## 4 Analysis of the Test Results
Presented values of the force distribution deviate from the theoretical values because of the differences in values of longitudinal strains in concrete and steel (\(\varepsilon c\
eq\varepsilon s\)). Results show that load transfer from the core RC column to the fill concrete and onto the steel tube starts to disrupt as a consequence of the slippage on the contact between two concrete. That means that the basic core column starts to carry more of the axial force, while both steel tube and fill concrete carry less of the applied load. The same behavior was registered in all three groups. After the experimental testing, the steel jacket was removed from the specimen to observe the failure mode and to record the cracks in fill concrete, as shown in Figure 18a. Figure 14.3 Comparative relationships for theoretical load distribution between three parts of the composite column. Figure 14.4 Experimental load distribution for strengthened specimens: B2x-Fill 1 (**a**), B4x-Fill 2 (**b**), and P0x-Fill 3 (**c**). In general, there were two failure modes for the strengthened columns: (A) the failure with a pronounced shear plane and diagonal cracks, as shown in Figure 18b, and (B) the failure with longitudinal cracks and indistinguishable shear plane, as shown in Figure 18c. The different modes of failure were attributed to the strength of fill concrete and to the level of confinement stresses which the steel jacket provides to the concrete core. Change of the failure mode from type (A) to type (B) contributed to the increase of fill concrete compressive strength and to the reduction of the confinement factor. For column models filled with concrete types 1 and 2, the column strength loss was determined by the compression failure, which occurred at the principal shear planes in the places where the shear stresses reached their maximum. After that, major diagonal cracks were formed, as seen in Figure 18b, which led to a decrease in the specimen load-bearing capacity. In addition, the steel jacket reached its yield stress and was not effective in restraining the relative slipping among formed shear planes. For specimens filled with high-strength concrete (Fill 3), which had the lowest confinement factor, the possible movement among the opposite shear slipping planes was restricted by the lateral confining pressure induced by the steel jacket. Therefore, fill concrete showed only the longitudinal cracks formed under high confining pressure. The confinement factor (\(\xi\)), also called the constraining factor, is commonly used to evaluate the compressive performance of steel tubes filled with concrete [21; 22; 23; 24]. It is believed to be one of the major factors affecting the composite interactions between the concrete core and the steel tube. The confinement factor for CFST was defined by [PERSON] et al. [36] as:  
Figure 14.5  
Similarly, to include the expected composite action between the two different concrete types, confinement factor was defined in the expanded term as:  
Figure 14.6  
where \(A_{s}\), \(A_{c,c}\), and \(A_{c,f}\) are the areas of steel jacket, basic (core) column concrete, and fill concrete, respectively;\(f_{y}\) is the steel yield strength; and \(f_{ck,c}\), and \(f_{ck,f}\) are the characteristic compressive strength of basic column concrete and fill concrete, respectively. The charac  
Figure 14.7 Strengthened specimens after removing steel tubes (**a**) and cracks in fill concrete—diagonal (**b**) and longitudinal (**c**). teristic compressive strength of the concrete is related to the average compressive cube strength as:  
Figure 14.8  
The values of concrete compressive strength presented in Table 3 are determined using the compressive strength of cubes. Detailed expressions for coefficients used in Equation (7) are given in [40].