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Due to the increasing population and lack of construction space, the use of high-rise buildings around the world is inevitable, so the use of sections with high strength to weight ratio that take up less space has been considered by researchers. One of the sections that has been considered by researchers and engineers to achieve this goal is composite columns, especially steel filled with concrete (CFT). These columns use the advantages of steel and concrete both and also have advantages such as eliminating column formwork in construction, providing complete enclosure conditions for concrete, preventing concrete parts from collapsing and tearing from the steel profile of the column and delay. The buckling of steel sheaths due to the presence of concrete is more common than other composite columns. CFTs also have higher strength, lateral stiffness and ductility than reinforced concrete columns of the same dimensions. The use of external stiffeners for replacement with continuity plates has been considered by researchers and design engineers and various forms have been proposed so far. The results of previous research have shown that external stiffeners are a good alternative to continuity plates.
In the present study, the behavior of steel frames with Hollow Steel Sections(HSS) and Concrete Filled Tube(CFT) Columns has been investigated. After launching the test setup and locating the displacement meters and dynamometers, the structure is subjected to cyclic loads according to the defined loading protocol. Three samples of two-opening and two-story frames with hollow box columns (HSS), concrete filled box (CFT) and concrete filled box with horizontal reinforcement nets have been evaluated experimentally and different parameters Their strength, ductility, hardness, energy absorption and cyclic behavior have been studied. In addition, the impact of the presence of concrete and horizontal reinforcement nets has been evaluated. On the other hand, in order to investigate the effect of using external stiffeners and horizontal bar mats in the frame, all connections of the two-story frame of the present study selected of this type. The results show that the use of concrete has improved the strength of connection due to the reduction of the buckling of the column plates, but the ductility has decreased. External stiffeners are used in all specimens, and as expected, the plastic hinge is moved into the beam, providing the idea of strong column-weak beam. In another part of the research, the behavior of the CFT frame and The presence of horizontal bar networks of the panel zone, which shows its effect of it in improving the behavior of the frame. The use of concrete in steel sections increases the strength of the frame, but since it is used in frames with CFT columns, beams with weaker sections show little reduction or little increase due to the use of concrete properties in pressure and Steel is in tension. On the other hand, the presence of concrete reduces the buckling of the column plate. In addition, the box-shaped sections around the concrete cause the concrete to be enclosed and ultimately increase the strength and energy absorption of the structure.

Keywords: Steel Frame, CFT, Horizontal Bar Mats, External Stiffeners, Envelope Curve.

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