| With the development of economy, traffic volume increase at a fantastic speed. The urban bridges built in the past decades now cannot satisfy the demand of the modern transportation development and face the reality of being demolished and reconstructed. In order to save the fund and realize the maximization of the social economic benefits, bridge widening and strengthening is the most reasonable and feasible method of improving traffic capacity. To meet the requirement of urban bridge widening, this paper proposes a new box girder widening method-steel member widening concrete box girder method without piers (SCWCBM) based on the characteristics of urban bridge and the combination of other usual widening method. SCWCBM has many advantages thereinafter:there is no need for the additional piers; the clearance of span is free and the SCWCBM can establish the safety and reliability of the operation. In order to validate the SCWCBM's validity and popularize it, the relevant theoretical analysis and experimental research are as follows:(1) Based on the reaction force analysis theory of beam on elastic foundation, the analytical expression of transverse external tendon theoretical action sphere is deduced considering the steel-concrete interface stress mechanism under critical load cases.(2) This paper proposed an idea for the optimization design of the SCWCBM based on the steel-concrete interface contact state. Firstly, the steel cantilever beam optimization mathematic model is established by the topology optimization theory. The shape and the hole layout of the steel cantilever beam is designed according to the optimization results. Secondly, the acting position of the transverse external tendon can be chosen according to the action sphere. Thirdly, the stiffener details are designed based on the above design. At last, the design indexes should be verified by the interface finite element calculation results.(3) In order to master the torsional strength of the box girder widened by SCWCBM, a theoretical model for ultimate torsional strength of reinforced concrete single box and multi-cell box girder in combined force with shear and torsion is presented based on the modified compression field theory and the thin-wall tube theory. The ultimate torsional strength calculation formula is deduced by employing the virtual work principles and the minimum potential energy principle. Computational procedure is also established to calculate more quickly. The calculation formula that is presented shows a good agreement with the experimental results. (4) The whole design process of SCWCBM-a case study of one real project is detailed, including the optimization design of the SCWCBM based on the steel-concrete interface contact state, flexural capacity, shear capacity and torsional capacity checking, stress checking, stability checking, fatigue analysis. By the optimization design and the check a satisfying design proposal is obtained, which can be used in the engineering. The design process provides important theoretical guidance for the practical engineering.(5) In order to prove the engineering feasibility of SCWCBM, the full-scale model test is carried out. Comparison of finite element results and experimental results shows that the finite element model can simulate the actual stress state of widened bridge. Thus, a conclusion is gotten that the optimization design is proved reasonable and the theoretical calculation results are reliable. Throughout the model construction process, a set of construction technology is summarized. The main points and key technology of main construction procedure can be applied to the practical bridge widened by SCWCBM construction.
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