| Segmental precast assembly technique, mainly applied in long-span bridge structures, is a method to assemble the prefabricated segments as a whole using prestressing technique. It is a construction technique of great importance in bridge structures, with the advantages of standard prefabrication, controllable quality, low construction machinery requirements, short construction period, little environment affection and so on. The essential difference between segmental structures and over-all structures is that the regular reinforcement bars are discontinuous at the seam crossings in segmental structures. As the seams are connected in different forms, the stress performance of the structures in ultimate state varies much and needs to be analyzed systematically. Existing theories on segmental precast assembly bridges focus mainly on a few key aspects as follows:cracking resistance, beam stiffness in different load stages, flexural capacity in ultimate stage, shear failure mode and the corresponding bearing capacity in ultimate stage. In the aspect of bending resistance, based on the essential cognition of the destruction of segmental assembly beams and over-all beams, this paper established calculation formulas for stress increment of the external prestressing tendons in both segmental assembly simply supported beams and continuous beams, analyzed internal force redistribution of segmental assembly continuous beams, and established calculation formulas of flexural rigidity at different load stages. In the aspect of shearing resistance, this paper conducted systematically analysis on the shearing failure mode of single shear connection and the failure mode of multiple shear connections, put forward corresponding numerical calculation models and their simplified models. Ulteriorly, shear failure mode of segmental assembly beams was studied and this paper came up with the calculation formula of their shearing capacity. To verify the theoretical analysis above,10 shear specimens with shear connections,4 simply supported beams and 1 three-span continuous beam were designed for experiments. The main research contents and results of this paper are as follows.(1) Rigidity calculation formula of segmental precast assembly beams in overall process was derived. A new concept of seam section rigidity influence region was proposed based on bond-slip theory. By ensuring the same rotation angle after equivalency, the equivalent stiffness calculation formula of segmental precast assembly beams under different load levels was derived. This formula, possessing clear physical significance and high calculation precision, takes in the form of triple line, which is divided into non-crack stage, normal service stage and ultimate stage.(2) Calculation formula for ultimate stress increment of the external prestressing tendons in segmental precast assembly beams was derived. This paper discussed existing methods for calculating stress increment of the external prestressing tendons, established calculation model of stress increment and ultimate displacement of overall external prestressing simply supported beams basing on equivalent plastic hinge theory. A concept of ultimate displacement proportional coefficient of segmental simply supported beam to overall simply supported beam was proposed. And the ultimate displacement calculation formula of segmental assembly beams was derived. According to the proportional relationship between the mid-span displacement and external prestressing tendon stress, the ultimate stress increment calculation formula of externally prestressing tendons in precast assembly simply supported beams was obtained and simplified.(3) The concept of correction factor χ of plastic hinge location in continuous beams was proposed. Basing on equivalent plastic hinge theory, the ultimate mid-span displacement calculation formula of continuous beams was derived, and the ultimate stress increment calculation formula of externally prestressing tendons in segmental assembly continuous beams was obtained further, both were verified by experimental results. A mechanical model, using fiber beam element, of segmental precast assembly continuous beam was built. This model considered section rigidity influence length in segmental beams and analyzed stress redistribution influence factors. The results provide reference for the design of segmental assembly continuous beams which consider stress redistribution.(4) 4 simply supported beams (1 overall and 3 segmental) and a three-span continuous beam were tested for flexural experiment. According to the results, the parameter values for crack resistance analysis of segmental assembly beams were given, and the established calculation formula of stress increment was verified.10 shear resistance experiments of segmental assembly beams with different seam types were conducted considering factors such as horizontal stress levels, dry joints and glue joints, shear key size, plain joints and shear key joints. Simplified calculation formula was verified by the experiment results.(5) By taking advantage of the CDP concrete model provided by finite element software ABAQUS, preliminary analysis was carried out to understand the failure mechanism of shear keys. Analysis results were compared with existing single shear key destruction tests, thus recommended parameter values for CDP model in shear resistance analysis were given. Being aware of the disadvantages of CDP model, this paper developed a new concrete shear analysis model basing on the subroutine VUMAT in ABAQUS. This model took into account concrete failure criterion, Ⅱ-type fracture energy and the influence of friction shear stress to the destruction of shear keys. On the basis of existing shear key destruction experiments, parameter values were defined and the model validity was verified.(6) Parameter analysis of shear key type specimens was conducted utilizing the developed concrete shear resistance model. Factors such as shear key shape, horizontal stress level, shear key amount and seam joint type, influencing the shear capacity of shear keys were considered. Sequentially, shear capacity calculation model of the seams was proposed and calculation formula of straight shear capacity for different seam types was derived. The formula obtained clear physical significance, uniformed format parameters and had high precision comparing to experiment results.(7) On the basis of difference analysis of segmental assembly beams and overall beams, this paper concluded different shear failure modes and failure mechanism of segmental assembly beams and analyzed the influence of different seam types to shear capacity of concrete. Basing on concrete transgression failure criterion and considering the contribution of compression flange to shear capacity, analysis procedure of seam straight shear destruction was programed. By adopting a simplified method, this paper deduced concrete shear capacity from the equilibrium equation and formed the calculation formula of seam straight shear capacity, which was verified by experiment results. According to trial calculation results of different examples, conditions for straight shear destruction in segmental assembly beams were provided and they were available for shear resistance design of bridges in preliminary stage. |
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