## 1 Introduction  
Although the concept of a steel special moment frame (SMF) is a relatively recent development in building codes, steel moment frames have been in use for more than one hundred years and date to the earliest use of structural steel in building construction [1]. Over time, the design and construction methods for these steel structures have experienced a constant update aimed at meeting the needs and challenges generated by new construction projects [2; 3]. In this context, building codes, such as AISC 358-16 and AISC 341-16 [4; 5], have adopted a design philosophy intended to provide safety by avoiding earthquake-induced collapse in severe events. This approach permits extensive structural and nonstructural damage through a systematized and endorsed design and construction of a considerable variety of frames that adapt to a large number of civil steel constructions [6]. At present, there is an increased demand for SMFs in civil construction due to their good dissipation response to seismic events and their versatility in architectural applications. These frames impose smaller forces on foundations when compared to other structural systems, making foundation systems more economical. Nonetheless, a design in compliance with seismic provisions must be guaranteed [7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. SMFs are composed of three basic elements: beams, columns and beam-column connections. Concrete-filled steel tubular (CFST) columns are widely used in architectural application proposals and in several industrial sectors due to their load carrying capacity under various loading conditions [17]. Connections with diaphragms are also usually used in SMFs to guarantee structural integrity [18; 19]. In this context, regulations such as AISC 358 present several prequalified connections. However, if a connection is not prequalified, it requires project-specific testing and must pass the criteria specified in AISC 341 [5], which defines the recommended seismic design criteria for SMF buildings, including connections with external diaphragms. Nonetheless, although connections with external diaphragms are widely used, the current regulations impose strong limitations in their design guidelines. In addition, it is necessary to address some issues and explore the influence of various design parameters. Experimental work has been conducted on different configurations of CFST column to steel beam connections to analyze their seismic behavior. For instance, [PERSON] and [PERSON] [12] conducted tests for six different types of circular CFST welded connections utilizing the quasi-static test method. The connections were fabricated by utilizing different elements such as diaphragms, plates, embedded weldable deformed bars, continuous flanges, and continuous through steel tubes. In this work, the connections were tested to failure. The results indicated that welding the connection-stub directly to the skin of the steel tube resulted in a large deformation demand on the tube wall. In addition, they found that large tube wall distortions made the beam flange, the flange weld, and the tube wall highly susceptible to fracture. Therefore, none of these connections qualified to be utilized in systems with special energy dissipation capacity. [PERSON] and [PERSON] [10] performed testing under cyclic loading on five circular CFST welded connections. They tested four types of joints characterized by the arrangement of weld from beam flange to the external diaphragm and by the tube walls of columns. The results showed that all the connections failed due to welding fracture during nonlinear behavior. According to the authors, the residual stress in the tube wall for this connection built up as a result of the welding of the diaphragm, causing a remarkable reduction in the deformation capacity of the column. [PERSON] et al. [14] performed testing under cyclic loads on concrete filled tubular (CFT) column to H-beam welded moment connections with external T-stiffeners.