Hybrid FRP Strengthening of Slender Steel Members for Buckling Control

In this paper, the structural properties and behavior of slender steel members strengthened against buckling by a hybrid system of fiber-reinforced polymer (FRP) shells filled with self-consolidating grout (SCG), in the form of buckling restrained bracing (BRB), were investigated. The goal of the hybrid system is to increase the load-carrying capacity of the slender member to reach the yielding load of the steel core through the addition of lateral support. A total of 36 small-scale specimens (27 strengthened specimens and 9 plain 25.4 mm x 6.35 mm steel cores) were prepared and tested in compression. Strengthened specimens were prepared with three different FRP shell lengths (300, 600, and 900 mm) and three outer shell diameters (41, 53, and 65 mm). A lubricant was applied to the steel core to allow the steel core to carry the majority of the axial load independently. The contribution of each component of the hybrid system to the overall load-carrying capacity was also calculated. The steel core was found to carry on average 86% of the load at yielding, with the grout and FRP carrying only 13.5% and 0.5%, respectively. A simple linear elastic model was created to predict the failure mode of the hybrid system that can also be used to design an optimized system. The model accurately predicted the failure mode for all 27 reinforced specimens. Overall, provided the hybrid FRP-strengthening system was sufficiently sized, the system was successful in changing the failure mode of the steel core from buckling to yielding.