## 1 Introduction  
A large number of reinforced concrete (RC) structures over time have their usage altered, and they experience deterioration or damage caused by different causes. RC elements are often degraded by enlarged loads, inadequate maintenance, a more aggressive environment, etc. Raised demands by renewed seismic codes often cause an older RC structure to have insufficient earthquake resistance. In all of the mentioned situations, forces in structures are increased, so it is necessary to strengthen the load-bearing elements. In the case of the columns, a steel tube, also called a jacket, can be placed over the entire height of the column, while the remaining space between the column and jacket is filled with additional concrete. Strengthened in this way, the RC column has increased load-bearing capacity to all additional permanent and live loads on the structure. The load-bearing capacity of the strengthened column is affected by the level of engagement of the composite cross-section parts, e.g., steel jacket, concrete fill, and basic RC column. Due to the coupling effect with concrete, the steel jacket enables the formation of a hoop stress state, which significantly increases the composite action and column load-bearing strength. The effect of the hoop stress causes the biaxial stress state in the steel tube and the triaxial state of stress in the concrete core. In addition to these states, the concrete core prevents any inward local buckling of the steel jacket. When the force simultaneously acts on the concrete core and the steel tube at exploitation load levels (usually up to 50% of ultimate load), different lateral expansion of concrete and steel can occur due to their different Poisson ratios. Then, the steel tube will have greater lateral strains compared to concrete, which can decrease the desired composite action of the steel-concrete cross section. If the load is applied only on the concrete part of the composite cross-section, the influence of different Poisson ratios can be diminished [1]. Over time, a great number of studies have been conducted on composite columns made as concrete-filled steel tubes (CFST). Static and dynamic behavior of stub and slender specimens was investigated on rectangular, square, or circular sections in order to determine their response to compression and to investigate the influence of the cross-section shape on the lateral confining stresses development, such as in [2; 3]. [PERSON] and [PERSON] [4] studied the effects of the concrete compressive strength and steel tube wall thickness on the behavior of concrete-filled steel tubes. [PERSON], et al. [5] presented results of the extensive research of the influence of the physical and mechanical properties of steel tubes in CFST members. In recent times, usage of high-strength and ultrahigh-strength concrete as filler of steel tubes was investigated [6; 7; 8; 9]. In those papers, researchers showed a linear relationship between the concrete compressive strength and the bearing capacity of the CFST columns. The ductility and strength of the specimens grow by increasing the concrete strength and confinement index. Usage of the high-strength steel was investigated in [10; 11; 12], while usage of stainless steel in CFST members was presented in [13; 14; 15]. Stainless steel composite members showed larger ductile behavior when compared with regular carbon steel CFST columns, and stainless steel members had higher residual strength. Innovative usage of composite columns, such as double skin CFST members [16; 17; 18], bimetallic tubes [19], or fiber-reinforced plastic tubes [20], is considered today. The investigation of the behavior of CFST stub columns with circular, square, and rectangular sections with the partially loaded cross-section was presented by [PERSON], et al. [21; 22; 23], [PERSON] [24], and [PERSON] [25]. The behavior of composite columns with the partially loaded area was considerably different from the fully loaded members, mainly due to the early development of the confining stresses in concrete.