| The concrete base plate of traditional composite box girder with corrugated steel webs is prone to tensile cracking,which seriously affects the durability and service life of the bridge.For this reason,domestic scholars have proposed a new structure,namely composite box girder with steel bottom plate and corrugated webs,which makes full use of the advantages of concrete compressive and steel tensile resistance by replacing the concrete bottom plate of traditional combined box girder with flat plate.Due to the lighter dead weight and higher prestressing efficiency of this type of combined box girder,it has been popularized so far with the gradual research on its mechanical properties.However,China has a vast territory with long coastlines and rivers along the eastern coast,and large areas of saline soils,salt lakes and saline groundwater in the northwest,etc.These environments contain high concentrations of chloride salts,and with the advancement of the whole life cycle design concept in the bridge design field,more and more attention has been paid to the durability design of bridge structures in chloride salt environments.In view of the rapid development of transportation and the health condition of box girder bridges in China’s chloride environment,it is more necessary and urgent to study the mechanical properties of composite box girder with steel bottom plate and corrugated webs bridges under the effect of chloride salt erosion.This paper is based on the project of Gansu Provincial Department of Transportation: "Research on the Whole Life Design and Application Technology of Highway Concrete Bridge Engineering in Gansu Province(2020-05)",aiming to investigate the vertical bending static and dynamic properties of this type of combined bridge under chloride salt erosion by using a combination of model test,theoretical analysis and numerical simulation,with a view to providing corresponding guidance for the durability design of combined box girders under chloride salt environment by the research results.The main research contents and results of this paper are as follows:(1)Two identical composite box girder with steel bottom plate and corrugated webs were fabricated in reduced-span simply supported models,one of which was not subjected to chlorine salt erosion while the other was fully immersed in chlorine salt solution for 17 months,and then the mechanical performance tests were carried out by means of concentrated loading,including(i)static bending tests,which analyzed the deflection and stress changes at the mid-span section of the two test girders under four levels of loading;(ii)self-vibration characteristics tests,which analyzed the effect of chlorine salt erosion on the self-vibration characteristics of the structure.The results show that chloride erosion has a greater effect on the static performance and self-vibration characteristics of the composite box girder with steel bottom plate and corrugated webs,and the weak point of the structure is the connection between the steel bottom plate and the corrugated web in the middle of the span,and compared with the concrete top slab,the chloride salt erosion has a greater effect on the stress of the steel bottom slab of the combined box beam.(2)The shear hysteresis effect and shear deformation effect are considered in the theoretical calculation to find out the total variable situation energy and load potential energy of this structural system.Based on the principle of energy variation,the controlling differential equations and boundary conditions of the combined box girder are derived;the effect of chloride salt erosion is considered as the reduction of the effective thickness of the concrete top plate and steel bottom plate of the combined box girder,based on which the evolution law of the mechanical properties of the composite box girder with steel bottom plate and corrugated webs under the effect of chloride salt erosion is explored,including the top and bottom plate stress,shear hysteresis and fold effect,and the change of vertical deflection of the combined box girder;Through theoretical analysis,finite element simulation and experimental exploration,the influence laws of shear hysteresis warpage stress and crease effect on the positive stress and deflection of the top and bottom plates under chlorine salt erosion were obtained.The results show that chlorine salt erosion basically does not change the stress distribution of the top and bottom plate of the combined box girder,so its shear hysteresis effect is negligible,but chlorine salt erosion increases the top and bottom plate stress significantly,especially the steel bottom plate,and the deflection of the new combined box girder,whether it is subject to chlorine salt erosion or not,basically conforms to the linear elastic law,while chlorine salt erosion increases the vertical deflection of the combined box girder;the theoretical calculated values of the positive stress and deflection of the top and bottom plate,finite element simulation values and experimental test values are basically consistent.(3)The self-vibration characteristics and dynamic response of the composite box girder with steel bottom plate and corrugated webs under the action of chloride salt erosion are investigated by applying the energy variational principle,finite element simulation and experimental tests.The results show that chloride erosion reduces the vertical stiffness of the combined box girder,and the results obtained by discounting the effective thickness of the airfoil are similar to the experimental condition,which indicates that the method of double approximation of stress and deflection of the top and bottom plates is effective in calculating the dynamic properties of the combined box girder under the action of chloride erosion.The effect of shear deformation on the flexural stiffness of the structure is greater than that of shear hysteresis effect;The self-oscillation frequencies of simply supported,continuous and cantilever composite box girders are reduced after chloride attack,and if the shear hysteresis effect is taken into account,the reduction of the inherent frequency of each order of cantilever composite box girders is greater than that of two-span continuous composite box girders,and the shear hysteresis effect has a greater effect on the higher order frequencies than the lower order;The dynamic stress values of the top and bottom plates of the composite box girder increase after chlorine salt erosion;in the simple harmonic bending forced vibration analysis,the shear hysteresis effect significantly affects the dynamic stress values of the top and bottom plates of the composite box girder,and the dynamic stress values at various points on the cross section of the span of the structure are unevenly distributed;The deflection of the span section of the simply supported beam increases with the increase of the simple harmonic concentrated force vibration frequency,and the shear deformation effect will affect the dynamic deflection of the span section of the simply supported composite box beam more significantly,while the shear hysteresis effect has a smaller effect.(4)The numerical simulation analysis of the composite box girder with steel bottom plate and corrugated webs was carried out by ABAQUS finite element software.By comparing the analysis with the experimental values,it can be seen that the results of the finite element model established by ABAQUS are more accurate,and by simulating the load carrying capacity analysis of the un-erosion model beam and the erosion model beam,it was found that although the maximum stresses of both the un-erosion and erosion model beams start from the middle of the span as the load increases the base plate to web joint starts to expand along the longitudinal,lateral and vertical directions of the beam until the steel base plate deforms plastically in the range of L/4 to 3L/4 in its entirety,but the chloride salt corrosion significantly reduces the load carrying capacity of the structure;At the same time,the displacements,stresses and strains of the composite box girder with steel bottom plate and corrugated webs at different depths of uniform erosion are compared and analyzed,and it is found that the elastic phase of the structure is shortened with the increase of the erosion depth,and the effect of the erosion depth on the load-deflection curve of this combined box girder is basically linear;combined with python programming to establish random erosion pits for the bottom plate of the structure,so as to simulate non-uniform erosion,it is found that the strain of the structure will increase suddenly near the pits,which may produce new damage points,but the connection between the bottom plate and the webs on both sides is still an important damage point of the structure. |
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