Study On The Structral Behaviors Of Prestressed Concrete-Filled Rectangular Steel Box Beams

At present, the development of concrete-filled steel tube (box) is one of the hot topics in the fields of steel-concrete composite structures. Because of the good mechanical performance and beautiful appearance, the concrete-filled steel tube (box) structures are widely applied in industrial and civil buildings. Though there are many researches on the concrete-filled steel tube (box) structures subjected to compression, but a few studies have been done on flexural behavior, moreover studying the bending behaviors of concrete-filled steel tube (box) structures is an important issue for actual bridge engineering structures. In this dissertation, supported by a research project "Study on the Key Technology of Design and Construction for Long Span Arch Bridge" sponsored by science and technology fund of Sichuan Province Communications Department, taking the Dongping bridge as study background, the author carries out systematic researches on the prestressed concrete-filled rectangular steel box beam. The dissertation includes five parts mainly:In the first part, based on a large number of literatures, a preliminary study on the bending behavior of concrete-filled circle (rectangular) steel box is carried out by means of the limit equilibrium theory and unified theory. At the same time, the advantages and disadvantages of prestressed steel structures are also discussed. Based on the end transverse beams of Dongping bridge, which adopt the prestressed concrete-filled rectangular steel box beams, a research project to study the behaviors of prestressed concrete-filled rectangular steel box beam is put forward.In the second part, static load tests on eight prestressed concrete-filled rectangular steel box beams have been done. The main test parameters are concrete strength (50 MPa and 60MPa), prestress ratio (0.26 and 0.40) and thickness of steel plate (6mm and 8mm). During the load test, deflection, stress state and slip between steel and concrete are measured. The test results show that the prestressed concrete-filled rectangular steel box beams have good working and deformation behaviors, and the steel plate is restrained against inward buckling by the infill concrete. The confinement effect of core concrete in longitudinal compression zone is proved due to the prestress and steel box. The maximum deformation at structure failure is nearly 200mm, namely is 1/25 of the span, so the ductibility is up to the hammer and in favor of meeting the requirement of aseismic design. Through the comparison of test results about the three interactional parameters as concrete strength, prestress ratio and thickness of steel plate, the member with prestress ratio of 0.40, thickness of steel plate of 8mm and higher concrete strength has higher ultimate strength and better plastic yielding ability. Although slip between concrete and steel occurred in final loading stage, and the local buckling also exists before the maximum bearing capacity is reached in most members, but the ulimate strength didn't decrease abruptly.In the third part, based on the stress-strain constitutive relations of materials, the interfacial bond strength relating steel and concrete is taken into account, a three-dimensional nonlinear finite element model using popular FEM software is established for analysing the performance of the prestressed concrete-filled rectangular steel box beams, and the calculated results are in good agree with the test ones. From the load-middle point deflection curves, they could be classified into the following three stages: elastic stage, yield stage and plastic stage. When the member is going to fail, local buckling takes place at many steel plates in longitudinal compression zone, there are many uniform distributing cracks in longitudinal tension zone, and the width of these cracks is still small though most of them have exceeded the half of the height of the section. Slip between steel and concrete has occurred and the maximum value is in the range between mid-span and the end of the beam. The shape of slip is similar to sinusoid curve and the author also provides a statistical equation from the results. Moreover, we could ignore the influence of slip because of a few slip will happen if there are enough perfobond shear connectors.In the fourth part, with reference to the relevant provisions in the design specifications of several countries in the world, a computational method is established. This method is divided into elasto-plastic and plastic one. The elasto-plastic method is good suitable for engineering design and the plastic method could be used to check the results of scientific research. The author suggests that we could use the provisions in the CECS code to calculate the initial stiffness. Ignoring the effect of initial cracking, the stiffness of prestressed concrete-filled rectangular box beam can be simplified as a tri-linear model. Equations of the bending stiffness are also proposed.In the fifth part, a fiber analysis method is provided to predict the behaviour and ultimate strength taking account of the influence of local buckling and residual stress. Some useful conclusions are obtained by the extending parametric study.