Research Of Spatial Effects Of Concrete Box Girder Structure With Long Span

RC and PC box-girder structure, with its various advantages, have been widely applied in bridge engineering at home and abroad. Considerable Research of spatial mechanical effects of box girder is under way throughout the world. On the basis of generalizing and improving the classical thin-walled beam theory, the aim of this dissertation is to deal with the existing problems in the calculation and analysis of restrained torsion and distortion of concrete box girder structure. Based on the typical concrete box girder bridges in china, in-depth research has been carried out on the following aspects.1. Comparative Analysis and improving of restrained torsion theory of box girder:Based on comparative analysis and improving of restrained torsion theory of box girder, a new theory of restrained torsion is presented based on the mixed variational principle, which fully take into account the secondary shear deformation effects. Based on the new theory, the uniformity between the new theory and the existed open and closed thin-walled theories is analyzed. Research shows that differences among these theories rooted in differences of taking into account the secondary shear deformation. The calculation results based on Umanskii's closed thin-walled theory and Vlasov's open thin-walled theory are conservative.2. Comparative Analysis and improving of distortion theory of box girder:Based on comparative analysis and improving of distortion theory of box girder, it is first proved that calculation results of the two kind of theory which respectively takes distortion deflection and distortion angle as displacement variable are consistent, and their reciprocal equations are established. A new approach determining generalized coordinates of box-girder distortion was put forward, and based on the mixed variational principle, a new theory of distortion is presented, which fully take into account the secondary shear deformation effects. Based on the new theory, the uniformity between the new theory and the existed theories is analyzed. The research clearly explained why, the first-order derivative of distortion angle can be used as warping function in the distortion analysis of closed thin-walled beam, while the first-order derivative of torsion angle can not be used as warping function in the restrained torsion analysis of closed thin-walled beam.3. Restrained torsion and distortion of thin-walled box girder with cantilever flanges:Based on the classical thin-walled box girder theory and analyzing the internal stress state of thin-walled box girder with cantilever flanges under restrained torsion and distortion, a practical theory analyzing restrained torsion and distortion of thin-walled beam with open and closed cross-section is established. The research shows that contribution of cantilever flanges of thin-walled box girder to its behavior in restrained torsion and distortion lies in the proportion of its warping moment of the whole, and cantilever flanges cause significant redistribution of restrained torsion normal stress and distortion normal stress, and the contribution can not be ignored for common concrete box girder of railway ang highway.4. Restrained torsion and distortion of thick-walled box girder:Based on the generalized coordinates principle, a analytical theory of the restrained torsion and distortion of thick-walled box girder is established, and then it's existing problems are found. At the same time, a layered approach for analyzing that is established. In the research, one new internal force(St.Venant's restrained torsional moment)is first put forward, the differences between it and St.Venant's pure torsional moment is that the new internal force participates in the balance of warping normal stress. At the same time, tow new internal forces (St.Venant's pure distortional moment and St.Venant's restrained distortional moment) are first put forward. It is proved that so-called Bredt's pure distortion does not exist in the distortion of box girder with single cell and single box section.5. The variable cross-section segment element model considering restrained torsion and distortion effects of box girder:Based on the above theories, taking the homogeneous solutions derived from the governing differential equations as the displacement patterns of finite segment, using second-degree shape function to interpolate cross-section characteristic parameter of the variable cross-section segment element, its stiffness matrix and load column matrixes are derived by means of variation principle. Thus a semi-solution beam-segment element model is established considering restrained torsion and distortion effects. The proposed approach can achieve higher calculation accuracy with extremely few discretization nodal unknowns, and conveniently bring into the program system for the conventional beam-bar elements as well as improve resolution efficiency, thus laying a foundation for the study of the restrained torsion and distortion of box girder.6. Research on the spatial mechanics effects in box girder and the influence on behavior of bridges:The dissertation has analyzed that restrained torsion and distortion effects of typical highway flat box girders, high-speed railway box girders, integral type box girders, and concrete continuous rigid frame bridges in china, and then has obtained their restrained torsion and distortion stresses and the amplification coefficients of stress. Based on the analysis of the results, some important conclusions have been made for the spatial mechanics effect of box girder bridges. Combining the structure type and size for box girder, the author has found that the factors such as the span, height to width ratio of cross-section, slope ratio of web plate, diaphragm have a great effect on restrained torsion and distortion, and brought to light the objective law governing restrained torsion and distortion of box girder.