| With the development of modern high speed and high haul transportation, newchallenges are imposed on railway bridges and highway bridges. High-strengthconcrete and steel fiber reinforced concrete, which have excellent mechanicalproperties, have been paid more and more attentions by engineers. To meet thechallenges of high speed and high haul transportation, scientists and engineersare attempting to construct railway bridges and highway bridges withhigh-strength concrete and steel fiber reinforced concrete. A research prOject'The StructuraI Behavior of Partially Prestressed High-Strength ConcreteVariable Box-Section Continuous Beam', which is sponsored by the NationalOutstanding Youth Tutor Scientific fund of Education Ministry of China, isundertaken by Department of Bridge and Structural Engineering of SouthwestJiaotong University. Several features are emphasized in the project. First, thetest model is two-span variable box-section continuous beam. Then, thepartially prestressed concrete is adopted. Third, C80 high-strength concrete isused in the test beams. Finally, to increase the ductility of the test beams afterthe crack of mid-support, steel fiber reinforced concrete is used in themid-support zone.As part of the whole research project, several works have been undertaken inthis dissert8tion. First, the constitutive model of concrete based on micropIanetheory is studied and the constitutive behaviors of high-strength concrete andsteel fiber reinforced concrete are described. Second, the material nonlinearfinite element analysis program of concrete plate, shell and box-sectionstructures is developed. Third, the sttuctural behaviors of test beams aresimulated by the program developed by the aathoLIn the first part, the constitutive model of concrete, a key matter of materialnonlinear finite element analysis of concrete structure, is studied. A lot ofimportant works have been done in this field since l960's. In recent years, Pro fBazant has been devoting to the work ofmicroplane theory Microplane theorya theory on the basis of micro-mechanics, has excellent ability of describing notis ivonly the concrete constitutive behaviors under non-proportional load but also the damage behavior of concrete under multi-axial stresses. Based on the author's research, some suggestions and modifications about microplane theory are proposed. Finally, the constitutive behaviors of high strength concrete and steel fiber reinforced concrete are described by the constitutive model developed by the author.Several works are included in the second part. First, the four-node degenerated shell element is adopted to simulate the nonlinear structural behavior. Degenerated shell element can deal with the crack and crush of concrete, the plasticity of steel bar in shell element conveniently. Second, to study the whole-process behavior of the concrete structure, the arc-length method is introduced in the nonlinear analysis program. In the past studies of arc-length method, the works mainly focused on the structural geometrical nonlinear analysis. In the case of the application to the material nonlinear analysis of concrete structure, especially when the concrete strain-softening behavior is included in the constitutive model, the studies are limited. In previous researches, the attentions were mostly paid to the local negative stiffness caused by concrete crack, and in this case the global stiffness matrix was rarely negative. Furthermore, and more important, in those studies, the constitutive model of concrete is comparatively simple. In this dissertation, the global negative structural stiffness is concerned, and a comparatively complex constitutive model, the microplane theory, is introduced in the numerical study. Finally, several examples are studied, and good agreements with test results are obtained.In the final part, the structural behaviors of six pieces of test beams are simulated by the program developed in the first part and the second...
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