Semi-Rigid Composite Connections for Gravity Loads

Double web angles, top and seat angles, and seat angle and web clips are some of the most common connections used for floor beams in braced steel frames. These connections are considered to be rotationally weak, and therefore design proceeds along the assumption that these connections can be idealized as pins. In reality, because these connections have a limited amount of moment transfer capability, they are semirigid connections. The economic and structural benefits of semi-rigid connections in braced frames are well known but seldom utilized. The main advantages they provide over simple connections are lower mid-span moments for the beam and lower effective length of the columns. Depending on the geometry of the connection and the size of the angles used, the moment capacity of these simple connections may range up to 20 percent of the plastic capacity of the beam alone, and lead to saving up to 10 percent of the steel required in floor beams. This moment capacity, however, is associated with large, nonlinear rotations that make serviceability and drift computations difficult. Therefore the current design assumption of pinned ends seems justifiable and conservative.