Recent field applications and research findings have demonstrated the effectiveness of concrete-filled fiber reinforced polymer (FRP) tubes (CFFT) as an efficient and promising hybrid system for designing main components such as pier columns, girders and piles for a bridge system. The vision was to provide a cost-competitive unified system composed of FRP/concrete hybrid members, which may act as a viable alternative to conventional reinforced and prestressed concrete structural systems. To achieve their broad-based implementation in civil infrastructure, understanding of their behavior and developing analytical tools under full spectrum of primary and secondary load demands are essential. Response characterizations under primary load demands namely, axial compression, flexural and axial-flexural, and seismic loadings have already been reported. However, investigations under primary shear and secondary fatigue load demands remain to be addressed.; The present study consists of two phases. In the first phase, an experimental and analytical investigation was undertaken to characterize the behavior of a CFFT beam. Study on shear was primarily focused on the deep beam behavior. Comparisons of behavior of deep, short and slender beams were also highlighted. A strut-and-tie model approach, pertinent to analysis of deep reinforced and prestressed concrete members, was proposed to predict the shear strength of deep CFFT beams. Prediction showed good agreement with test results. It was concluded that shear failure mode is only critical for beams with shear span less than their depth.; In the second phase, a detailed study on flexural fatigue behavior and modeling was undertaken. The main objective was to evaluate the performance of beams under four basic criteria; (i) damage accumulation (ii) stiffness degradation, (iii) number of cycles to failure, and (iv) reserve bending strength. Effects of laminate fiber architecture, reinforcement index, load range, and end restraint on the fatigue response of CFFT beams were addressed. A fiber element was developed, capable of simulating sectional strain profile and moment curvature at any given time or number of cycles under single and two stages of loading. The model can also predict deflections at mid-span, and can analyze the reserve bending response of a fatigued CFFT beam. Parametric study revealed that flexural fatigue performance of CFFT beams could be enhanced by increasing reinforcement index and the effective elastic modulus in the longitudinal direction.
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