# Research on Compression Behavior of Square Thin-Walled CFST Columns with Steel-Bar Stiffeners
## 1 Introduction  
Thin-walled concrete-filled steel-tube (CFST) columns have received extensive attention from scholars and engineers, because of their significant advantages to reduce the amount of steel consumption and workload of welding. [PERSON] studied the local buckling restraining behavior of concrete-filled steel tubular columns under seismic loads and the interaction between steel tube and concrete was discussed [1]. [PERSON] et al. studied the evaluation of the ultimate behavior of concrete-filled tubular (CFT) members subjected to nonuniform bending moments and presented a fiber model able to predict the ultimate response of CFT members [2]. [PERSON] et al. presented a new method to determine new ductile slenderness limits suitable for plastic design of structures based on the measured strains in plastic-bending tests on concrete-filled tubes [3]. [PERSON] et al. showed, through experimental and numerical investigations, that thick flanges of high-strength steel, thin webs of mild steel, and electrodes for the thin and mild steel can be used to increase cost efficiency without performance degradation and the hybrid rectangular CFT (RCFT) sections can develop full plastic strength [4]. [PERSON] et al. evaluated the seismic behavior of CFS buildings sheathed with gypsum panels by shaking-table tests and developed a numerical model able to simulate the dynamic/earthquake response in an OpenSees environment [5]. [PERSON] et al. studied the structural behavior of timber-filled and carbon fiber reinforced polymer (CFRP) jacketed circular steel tubes and clarified the effect of each material on structural behavior [6]. Then, they studied the effect of timber cores on the structural response of concrete-filled circular tubes under compression [7]. [PERSON] et al. established finite element models of concrete-filled steel-tube stub columns under localcompression and proposed precise and concise formulas [8]. The steel tube can be used as the concrete formwork and support the stirrup inside as well. But, mechanical deficiencies still exist, so that local buckling is easy to form into the thin-walled steel tube, thus making it unable to completely use the bearing capacity of the core concrete inside the tube. Therefore, the improvement of local-buckling performance of the thin steel tube becomes a critical problem in the study of thin-walled CFST columns. Some experiments and analysis have been conducted to resolve the problem. [PERSON] et al. used a finite strip model for elastic local buckling to study the behavior of steel plates in composite steel-concrete members, considered the buckling of the steel skin and suggested the width-to-thickness ratios suitable for design [9]. [PERSON] et al. presented an efficient mathematical model for studying the global buckling behavior of CFST columns with compliant interfaces [10]. [PERSON] et al. proposed a new stress-strain model considering the local-buckling effect [11]. [PERSON] conducted buckling tests of square concrete-filled steel tubes under concentric axial compression [12]. [PERSON] et al. studied the local-buckling behavior of eccentrically loaded rectangular CFT columns and derived the formulas for critical buckling stress of the steel plates [13]. [PERSON] et al. investigated factors that affect the buckling and postbuckling behavior of steel flanges of partially encased composite columns [14]. [PERSON] et al. considered the influences of local buckling and proposed a fiber model to analyze CFST beam columns [15]. On the other hand, some structural measurements were proposed to resist local buckling in previous studies, such as setting straight ribs [16], tensile sheets [17], longitudinal and transverse stiffeners [18,19], binding bars [20], angle braces, and other kinds of buckling restrained braces [21,22]. Such measurements can improve the effect of steel tubes on confining concrete, but also result in construction operation difficulties, like the increase of welding workload and a decrease in construction quality.