Conceptual Investigation of Partially Buckling Restrained Braces

Although in its infancy, leveraging high strength fiber reinforced polymer (FRP) materials for retrofit of steel structures has been the focus of recent investigations. Studies include the application of FRP to steel for flexural and fatigue or fracture retrofit as well as improving steel member stability. The research presented in this thesis attempts to introduce the concept of an FRP-stabilized steel member through a retrofit application creating a Partially Buckling Restrained Brace (PBRB). A PBRB seeks to increase steel brace stability and hysteretic energy dissipation during a seismic event through the strategic application of bonded FRP materials along its length. Six 65 ½" long A992 Gr. 50 WT6x7 steel braces were tested under cyclic compressive loading to failure. Two braces were retrofitted with carbon FRP (CFRP) and two braces were retrofitted with glass FRP (GFRP). One brace was encased in an HSS 7 x 0.125" steel tube and filled with grout to create a conventional Buckling Restrained Brace (BRB). The final brace was an unretrofit control specimen. Two arrangements of FRP materials were used for both the CFRP and GFRP retrofit braces: (1) 2" wide strip was applied to each side of the stem of the WT, and (2) 1" wide strips were applied to each side of the stem in an effort to optimize the retrofit application. The GFRP specimens increased the axial capacity of the brace by 6% and 9%, whereas the CFRP specimens had no effect. The observed variability in axial capacity was largely a result of initial loading eccentricities. The GFRP specimens did however show greater control over residual deflections suggesting that the retrofit can delay the formation of a plastic hinge within the brace and maintain compressive capacity through several cyclic loading loops. All of the FRP-retrofit specimens reduced weak-axis lateral displacement of the braces and showed increased control of local behavior. However, the brace is not dominated by local behavior due to its length, and this application may be better suited to shorter braces, similar to those found as cross frames between bridge girders, or to control local buckling in steel I-shaped beams