Flexural Performance and Moment Connections of Concrete-Filled GFRP Tubes (CFFTs) and CFFT-encased Steel I-Sections

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Zakaib, Sarah
CFFTs , structural engineering
The first part of this thesis addresses a new hybrid system, concrete-filled FRP tube (CFFT)-encased steel I-sections. The embedded steel section enhances flexural strength, stiffness and ductility, and facilitates connection of the CFFT member to footings or other members. Phase I addresses the flexural behaviour of the system through the testing of beam specimens with GFRP tubes which vary in thickness and laminate structure. The steel section enhances performance considerably, especially ductility, in tubes with cross-ply laminates, where significant sustained reserve strength remains upon fracture of the tube. CFFTs with angle-ply tubes show considerable inherent ductility on their own, although adding the steel section enhances strength and stiffness. Phase II addresses the development of a moment connection through cantilever tests. The connection consists of steel base plates welded to the steel sections, which are embedded into CFFT members at various length-to-span (Ls/L) ratios between 0.1 and 1.0. Three distinct failure modes are observed. At (Ls/L) ratios below 0.17, premature bond failure occurs. At ratios of 0.17 to 0.47, flexural tension failure of the tube occurs just beyond the free end of the steel section. Beyond a 0.47 ratio, the plastic hinge capacity is developed at the fixed end. A simple design-oriented model to predict strengths of the connection at the full range of (Ls/L) ratios is developed and validated. Also, a readily available computer program is adopted to model flexural behaviour of the CFFT-steel member itself. The second part of the thesis investigates unreinforced CFFT members, with emphasis on moment connections to concrete footings. The study explores the effect of maximum shear and maximum moment, both occurring at the same location, on the ultimate strength of CFFTs. Testing involves simple beams and cantilever specimens with varying shear spans and fixed end arrangements. End conditions consist of either direct embedment into concrete blocks with steel dowels, or mechanical clamping. For the cross-ply GFRP tubes used, the presence of shear at the location of maximum moment near the connection of the cantilevers does not reduce flexural capacity. Slip can prevent the CFFT member from attaining the potential moment capacity in spite of the tube failing due to tensile rupture.
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