With the development of modern high speed and heavy haul transportation, new challenges are confronted by railway bridges and highway bridges. Highstrength concrete and steel fiber reinforced concrete, which have excellent mechanical properties, have called more concern, and have wideranging application in railroad and highway bridge. The dissertation focuses on numerical simulation of the nonlinear structural behavior of prestressed highstrength concrete boxsection continuous beam. It consists of: (1) The constitutive model of concrete based on microplane theory is studied. Microplane theory, a theory based on the micromechanics, has been developed in the last ten years. On the basis of the author′s research fruit, some modifications about the effect of the principal stress combination on the shear stiffness of the microplane and loading/unloading criterion of the microplane are proposed. In Prof. Bazant′ paper, the confining stress is used to reflect the effect of the principal stress combination on the shear stiffness of the microplane. In this dissertation, the author adopts the direction cosine between vector and vector to describe the effect of the principal stress combination on the shear stiffness of the microplane. New loading/unloading criterion of the microplane is developed. The new criterion includes all cases that may happen in loading or unloading, and effectively overcomes the drawbacks that the unloading stress may exceed the limit stress of microplane. On the basis of those researches, the constitutive model of concrete is established. To verify the accuracy and efficiency of the model, several classical tests are studied and excellent agreements are obtained. (2) The basic mechanical behaviors of highstrength concrete and steel fiber reinforced concrete are studied. The compressive strength, tensional strength, elastic module, axial stress and strain relationship, the failure criterion under multiple axial stress, section ductility
