Low-rise steel moment frames are commonly found in seismic-prone regions because of their ability to efficiently resist lateral forces through bending and shear in the beams and columns. This type of frame systems in the United States often utilizes wide flange members due to their good bending resistance about their strong axis and the use of exterior seismic moment frames to resist lateral loads in each orthogonal direction. Although wide flange members are still most commonly used for seismic moment frames, recent interest in taking advantage of the high strength to weight ratio, good bending resistance, and excellent torsional behavior of hollow structural sections (HSS) has inspired new research into their applications within earthquake engineering and, in particular, HSS-based seismic moment frame systems. Studies have demonstrated that HSS beam to HSS column seismic moment connections are capable of meeting special and intermediate seismic moment frame criteria, but these diaphragm plate type connections commonly require considerable field welding which can be costly and can affect the reliability of the connection.
To address constructability concerns of HSS-based low-rise moment frame systems, an HSS-based moment resisting collar connection is considered. Previous studies around an exterior moment frame connection suggest that the collar connection can achieve stable hysteretic behavior under large cyclic drifts. Such a connection allows for the use of HSS as both beam and column elements, while having a high degree of efficiency in terms of construction speed due to its relatively simple assembly process. This is achieved through a design which reduces the amount of field welding necessary, limiting it to mainly fillet welds. To further optimize the design of the collar connection, a detailed finite element (FE) study is conducted to assess their expected behavior as a major component of low-rise moment frames. The FE study explores collar connection performance in the case of both exterior and interior moment frame connections, different weld lengths and configuration, and different levels of column axial load. The results of this study provide significant insight into design implications for the HSS collar connection and the range of applications of a collar connection approach for low-rise HSS-based steel moment frames.