The use of high strength materials for high-rise structures has become popular, which in turn leads to smaller member sizes for compression members, which in most cases result in slender columns. However, most of the current design codes, such as Eurocode 4, are still limited to normal strength concrete (C20/25-C50/60) and normal strength steel (S235S460). This thesis discusses the behavior and strength of slender fully encased composite (FEC) columns made with normal and high strength materials under eccentric loading. A total of 150 FEC columns are numerically simulated, among which 86 are made with high strength materials to address the limitation of EC 4 and strain compatibility issues between concrete and steel. The finite element package ABAQUS 6.13 software is used, in which material and geometric nonlinearities are considered. Validation of the FE model is done by using experimental reports Zhao et al. (2010) and Kim et al. (2012). The finite element results have shown a good agreement with experimental results. The critical parameters investigated in this study are column slenderness ratio (L/D), compressive strength of concrete, structural steel contribution ratio, yield strength of steel, e/D ratio, and tie reinforcement spacing. The ultimate load capacity of FEC columns made with normal and high strength materials is reduced by 65.04% and 76.13%, respectively, with the increment of L/D ratio (from 10 to 35). On the other hand, for columns made with normal strength concrete (NSC) and normal strength steel (NSS), 14.18%, 27.14%, and 12.93% improvement is noticed in ultimate load carrying with the increment of compressive strength, structural steel contribution ratio, and yield strength of steel respectively. Similarly, when the compressive strength of concrete is increased from C60/75 to C120/140, the ultimate load capacity of FEC columns made with high strength concrete/high performance concrete (HSC/HPC) is enhanced by 35% and 21.96%, respectively, for columns with L/D ratio of 10 and 25. Increasing the yield strength of structural steel has no beneficial effect for columns made with a combination of materials other than NSC and NSS. It is also shown that the strength formula proposed in Euro code 4 could be extended for slender FEC columns made with HSC/HPC and HSS.