## 1 Introduction
The simplified calculation models to determine the ultimate strength of CFST columns loaded on a partially stressed cross-sectional area were presented in [23; 24; 25]. Usage of the steel tube as an external jacket in the strengthening of RC bridge columns was investigated by [PERSON], et al. [26; 27]. Strengthened members showed increased axial and shear capacity and greater ductility compared to plain RC columns. [PERSON] and [PERSON] [28] showed the results of an experimental analysis of square RC columns retrofitted by the steel jacket. [PERSON] and [PERSON] [29] investigated axially loaded RC columns strengthened with various jacketing methods. Jackets made from steel or carbon fiber composites with circular, square, and octagonal shapes were used on square RC columns. It was found that the steel-jacketed specimens showed greatly enhanced load carrying capacity and pronounced ductile behavior. [PERSON] and [PERSON] [30] studied the axial behavior of circular reinforced concrete columns strengthened by steel jackets and fiber-reinforced polymer composites. Moreover, they investigated a method of strengthening columns with concrete jackets reinforced with spiral steel rebar, premade welded steel wire fabric, and special steel reinforcement. All considered strengthening methods effectively increased the column's capacity and stiffness compared to the reference specimen. [PERSON] et al. [31] presented a study of a seismic retrofitting of RC frames using welded concrete-filled steel tubes. The RC frames strengthened with the CFST system showed increased bearing load, effective stiffness and capacity to dissipate energy, with values approximately two to three times greater than those obtained on the unstrengthened RC frame. Lately, there have been some innovative usage of fabric reinforced cementitious mortar as a way to confine concrete columns [32]. From the presented literature review, it can be noticed a lack of information on rectangular cross-section reinforced concrete columns strengthened with a steel jacket. This paper is, thus, an attempt to investigate the behavior of strengthened axially compressed RC columns. The main objectives of this paper are: to present experimental results of the behavior of the stub RC columns strengthened with the steel tube jacket and the concrete filling; to analyze the influence of compressive strength of the filling material on the behavior of strengthened specimens; and to predict the load-bearing capacity of the strengthened stub RC columns by using existing codes or sets of simplified expressions. The novelty of the study is the applicability of the presented method for strengthening the RC columns and confirmation of Eurocode 4 results in determining plastic resistance of the strengthened column. The paper is structured as follows: in Section 2, test specimens from the experimental program are presented. Next, experimental studies to determine the properties of the used materials are conducted, and the test setup was defined. In Section 3, results of the experimental testing on plain RC and strengthened column are presented. A discussion of the experimentally obtained results is provided in Section 4. The applicability of three different design codes to predict the axial bearing capacity of the strengthened columns is presented in Section 5. Section 6 summarizes the final remarks and the conclusions of the paper.