Effect of beam-column connection fixity and gravity framing on the seismic collapse risk of special concentrically braced frames

In concentrically braced frames (CBFs), braces are typically connected at beam-column connections through gusset plates, which also increase the rotational stiffness and moment capacity of the beam-column connection. This fixity provides a reserve lateral force resisting capacity that may improve the seismic collapse capacity of the system, but that is not considered in design. Recently, a new brace connection type has been proposed that does not include a gusset plate that would stiffen and strengthen the beam-column connection. To address the implications of the range of possible connection design alternatives, this paper assesses the effects of the fixity of beam-column connections on the behaviour of three special concentrically braced frames of different heights. The results show that flexural strength and stiffness at the beam-column connections reduces the collapse probability when the gravity framing contribution is ignored, but this influence is minor for low-rise buildings and is typically much less significant than the influence of the gravity framing's stiffness and strength. Simple design recommendations are presented regarding the beam-column connection fixity within the braced bay.