Experimental Evaluation of the Seismic Vulnerability of Braces and Connections in Older Concentrically Braced Frames

Concentrically braced frames (CBFs) have been used as seismic force resisting systems in steel structures for many decades. CBFs designed prior to about 1988 were not capacity designed for the expected brace strength, and many are therefore prone to undesirable failure modes when subjected to large earthquakes. The consequences of other factors, such as weld toughness, gusset plate clearance to permit brace end rotation, and local slenderness of the brace, were also not fully understood, so older CBFs may also exhibit premature failure relative to modern, special CBFs (SCBFs). Thus, these older CBFs are expected to exhibit low inelastic deformation capacity and are termed nonductile CBFs (NCBFs) here. An infrastructure review showed that many NCBFs are deficient by SCBF standards and an experimental program was undertaken to explore the impact of brace and connection deficiencies in particular. Brace local slenderness and brace-to-gusset plate weld deficiencies were found to be highly detrimental to drift capacity. Beyond these concerns, welds joining the gusset plate to the frame were found to be vulnerable to fracture even if they had sufficient strength based upon the yield capacity of the brace. Bolted connections were found to be generally more robust, especially if bolt bearing and fracture resistances were similar. The findings presented here will aid engineers in determining need and type of retrofit for NCBFs.