Strengthening by Stiffening: Fiber-Reinforced Plastic Configuration Effects on Behavior of Shear-Deficient Steel Beams

Abstract

Restoring the capacity of deficient structural members to meet original design requirements or new higher demands is common. For that purpose, research efforts to use composite materials in retrofitting steel structures have been gradually gaining ground in recent years, with more focus on inhibiting buckling modes by increasing local and/or global member stiffness. In this paper, the authors experimentally investigate retrofitting alternatives for shear-deficient steel structures. The proposed alternatives fall on what is referred to as strengthening by stiffening (SBS), in which thin-walled steel beams are retrofitted by bonding pultruded glass fiber reinforced polymer (GFRP) sections to the buckling-prone web panels. Different GFRP stiffener orientations, types, and adhesives were investigated in addition to conventional welded steel stiffeners, and bonded biaxial carbon fiber reinforced polymer (CFRP) sheets were tested in this study. The experimental results showed that gains in shear strength of up to 81% compared to non-retrofitted beams were achieved with bonded CFRP sheets, whereas an increase of about 50% in load capacity was achieved for bonded GFRP and welded steel stiffeners. The energy ductility of the retrofitted beams was found to be higher when a ductile epoxy type was used. Recommendations regarding the number of GFRP stiffeners, their orientation, type, and adhesive properties were drawn based on the findings from this paper. The results also showed that it may be possible to use SBS in new construction to eliminate conventionally welded steel stiffeners, which is known to introduce stress concentrations and their associated fatigue problems.