Hollow Structural Sections (HSS) are efficient members to use in a variety of applications, including moment frames. When the beams and columns are HSS members, moment connections can be designed using the provisions in the American Institute of Steel Construction (AISC) Specification for Structural Steel Buildings Chapter K. The tables in Chapter K cover various connection configurations and provide the limit states with the applicable strength equations. When the beams are wide-flange sections and the columns are HSS, there are multiple options for moment connections and these will be discussed in this article.
It is important to remember that connecting to an HSS column is different than the considerations of connecting to the flanges of a wide-flange column. The moments in the wide-flange beams are resolved into concentrated forces at the beam flanges that must be transferred into the column. The main difference between an HSS and wide-flange column is how the forces from the beam flanges are transferred into the column webs to be resisted as shear. In a wide-flange column, the web (and thus the stiffness) is located at the center of the column flange. In an HSS column, the forces applied to the column face must be transferred to the side-walls, which act as the webs. Due to the fact that HSS walls are generally thinner and must transfer the forces to the sidewalls, the thickness of the HSS column wall becomes a critical consideration for the strength and stiffness of a moment connection between an HSS column and a wide-flange beam.
Given these considerations, the common connection types discussed in this article and the recommendations that accompany them are generally aimed at having the concentrated forces from the beam flanges applied as close to the sidewalls of the HSS column as practical. Two general recommendations can be made for all connection configurations; design the column to eliminate the need for reinforcement at the connection and keep the ratio between the column width and the beam flange width close to one.
When possible, the connection configuration and forces applied to the HSS columns should be considered when selecting the column size. Thicker walls and/or narrower column face dimension can strengthen and stiffen the column wall and remove the need for costly stiffeners or column reinforcing. Put more simply, it is typically more economical to have heavier columns than to have reinforced connections. This is true of connections with wide-flange columns as well. There are requirements for applying concentrated forces to the flanges that may lead to stiffener or doubler plates at the moment connections (when the column web or flanges are not thick enough). AISC recommends using heavier column sections to avoid costly connection reinforcement.
The second recommendation is to keep the column width and beam flange width at optimal ratio. Narrow beam flanges (as compared to the width of the column face) concentrate the force to the central portion of the HSS wall, making the thickness of the wall more critical.
Five of the most common connection types are discussed in this article, but there are several other viable configurations discussed in the resources sited. This article is focused on low-seismic applications, but there are connections suitable for high-seismic applications.
(This article comes from Steel Tube Institute edit released)