Development and Evaluation of Seismic Retrofit Alternatives for Older Concentrically Braced Frames

Concentrically braced frames (CBFs) have been used for seismic-lateral-force resisting systems for many years, but prior to approximately 1988, they were not designed to promote ductile response using capacity-based design. These older, nonductile CBFs (NCBFs) have been investigated in prior research that identified severe deficiencies in these systems, but some deficiencies had more detrimental consequences than others. This paper describes a research study to evaluate retrofit strategies for NCBFs. Welded and bolted continuous and split shear-plate connections, end-plate connections, split double-angle connections, and integrated gusset-shear plate connections were investigated. Retrofit strategies included brace replacement, concrete in-fill of a locally slender brace, weld overlay, bolt reinforcement, development of in-plane brace buckling, and the use of buckling-restrained braces. In all cases, system deformation capacity was enhanced, and the extent of improvement depended on connection configuration, connection demand-to-capacity ratios, and bracing type.