## 1 Introduction  
Frames with concrete-filled steel tube (CFST) columns have been widely used in modern construction, primarily because of the high structural efficiency of this type of column and the substantial savings with respect to cost and building time of such a construction [1]. For fully welded connections, the construction progress is slow and extensive erection and welding work is required, resulting in high labor costs; additionally, the on-site welding quality is difficult to guarantee. Moreover, according to the post-earthquake surveys conducted after the 1994 Northridge and 1995 Kobe earthquakes, severe damage and brittle fractures in welded steel moment-resisting structures were observed due to the poor performance of welding in beam-to-column connections [2]. Moreover, the fracture of steel structures under earthquake action is a low-cycle fatigue problem, and the fatigue strength of steel is greatly affected by fine cracks in the welding seam and its heat-affected zones. The bolted connections can meet the seismic performance requirements, but it is hard to access inside the steel tube when connecting beams to CFST columns, as openings in the steel tube are costly. To overcome the complexities associated with utilizing open section beams connected to hollow section columns, a new blind-bolting technique has been developed within the past few decades. This technique allows a fastener to be tightened from one side only, without the demand for access into the hollow section column. Although the blind bolts have solved the bolted connection problem of CFST column joints, the tensile area in a square steel tube often tends to be susceptible to failure when the joint is resisting moment. For moment-resisting blind bolted CFST column joints, the infill concrete functions as a local stiffener for the column walls in the compressive region, while severe outward plastic deformation develops much more easily in the tensile region, which is owing to the weak outward flexural rigidity for the tube face. [PERSON] et al. [3] and [PERSON] et al. [4] performed monotonic and cyclic loading tests on bolted end-plate connections to square CFST columns. For most tested joints with square CFST columns, failure occurred when the corner of the steel tube cracked and the tube face bulged severely. Especially for the specimen with the thinner steel tube, only slight flexural deformation of the end-plate was observed at failure due to the excessive outward deformation of the tube face. Thus, it is concluded that premature local failure of the square steel tube induces a flexural resistance below the design requirement, accelerating the destruction of the overall structure and preventing other parts of the connection from working at full capacity. Several measures have been suggested by contemporary researchers for enhancing the tensile performance of CFST columns to prevent local tube failure. In some studies, this enhancement is realized by modifying the steel tubes. [PERSON] et al. [5,6] extended an experimental and analytical study of a new bolted moment-resisting connection with T-stubs connecting open section beams to unfilled hollow section columns. The load from the beam end can be transmitted to the sidewall or back of the column to reduce the outward deformation of the tube face directly subjected to moment. [PERSON] et al. [3] proposed several strengthening methods, including using binding bars to tie the opposite surfaces of the tube, adding internal rings, externally welding two C-shaped channels, and internally embedding a short segment of I-section. The tensile force applied to the blind bolts was not effectively transferred, therefore the first two methods had only limited influence to restrain the bulging of the tube face; however, the last two methods effectively improved the performance of blind bolted joints to square CFST columns.