Mechanical Performance Analysis Of Trussed Longitudinal Diaphragm In Steel Box Girder Of Cable-stayed Bridge

At present,streamlined flat steel box beams are widely used in the main beam section of modern large-span cable-stayed bridges.Flat steel box beams have the advantages of large torsional stiffness in section,excellent wind resistance stability,and standardized production.Considering the factor of shear lag,two longitudinal partitions are often added to the flat steel box girder.Longitudinal partitions can enhance the bending,shearing and torsion resistance of the steel box girder,reduce the local deformation of the bridge deck,and the deformation and stress of the cross section,thereby enhancing the safety reserve of the steel box girder.Longitudinal partitions are divided into two types: plate type and truss type.In order to save steel consumption,truss type longitudinal bulkheads are usually installed at the positions other than the tower beam joint position and the auxiliary pier top.However,many bridges with trussed longitudinal partitions have been inspected and found a large number of cracks after many years of operation,affecting the normal operation of the bridge.At present,few scholars have studied the mechanical performance of the longitudinal partitions of steel box beams.This article takes a large-span cable-stayed bridge in actual operation as the engineering background.In this paper,the finite element software Midas Civil and Abaqus are used to establish a full-scale multi-scale model and segment model to analyze the stress mechanism,fatigue performance and the causes of cracks of different types of longitudinal diaphragms.This article mainly did the following research:(1)This article introduces the basic structure of flat steel box girder and the current research situation at home and abroad,and expounds the basic theory of multi-scale modeling method for long-span bridges.This paper also establishes a verification model to compare and verify the multi-scale model of beam-slab element using the multi-point constraint equation method;This article introduces the basic theory of fatigue performance research,as well as fatigue check calculation and fatigue life assessment process;This article compares and summarizes the construction deviations in national codes and local acceptance standards.(2)In this paper,based on the actual bridge engineering example,the distribution of cracks in the longitudinal diaphragm of the steel box beam is statistically analyzed.In this paper,Midas civil is used to establish a multi-scale model,and the most unfavorable position of the truss longitudinal diaphragm is obtained through trial calculation.The stress change trend of the key members of the diaphragm in the multi-scale model is consistent with thetrend of the crack distribution of the diaphragm in the actual bridge inspection.By comparing the mechanical properties of the key members of different types of longitudinal diaphragms,it is found that the secondary stresses will be generated due to the constraint of the web rods due to the out-of-plane rotation of the upper node plate,and three types of longitudinal diaphragm have different levels of stress concentration effects.The stress concentration effect of the corner point of the upper node plate of the steel tube type vertical diaphragm and the top end of the steel tube is significant,and the stress diffusion of the channel steel and angle steel vertical diaphragm is relatively stable and smooth.(3)In this paper,Abaqus was used to establish steel box girder segment models of steel tube,channel steel and angle steel truss longitudinal diaphragms.This article selects the fatigue load in the Chinese code for loading,compares and analyzes the transverse and longitudinal stress history of each fatigue detail,and then performs fatigue check calculation and fatigue life assessment.The results show that under the repeated load of the vehicle,the nodal plates and webs are constantly subjected to alternating tensile and compressive forces,which is an important cause of the expansion of the corner cracks of the nodal plates along the main compressive stress trace and the circumferential fracture of the webs.The fatigue detail stress amplitude of the steel tube type longitudinal diaphragm joint plate is greater than the fatigue strength,so this position is the starting position for fatigue cracks to occur in actual detection.The assessment shows that the fatigue details of the steel-tube longitudinal diaphragm reach the fatigue life after 7-11 years,which is consistent with the results of the2013 test report;The individual fatigue details of the angle steel longitudinal diaphragm will also produce fatigue cracks when the number of stress cycles is sufficient;The fatigue performance of channel steel longitudinal separator is better than that of angle steel longitudinal separator,and the fatigue performance of angle steel longitudinal separator is better than that of steel pipe longitudinal separator.(4)This paper analyzes the influence of construction deviation and design parameters on the mechanical performance of trussed diaphragm based on the segment model and the design data of diaphragm.The results show that the construction method of forcibly correcting the joint plate should be avoided during the assembly process of the steel box girder,so as to avoid the damage of the longitudinal diaphragm caused by large additional stress;The stress and stress concentration effect of the welding area between the nodal plate and the steel pipe can be reduced by appropriately increasing the thickness of the nodal plate;The overall force of the longitudinal partition is preferably when the thickness of the steel pipe is maintained at about 6.5mm...