Strengthening Slender Steel Compression Members Using a Fibre-Reinforced Polymer Buckling Restrained Bracing System

Abstract

In this thesis, the structural properties and behaviour of slender steel members strengthened against buckling by fibre-reinforced polymer (FRP) shells filled with self-consolidating grout, in the form of buckling restrained bracing (BRB), were investigated. The goal of the FRP-BRB 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 27 strengthened specimens and 9 plain steel cores were prepared and tested. The strengthened specimens were prepared with three different length FRP shells, namely, 300 mm, 600 mm and 900 mm as well as three outer diameters, namely, 41 mm, 53 mm, and 65 mm. Hot rolled steel bars, 25.4 mm by 6.35 mm, with lengths corresponding to 30 mm longer than the FRP shell length were prepared with 45 mm long tapered tabs. The self-consolidating grout for filling the shells was prepared to have a compressive strength of 20 to 35 MPa. A lubricant was applied to the steel core to inhibit bonding with the grout. Specimens were tested under uniaxial compression up to failure. Through strain data the contribution of each component of the system to the overall load carrying capacity was calculated. It was found that at the yielding point the steel core carries on average 86% of the load with the grout and FRP carrying only 13.5% and 0.5%, respectively. A simple linear elastic model was created in order to predict the failure mode of the FRP-BRB 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 FRP-BRB system was sufficiently sized, as determined by the model, the system was successful in changing the failure mode of the steel core from buckling to yielding.