Study On Mechanical Properties Of The Thin-walled Box Girder With Dense Diaphragm Plates Composed Of Ultra-high Performance Concrete

In order to improve the continuous beam bridge with main span of 400 m and solve the ever-existing problems of long-span pre-stressed concrete(PC)bridges,such as the concrete beam being subject to cracks and the difficulty in controlling the long-term deflection at mid-span,study on mechanical properties of the thin-walled box girder with dense diaphragm plates is carried out in this dissertation,based on the conceptual design of the new continuous box girder bridge composed of ultra-high performance concrete(UHPC).The conceptual design and preliminary experiment of the new continuous box girder bridge,stability of flange panel in the box girder under compression and stability of the web panel under shear were studied.The main contents of this dissertation are described as following:(1)A new super-long-span continuous box girder bridge composed of UHPC was proposed.With the organic combination of UHPC,thin-wall box girder with dense diaphragm plates,and partial external-prestress technique,the new box girder system transforms the traditional three-dimensional prestresses into a new one-way prestress in longitudinal direction.It not only improves the main span of beam bridges,but also eliminates the ever-existing problems of long-span PC bridges,such as the concrete beam being subject to cracks and the difficulty in controlling the long-term deflection at mid-span.The experiments of model beams were carried out to explore the creep deformation and shear capability of UHPC beam,the results show the creep deflection of UHPC beam is only 20% of the ordinary concrete beam’s,and the shear capability of web can be further improved by setting up dense diaphragms.Using UHPC and the thin-walled box girder with dense diaphragm plates,the new box girder bridge has good economic index,and its dead weight of superstructure can lighten largely,compared with the traditional PC bridge.It is applicable to the continuous beam bridge with main span of 400 m.(2)The stability behavior of flanges in the thin-walled box girder with dense diaphragm plates composed of UHPC is studied by theoretical analysis,simplifying the supporting role of flexible diaphragm plates in stability of flange panel in the box girder under compression with the elastic supports.The analytical methods for the elastic buckling analysis of multi-span continuous thin plates with elastic supports are proposed.By the proposed method,this paper investigates the following parameters that influence the elastic buckling,including the stiffness of the elastic supports,the arrangement of the position and spacing of the elastic support,and the in-plane compressive loads condition.The results demonstrate that the elastic supports have a significant influence on the elastic buckling of a continuous plate,and the buckling coefficient increases non-linearly as the support stiffness increases.Elastic supports with equal spacing and equal stiffness are beneficial to the stability of a continuous plate,while the significant difference of the support stiffness or the support spacing will result in obvious unfavorable influence to the stability of the continuous plate,and the larger difference the more unfavorable.In order to improve the stability behavior of flanges in the thin-walled box girder composed of UHPC,it is suitable to set up the diaphragm plates with equal spacing and equal stiffness,and to increase the stiffness of diaphragm plates properly.To improve the stability behavior of bottom plate in the box beam,the angle between bottom plate and diaphragm plates should be restricted to a certain range.(3)The stability behavior of flanges in the thin-walled box girder with dense diaphragm plates composed of UHPC is studied by finite element analysis,including the elastic buckling mode and the buckling load.The results show that the buckling load increases as the stiffness of diaphragm plates increases.The buckling load increases slowly when the half-wave number of the buckling mode increases to the number of diaphragm plates added one.Due to the joint influence of the curvature of bottom plate and diaphragm plates,the position of maximum out-plane displacement in the buckling mode of the bottom plate is not the mid-span,but where the capacity of resisting buckling is relatively weak.The ultimate bearing capacity of the flanges in the trial-design box girder is controlled by the buckling behavior of its flanges under compression,and we should pay more attention to the stability behavior of its flanges in the design of thin-walled box beam composed of UHPC.(4)The stability behavior of webs in the thin-walled box girder with dense diaphragm plates composed of UHPC is studied by theoretical analysis and finite element analysis.An analytical method for the shear buckling analysis of multi-span continuous rectangular thin plates with elastic supports is proposed.The present method is verified with the result of the finite element method.The results show that the shear buckling mode of the multi-span continuous rectangular thin plates with elastic supports lies between that of the non-supports plate and that of the continuous rectangular with rigid supports.The shear buckling load of its web can be improved by setting up flexible diaphragm plates.The ultimate bearing capacity of the webs in the trial-design box girder is controlled by the material strength of box beam,and we should pay more attention to the anti-shear capacity of its webs in the design of thin-walled box beam composed of UHPC.