## 1 Introduction
[24] conducted experimental tests to evaluate the seismic performance of concrete-filled double-skin steel tube (CFDST)-steel beam frames with different construction details. The authors concluded that the CFDST structures exhibit better seismic performance, fire and corrosion resistances by comparing with the CFST structures. However, the application of the CFDST structures is much less common than the CFST structures due to the traditional construction processes of the CFDST structures, which are complicated and tedious. [PERSON] et al. [25] conducted experiments to investigate the ultimate strength, deformation capacity and failure modes of a new irregular connection with reinforced concrete (RC) wing-walls. The hysteresis behavior of the connection was tested subjected to cyclic load with constant axial force. It was concluded that reinforced concrete wing-walls enabled the enhanced ductility and safety margin of the hybrid beam-to-column connections. [PERSON] et al. [26] studied the seismic performance of a steel-reinforced concrete column-steel beam composite joint (MPCJ). They analyzed three different beam-column connection types: bolted, welded, and bolted-welded. The connections were subjected to low-cycle reversed loading to investigate the elastic and elastoplastic development trends, failure characteristics, and seismic response. The results showed bending failure at the end of the beam that follows the buckling of the flange connecting plate. This, while exhibiting stable hysteretic curves, also had reasonable strength and stiffness degradation, as well as good ductility and energy dissipation performance. [PERSON] et al. [27] investigated the seismic performance of a steel frame with an external diaphragm joint between a CFST column and H-shaped steel beam. A quasistatic test was conducted to analyze the joint stiffness, beam-to-column stiffness ratio and concrete strength. Failure mode, lateral load versus displacement hysteretic curve, skeleton curve, strength and stiffness degradation effects, ductility, and energy dissipation capacity were analyzed. The experimental results showed hysteretic curves with a plump shuttle shape. In addition, the strength degradation effect and stiffness degradation effect were relatively feeble, and the ductility and energy dissipation capacity was excellent. [PERSON] et al. [28] conducted experimental investigation on the seismic behavior of a novel connection between a beam and a reinforced concrete-filled steel tube (RCFST) column. The connection was tested under cyclic loading to evaluate the failure modes, hysteretic performance, stiffness degradation, strength degradation, energy dissipation capacity and the strain responses. The hysteretic curves showed obvious pinch phenomena because of the slip around the transfer sleeve which was identified as the key factor to affect the strength of the connection. [PERSON] et al. [29] studied a novel U-shaped diaphragm connection designed to transfer the moment at beam ends in the frame with special-shaped CFST columns and steel beams. The connection was tested under low-cycle horizontal loading. Based on the experimental results, the strength, ductility, and strain distribution of the connections were calculated. The results showed that this connection can develop plastic bending capacity in the beam, satisfying the design code requirements of America, Great Britain, and China. The aforementioned research highlights the need to explore different types of connections in order to meet earthquake-resistant criteria while improving manufacturing processes; in this way, expanding the catalog of connections to be used depending on the different construction and architectural design requirements. Despite all the work done so far, the steel beam-to-concrete-filled steel tubular column connection using external diaphragms represents a significant contribution to the state of the art since this connection has not been previously studied, despite its simplicity in the design and multiple constructive advantages.