DUCTILITY · Enhancing plastic rotation capacity of beam-column connections in existing steel moment frames by glass fiber reinforced polymers

Final Activity Report Summary - DUCTILITY (Enhancing Plastic Rotation Capacity of Beam-Column Connections in Existing Steel Moment Frames by Glass Fiber Reinforced Polymers)

Existing steel moment frame beam-column connections are vulnerable to beam flange and web local buckling at the plastic hinge region of the beams prior to reaching the desired plastic rotation capacities specified for intermediate or special moment frames. As the use of fibre reinforced polymers (FRP) have increased in strengthening and repair of steel members in recent years, using glass FRPs (GFRP) in stabilising local instabilities have also attracted attention. The main objective of this research project was to investigate the behaviour of cyclically loaded beam-column assemblies modified by a welded haunch (WH) or reduced beam section (RBS) at the beam bottom flange and reinforced with GFRP strips at the beam plastic hinge location. The investigation was both experimental and analytical. Experimental study included small-scale GFRP and polymer standard tests, as well as full-scale cantilever beam tests. The cantilever beams were modified by a triangular welded haunch at the beam bottom flange, similar to the modification applied to existing steel beam-column connections, and reinforced by GFRP at the plastic hinge region. Computational studies included finite element analyses on cyclic response of cantilever beams modified by a WH or RBS and reinforced with GFRPs. The slenderness ratios of the beams exceeded the limits set forth in current seismic codes. The main conclusions achieved from the research investigation are: 1- The effect of GFRP reinforcement highly depends on the bond strength between steel and GFRP. The recommendations given here are based on a minimum interfacial shear strength of 13.5 MPa, a minimum interlaminar shear strength for GFRPs of 18 MPa, and a minimum elastic modulus for GFRPs of 10000 MPa. 2- Based on the small scale GFRP and steel-GFRP lap shear tests it appears that the weakest link in a steel-GFRP hybrid system is the bond between the steel surface and GFRP. The computational and large-scale experimental studies indicate that the contribution of the GFRPs is controlled by the interfacial shear strength of the steel-GFRP surface. 3- The effect of GFRP reinforcement is more for deeper beams (depth = 700 mm) then for shorter beams (depth = 400 mm). 4- For welded haunch modified beams with a depth of 700 mm the rotation capacities of the following beam sections can be brought up to 2%, specified for intermediate moment frames: Beams with flange slenderness ratios of 8-9-10 and web slenderbess ratios equal to or smaller than 45. The thickness of GFRP should be at least 3 mm. 5- For welded haunch modified beams with a depth of 700 mm the rotation capacities of the following beam sections can be improved but cannot be taken up to 2%: Beams with flange slenderness ratios of 8-9-10 and web slenderbess ratios greater than 50 and smaller than 65. The thickness of GFRP should be at least 4 mm. 6- The use of GFRP for beams modified by a welded haunch and with flange slenderness ratios of 11 and 12 is not recommended. No significant improvement in the cyclic behaviour of these beam sections were observed. 7- For welded haunch modifed beams the GFRPs should be placed on the top and bottom of both of the flanges at the plastic hinge region with a length equal to the depth of the beam. GFRP should also be placed inside the welded haunch region at top and bottom of the top flange. Raping GFRP all around the plastic hinge is desirable. 8- The use of GFRP for beams with a depth equal to 400 mm and modified by a welded haunch, and for beams modified by an RBS at the bottom flange is not recommended. No significant improvement in the cyclic behaviour of these beam sections were observed.

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