Seismic Design of Steel Special Concentrically Braced Frame Systems

Concentrically Braced Frames (CBFs) are a class of structures resisting lateral loads through a vertical concentric truss
system, the axes of the members aligning concentrically at the joints. CBFs tend to be efficient in resisting lateral forces because they can provide high strength and stiffness. These characteristics can also result in less favorable seismic response, such as low drift capacity and higher accelerations. CBFs are a common structural steel or composite system in areas of any seismicity.

Special Concentrically Braced Frames (SCBFs) are a special class of CBF that are proportioned and detailed to maximize
inelastic drift capacity. This type of CBF system is defined for structural steel and composite structures only.

The primary source of drift capacity in SCBFs is through buckling and yielding of diagonal brace members. Proportioning and detailing rules for braces ensure adequate axial ductility, which translates into lateral drift capacity for the system. Special design and detailing rules for connections, beams, and columns attempt to preclude less ductile modes of response that might result in reduced lateral drift capacity.

This Guide addresses the seismic design of steel SCBFs in typical building applications.