Bonded FRP Plates for Strengthening Rectangular Hollow Steel Section T-Joints against Web Buckling Induced by Transverse Compression

This paper addresses the strengthening of T-joints of rectangular hollow steel section (HSS) chords against web buckling induced by transverse compression loading of welded brace members. Twelve T-joints of 1,200 mm long, $203\times 76\times 3.09$ (or 5.92) mm HSS chords, welded to 400 mm long, $76\times 76\times 8.9\mathrm{mm}$, HSS braces were tested under brace axial compression. Glass fiber reinforced polymer (GFRP) plates, 9.4 mm thick, of different configurations, with and without edge tapering, and 2 mm thick high-modulus carbon-FRP plates of equivalent stiffness, were adhesively bonded to the webs of the chord near the brace. Strength gain increases significantly, from 9–38%, as the web height-to-thickness ($h/t$) ratio of the HSS chord increases from 34 to 65. In thin-walled HSS ($h/t=65$), retrofitting provided significant gains in strength but not in ductility. Fiber-reinforced polymer (FRP) plates delayed and shifted local buckling away from brace location but did not prevent it. Square-edged and tapered plates behaved similarly. Plates as small as half the chord depth and of width equal to the brace provided a 35% strength gain. Very wide plates, 0.4 of the span, provided a 53% gain. In thick-walled HSS ($h/t=34$), retrofitting provided little strength gain but enhanced ductility, especially with properly bonded plates extending on the brace. Generally, plates fractured under local bending or delaminated within plate layers while the bond was fully intact.