Preliminary Study On Strengthening Design Theory And Decision Making Methods Of Medium/Short Span Concrete Beam Bridges

In this thesis, prestressed concrete hollow plate girder bridge was taken as example to study the behavior of its superstructure mainly from the aspect of spatial stresses by using theoretical and numerical analysis method, and the design theory of the substructures built in soft ground was proposed. Besides, a reinforcement decision support system for the small and miduem-span concrete beam bridges was developed. The main conclusions are as follows:1) An accurate three-dimensional finite element model of a typical fabricated hollow slab beam bridge was built and the stresses in the control sections of the bridge were analysed to conclude that the main cause of longitudinal cracks along the hinge joints is the excess of transverse stress. Two schemes that involve transverse prestressed steel wire system to strengthen and improve the transverse load bearing capacity of such kind of bridges are presented, and numerical analyses and comparison of the two strengthening schemes are conducted. It can be drew from the results that both strengthening schemes can effectively reduce the transverse tensile stresses in the hinge jointsand the scheme with the prestressed steel wire set in the area25cm from the bottom of the beam is more effective and has more advantages in durability and security.2) In order to simplify the lateral load distribution of the strengthened bridges in1), the slab beams of these bridges were simulated as an orthotropic plate. An assumption was introduced to calculate the longitudinal and lateral stiffness of the simulated orthotropic plate. The modified G-M method was proposed to predict and analyse the lateral load distribution of the bridges strengthened by this kind of method. Meanwhile, two typical hollow slab bridges were taken as examples to conduct analysis with the modified G-M method and finite element method. The analysis results were compared with the static load test data and showed that there were mean error less than6%between the load transverse distribution coefficients calculated by the modified G-M method and numerical finite element method. These results accord well with the experimental test data. As for the side beam, the result calculated by the modified method is3.9%larger than the experimental result, which gives a considerable safety stock. Thus, the validity and feasibility of the stiffness assumption and the modified G-M method are verified. It provides a convenient and practical way to evaluate the lateral load distribution of the bridges strengthened by the kind of method.3) In designing the Pile foundation, the passive lateral compressive stress should be fully considered and the design should be guided by the ultimate lateral compressive stress in the body of the piles to achieve enough safety stock. From the calculation results,"the flow resistance method" and "the lateral pressure coefficient method" both can reflect the current situation of the foundation soil and the influences of roadbed fill on the foundation, thus both are feasible to calculate the additional horizontal stress value within all the depth under the ground. But when the condition of the soil is quite poor,"the flow resistance method" could fully take the impact of lateral soil movement on the pile into consideration, thus is more feasible when the roadbed fill is high for the safty consideration.4) In this thesis, the knowledges of main failure mechanism of small and medium-span concrete bridges, strengthening method, hierarchical analysis, fuzzy comprehensive evaluation and software development technology were combined, Microsoft Access2007, Visual Basic6.0and Matrixvb were employed as tools, and object-oriented programming method was used to develop the reinforcement decision support system for small and medium-span bridges on the Windows platform.