Compressive performance of steel fiber-reinforced concrete-encased steel composite stub columns

Concrete and steel are materials with extensive use in human construction activities. Concrete is a material with high stiffness which is less expensive than other available construction materials, and steel is a material with high strength and ductility. Steel-concrete composite structural systems have been utilized in the construction of high-rise buildings due to their superior structural behavior. Fibrous concrete-encased steel columns are one of the most important composite structural members in which the axial load is carried by the steel and concrete at the same time. These columns are attracting the interest of many researchers due to their excellent structural performance under both static and seismic loading conditions. The steel-concrete interaction enhances the performance when carrying monotonous and earthquake loading. Reinforcing steel fibers help control crack propagation and prevent brittle failure in concrete through improving aggregate interlocking and thus enhance the properties of concrete including the tensile strength and ductility. This paper aims to investigate the axial capacity of fibrous concrete-encased steel composite stub columns. A total of 36 specimens with different cross-sectional shapes of steel profiles, including H-shaped and C-shaped, were tested, and axial parameters and compressive behavior were investigated. The variables of the research included the shape of the steel profile (H-shaped and C-shaped), steel fiber volume ratio (0%, 0.75%, and 1.25%), and the stirrup spacing (40, 65, and 130 mm). The results showed that the loading capacity of fibrous concrete-encased steel columns was affected by the shape of the steel profile inside. In this regard, the use of the H-shaped steel profile in the columns led to a higher axial loading capacity than the use of the C-shaped steel profile, due to greater confinement provided by concrete in columns with this section type (H-shape). Moreover, the addition of fibers significantly increased the ductility of these columns in comparison with those without fibers, and also, the addition of fibers increased the axial capacity of the steel-concrete composite columns by 6%. On the other hand, given the results, it is found that the stirrup spacing had a considerable effect on the load-carrying capacity of these columns, in that by increasing the stirrup spacing, due to the lower confinement of the column, the axial load-carrying capacity declined. In this regard, as the stirrup spacing increased, the decline in this parameter reached up to 11% for the columns with the H-shaped profile and 9% for the columns with the C-shaped profiles. Furthermore, the results of this study showed that the specimen with the H-shaped steel sections, 1.25% fibers, and the stirrup spacing of 40 mm generally were the optimal specimens in terms of the axial load-carrying capacity and ductility in comparison with the other specimens under study. All the specimens had almost similar damage patterns up to their failure. The difference was that the specimens containing fibers experienced failure mainly in the form of the crushing of concrete cover and its breakage from the middle of the column height, due to greater integrity of the concrete structure in these specimens. However, in the specimens without fibers, a considerable portion of the concrete cover was completely detached from the column.