Experimental Behavior of Low-Ductility Brace Connection Limit States

In low-ductility, concentrically braced steel frames (CBFs) with traditional fillet welds between the gusset plates and slotted HSS braces, fracture of the fillet welds or net section rupture of the tube are viewed as the most likely connection failure modes. After such failure, the braces lose all their tensile strength but may still re-engage in compression to provide some degree of reserve strength to the structure. A test program was recently completed to verify the behavioral limit states and probable re-engagement strength of brace connections. Six specimens with slotted-HSS welded-gusset plate connections were examined in this research. The connection specimens were part of a 9-story prototype building with chevron CBFs located in eastern North America and designed with an R factor of 3.0. The connection specimens were proportioned to exhibit weld or net section rupture failure modes. The tests showed that connections can re-engage in compression after weld or net section rupture when the edge of the HSS slot comes into contact with the edge of the gusset plate. For slender gusset plates, connection buckling occurred due to the eccentricity resulting from unsymmetrical damage modes - such as coupled weld and net section ruptures - before substantial connection re-engagement can develop in bearing.