Retrofitting tubular steel T-joints subjected to axial compression in chord and brace members using bonded FRP plates or through-wall steel bolts

T-joints of Hollow Steel Sections (HSSs) are vulnerable to local instabilities of the web under the orthogonal compression in both members. Unlike W-sections stiffeners cannot be installed inside the closed sections. Therefore, alternative strengthening methods are needed. This experimental study explored the effectiveness of two retrofitting methods by controlling the web buckling of the longitudinally compressed 203 × 76 × 3.09 mm chord, which is also subjected to transverse axial compression loading through the brace member. The web height-to-wall thickness (h/t) ratio of the chord is 65. In the first method, 8 mm diameter through-wall steel bolts were used to brace the webs of the chord at the vicinity of the brace. In the second method, 76 × 185 × 9.5 mm glass fiber reinforced polymer (GFRP) plates were adhesively bonded to the two webs of the chord at the brace location. Two levels of sustained axial compression load were induced in the chord, representing 45% and 80% of its full axial capacity, in addition to control specimens without axial loads. The transverse brace load was then gradually increased to failure. The through-wall steel bolts increased the joint capacity by 13–25%, depending on the chord’s axial load level, while the bonded GFRP plate increased the capacity by 38–46%.