Cyclic Testing of a Buckling Restrained Braced Frame with Unconstrained Gusset Connections

Abstract

Buckling restrained braces are intended to yield in both axial tension and compression. The gusset plates connecting them to the adjacent beams and columns are thick and often stiffened to prevent buckling, and as a result increase the stiffness of the beam-to-column connection substantially. The increased stiffness of the beam-to-column connection negatively impacts the seismic performance of the system by: increasing the portion of the base shear force that is resisted by the frame relative to that resisted by the braces; increasing the maximum base shear force that the system is subjected to for a given earthquake motion; and transferring moment to the braces for which they were not designed. Further, failure of gusset welds partly due to the opening and closing of the beam-to-column joint has been observed in recent experiments. To mitigate these effects and increase the efficiency of buckling restrained braced frames, a novel connection where the gusset is only connected to the beam and is offset from the column face is proposed and tested in a three-story frame under quasistatic loading. The connection is shown to accommodate frame drifts as large as 3% and satisfy the buckling restrained brace performance requirements of the 2005 AISC Seismic Provisions for Steel Buildings. Additionally, the portion of base shear force resisted by the braces relative to that resisted by the surrounding frame is shown to be consistent with design assumptions. This connection configuration could be particularly useful for retrofit of seismically deficient moment resisting frames.