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Concrete-encased concrete-filled steel tube (CFST) has been presented to integrate reinforced concrete (RC) and CFSTs that have been used increasingly in high-rise buildings and bridges in the world. Concrete-encased CFST exhibits higher confinement of the concrete core, high stiffness and strength, better durability and ductility, small sectional size, higher fire resistance due to the protection of the outer RC encasement compared to CFST, avoidance of local buckling and corrosion of steel tube, and easier connection with steel RC beams. On account of the insufficient research and the unknown behavior of these beam types, in this research, prestressed concrete-encased CFST (PCE-CFST) beams that incorporate CFST in the compression zone to improve the strength of concrete, and prestressed strands in the tension zone to control cracks in reinforced concrete (RC) beams are numerically investigated. The objective of this study is the finite element analysis of parameters that are not feasible to be examined through experimental specimens. Hence, the experimental study has been done to validate the nonlinear finite element modeling and a full-scale model is constructed to explore the flexural behavior of the cross-section. The model is then developed to include parameters such as the longitudinal rebar ratio, prestressed strand ratio, core concrete ratio, and the steel tube ratio indices. Based on findings, a good agreement was observed in the moment-deflection diagrams and the failure modes between the experimental and numerical results. Then the model was developed and 9 PCE-CFST beams is modeled by finite element software of ABAQUS to investigation of longitudinal rebar ratio (0.0257, 0.00856, 0.00286), prestressed strand ratio (0.00228, 0.00912, 0.000569), core concrete ratio (0.0206, 0.07799, 0.0281), and the steel tube ratio (0.01385, 0.00615, 0.000385) indices. The beam specimens were subjected to four-point loading and the parameters of bearing capacity, moment-deflection curve, energy absorption, ductility, failure mode, bending stiffness were investigated. Examination of indices revealed that as the prestressed strand ratio increases, displacement ductility, flexural stiffness and ultimate moment increase by 1.47, 1.06 and 3.22 times, respectively. Further, the elastic and entire absorbed energy of cross-section escalate by 1.04 and 3.22 times respectively, with increasing prestressed strand ratio. Likewise, by increasing the index of longitudinal rebar ratio, flexural stiffness and ultimate moment are 1.18 and 1.22 folded, respectively. In addition, the elastic absorbed energy is increased by 2.85 times as the longitudinal rebar ratio increased. As the ratios of core concrete and steel tube increase, the flexural stiffness is reduced by 5% and 6%, respectively. While, by increasing the core concrete and steel tube ratios, the ultimate moment grow by 1.05 and 1.29 times, respectively. The only effective index on the cross-section ductility and the entire absorbed energy is the prestressed strand ratio. The longitudinal rebar ratio has also the greatest increasing impact on the flexural stiffness and the elastic absorbed energy. Moreover, the core concrete ratio has the least effect (less than 10%) on the flexural stiffness. The prestressed strand and core concrete indices have respectively the highest and lowest escalating effects on the ultimate moment. As a consequence, an increase in the prestressed strand and longitudinal rebar ratios lead to a rise in the flexural stiffness and ultimate moment. On the contrary, an increase in the steel tube and core concrete ratios, decrease the flexural stiffness and gives a marginal increase to the ultimate moment. It was also unveiled that the failure mode of full-scale beams is flexural, and shear crack and shear capacity govern the behavior of PCE-CFST beams with shear span-to-depth ratios of less than 2. As shear span-to-depth ratio increases, that is, the shear failure mode shifts to flexural, flexural stiffness decreases, yet the ultimate bending moment increases. Additionally, a strut-and-tie model was proposed to describe the load transfer mechanism of PCE-CFST beams.

Keywords: Finite Element Analysis, concrete-filled Steel Tube (CFST), Prestressed Strand, Flexural Behavior.

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