filled steel tube (CFT) beam-columns was experimentally investigated. The CFT specimens were nominal B/t ratios of 20, made from either conventional (SM490) or high strength (HSB600, VCs 800) steel, filled with high strength (83, 91, and 120MPa) concrete. The studies were aimed at investigating the effect of higher material strengths on the structural behavior and creating mathematical models of the steel tube and filled concrete. The study focuses on load-deformation analyses for CFT columns with different stress-strain models for both concrete and steel. The predicted initial stiffness and moment-rotation responses of square CFT columns under flexural or eccentric loading are compared with existing experimental results to examine the accuracy of the theoretical model developed.