## 1 Introduction  
The use of rectangular hollow structural section (RHS) trusses, as shown in Figure 1a, is increasing as an economical roof structure for long-span building structures such as factories and warehouses. In such RHS trusses, medium-size sections of 150 mm-300 mm in size are used as top and bottom chord members, and small-size sections of approximately 100 mm in size are used as diagonal and vertical branch members. Particularly for diagonal members that are subjected to large compression force, RHS members with greater buckling resistance may be advantageous. Furthermore, by filling concrete inside the hollow steel section, the resistance of RHS trusses can be enhanced (see Figure 1). For example, concrete-filled composite members have higher elastic buckling strength (\(P_{e}\)) and compressive resistance (\(P_{no}\)), as the filled concrete provides additional resistance. Thus, the compressive strength including length effects (i.e., \(P_{n}=P_{no}\cdot 0.658^{P_{no}/P_{e}}\)) can be greatly increased [1; 2]. Unlike diagonal and vertical branch members that are made of concrete-filled composite sections, top and bottom chord members are often made of hollow steel sections to make the roof truss lighter. At the connection where the filled composite branch members are joined with the hollow chord members, the thin RHS walls of the branch and chord members need to be directly welded to each other, or alternatively, longitudinal plate connection (i.e., plate-to-RHS connection, see Figure 1b) can be used [2; 3; 4]. The plate-to-RHS connection may be more convenient to assemble and erect the trusses and their members because the branch members are connected to the plate by bolting without field welding. However, the thin RHS wall of the chord member is weak to out-of-plane deformation, and consequently, the strength of the truss connection can decrease (see Figure 1b). This means, unless the connecting chord face is not appropriately stiffened, the excellent compression resistance of the composite branch member cannot be fully exploited. In the past, many studies have been conducted to investigate the behavior and design method of plate-to-RHS connections [5; 6; 7; 8; 9; 10]. [PERSON], [PERSON], and [PERSON] [5] investigated the strength of the plate-to-RHS connections with chord face stiffener by finite element analysis (see Figure 2b). The stiffened truss connections showed ductile behavior, and the strength increased as the deformation of the chord face increased. The connection strengths determined according to the \"3% \(b_{o}\) rule\" were in good agreement with the theoretical strengths by chord face plastification. The 3% \(b_{o}\) rule means that the connection strength is determined as the strength when the out-of-plane deformation of the connecting chord face reaches 3% of the width of the chord face (\(b_{o}\)). [PERSON], [PERSON], and [PERSON] [6] developed a design guideline for unstiffened plate-to-RHS connections (see Figure 2a) based on experimental and analytical investigations. They considered chord face plastification and branch yielding as governing design limit states and suggested using the strength resistance factor of 1.0 based on the investigation result that, due to hardening behavior, the actual strength was sufficiently larger than the theoretical strength. [PERSON] and [PERSON] [7] studied the behavior of the \"through plate\"-to-RHS connections, in which the longitudinal plate was extended up to the opposite wall of the connecting chord face. The truss connections stiffened by the through plate showed deformation-controlled or ductile behavior, and the test strength was larger than the theoretical strength by chord face plastification (strength resistance factor = 1.0). [PERSON] and [PERSON] [8] investigated the behavior of longitudinal plate-to-RHS connections stiffened by chord face stiffener (see Figure 2b). The minimum thickness of the chord face stiffener was proposed to satisfy the limit states of strength and serviceability.