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Research
Fatigue Life Study for Floorbeam Cutout Using the Theory of Critical Distances
Zhuoyi Chen, Yu Yang, Chuanxi Li, Zhenhai Zeng, Jianbo Zeng
2021, 36(10): 1-6.   doi: 10.13206/j.gjgs21062201
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Abstract:
Orthotropic steel bridge deck is widely used in civil engineering because of its light weight, high ultimate bearing capacity and wide application range. But its fatigue problem is serious. Steel box girder of diaphragm plate arc incision anti-fatigue design is still a difficulty, to effectively predict the fatigue life of the typical fatigue vulnerability details, using ABAQUS finite element analysis software, set up three kinds of fatigue specimen the simplified finite element model of arc incision, through three different radius arc incision specimen under tensile load of 50 kN finite element model, the independent stress nephograms were obtained respectively. It can be seen that the first principal stress at the notch decreases with the increase of the notch radius. When the radius increases from 10 mm to 20 mm, the first principal stress decreases from 319.8 MPa to 253.7 MPa, with a reduction of 66.1 MPa. When the radius increases from 20 mm to 30 mm, the first principal stress decreases from 253.7 MPa to 225.6 MPa, with a decrease of 28.1 MPa, which becomes gentle compared with before. Then the critical distance theory is introduced to analyze the characteristic stress in the stress concentration area of curved notch, and the point method and line method are used to calculate the characteristic stress. ABAQUS was used to set the path from the maximum principal stress peak according to the direction of the highest stress gradient to extract the first principal stress at each point and the distance between the point and the maximum principal stress peak. On the hot spot path, the maximum principal stress is inversely proportional to the distance. Within the critical distance of 1.2 times, the smaller the notch radius, the higher the stress level; In the region beyond 2 times the critical distance, the maximum principal stress curves of the notch radius of 20 mm and 30 mm show a close trend, indicating that the notch is no longer the key factor affecting the stress level. Thus, key parameters such as the critical distance and the threshold value of crack growth are obtained. Combined with the results of fatigue limit and FE-safe life, the fatigue life prediction models of three notched members were established. The accuracy of the simplified model was verified by the fatigue tests of the diaphragmcurved notch segments. The influence of different notch radius on the fatigue life of the specimens was studied.
The results show that:1) the predicted stress value based on the point method is 7%~13% higher than that based on the line method, and the predicted fatigue life value based on the point method is more than 50% lower than that based on the line method at the arc notch of the steel box girder diaphragm. The point method is more conservative than the line method; 2) the fatigue prediction model of the steel box girder diaphragm arc notch proposed in this paper has a high accuracy, and the error between it and the fatigue test results of the section model is basically within the range of 20%; 3) no matter using point method or line method, the calculated characteristic stress decreases with the increase of notch radius, the fatigue life increases with the increase of notch radius, the fatigue life of steel box girders can be improved by increasing the radius of the curved notch; 4) the fatigue specimens used in this paper are all smooth curved notch specimens. For the components with initial defects, the calculation model of fatigue life evaluation using critical distance theory needs to be further studied.
Numerical Simulation of Distortion-Induced Fatigue Effect in Steel Plate Girder Bridges
Yuzhu Wang, Chang Shu, Peng Wang, Xingguang Zhou, Chunsheng Wang
2021, 36(10): 7-15.   doi: 10.13206/j.gjgS21091001
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Abstract:
For two type of distortion-induced fatigue details at web gaps(vertical stiffener web gap and horizontal gusset plate web gap) in the steel plate girder bridges, finite element models of the straight, skewed and curved three-span continuous steel plate girder bridges were built. The out-of-plane distortion and fatigue stress of the web gap fatigue details of steel plate girder bridges with different spans, oblique angles and horizontal curve radius are investigated by numerical simulation. Parametric analysis of fatigue effect in skewed and curved bridges is carried out. The simulation results show that when under the vehicle loads, the relative out-of-plane deformation and fatigue stresses will occur at the web gap details. The relative out-of-plane distortion and the peak fatigue stress of the vertical stiffener web gap details in the straight bridge are 0.079 mm and 105.6 MPa; the relative out-of-plane distortion and the peak fatigue stress of the horizontal gusset plate web gap details in the straight bridge are 0.006 mm and 10.9 MPa. The distortion-induced fatigue effects of two types of web gap details in the skewed bridge and the curved bridge are significantly greater than that of the straight bridge, and the out-of-plane distortions are larger than that of the straight bridge. The peak fatigue stress of vertical stiffener web gap in the skewed bridge and the curved bridge are 2.4 times and 1.7 times as much as that in the straight bridge; the peak fatigue stress of horizontal gusset plate web gap in the skewed bridge and the curved bridge are 2 times and 2.9 times as much as that in the straight bridge. The relative out-of-plane deformation of the fatigue details has a strong correlation with the stress level at the fatigue details, and small relative out-of-plane deformation can cause high fatigue stress. In skewed bridges, only 0.15 mm relative out-of-plane distortion at the horizontal gusset plate web gap details can cause 250 MPa vertical bending tensile stress. Parametric analysis results show that the fatigue stress at details in skewed bridge increases with the increase of oblique angle, while the stress at fatigue details in curved bridge increases with the decrease of horizontal curve radius. In the straight bridge with a span arrangement of(45+70+45) m, the maximum tensile stresses at the vertical stiffener web gap details and the horizontal gusset plate web gap details are 105.6 MPa and 10.9 MPa. The maximum tensile stresses at the corresponding details in the skewed bridge and curved bridge with the same span are 250.5 MPa and 21.8 MPa, 176.5 MPa and 31.9 MPa, respectively, which are significantly greater than the maximum tensile stress at the corresponding details in the straight bridge with the same span.
Research on Extremely Low-Cycle Fatigue Crack Initiation and Propagation of Thick-Walled Steel Box-Section Bridge Piers
Ting Zhu, Shengbin Gao
2021, 36(10): 16-24.   doi: 10.13206/j.gjgs20061203
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Abstract:
In order to investigate the Extremely Low-Cycle Fatigue(ELCF) crack initiation life and crack propagation process of thick-walled steel box-section bridge piers, a series of thick-walled steel box-section piers subjected to a constant vertical load and cyclic lateral loading are numerically simulated by using ABAQUS software package. To improve the computing efficiency, 1/2 of the pier is modelled and the method to combine the shell element and beam element is employed to simulate the lower part and upper part of the piers, respectively. The crack initiation criterion based on Rice-Tracey model and the combined hardening model of steel is utilized to predict ELCF crack initiation life. A method based on ultimate fracture displacement is used to simulate ELCF crack propagation process. The accuracy of the crack initiation criterion and the crack propagation criterion to predict the ELCF crack initiation life and crack propagation process is verified by comparing the analytical results with test results. The factors affecting the ELCF crack initiation life and crack propagation process of the piers(i.e., normalized flange's width-thickness ratio, normalized slenderness ratio, and loading pattern) are parametrically studied, and the failure modes of the piers under different structural parameters are investigated. The difference between the crack initiation life and the ELCF failure life is defined as the ELCF residual life. The effect of structural parameters on the ELCF residual life of the piers is discussed. The agreement of the crack initiation life between the simulation result and test result indicates that the crack initiation criterion based on the Rice-Tracey model and the combined hardening model of steel can accurately predict the crack initiation life of the piers. The agreement of the crack growth length between the simulation result and test result demonstrates that the crack propagation criterion based on the ultimate fracture displacement can accurately predict the ELCF crack propagation process of the piers. On the basis of verifying the accuracy of the numerical analysis method, three types of failure modes(i.e., ELCF failure mode, local buckling failure mode and mixed failure mode) under different structural parameters are proposed by comparing the occurrence time of the ELCF failure and the local buckling failure, as well as the crack growth length based on the parametric study.
It is concluded that ELCF failure often occurs when the normalized flange's width-thickness ratio and the normalized slenderness ratio become relatively small, while the local buckling failure occurs when they are relatively large. The ELCF residual life of the piers is not only related to the normalized flange's width-thickness ratio and normalized slenderness ratio, but also related to the loading pattern. The research results can provide significant reference value for the seismic design of such steel bridge piers.
Study on Effect of UIT on Welded Residual Stress of Innovative Doubly-Welded Rib-to-Deck Joint in Orthotropic Steel Decks
Hongsheng Liu, Yongchuan Luo, Shaohui Han, Qinghua Zhang
2021, 36(10): 25-33.   doi: 10.13206/j.gjgs20092301
Abstract(0) PDF (11275KB)(0)
Abstract:
Compared with traditional singly-welded rib-to-deck joint(SRJ), there exists a greater number of welds and more complicated welded residual stress(WRS) distribution in innovative doubly-welded rib-to-deck joint(DRJ), and it is of great significance in further promotion and application of DRJ to develop applicable post-treatment measures for adjusting WRS in DRJ. In order to study the effect of ultrasonic impact treatment(UIT) on WRS of DRJ, the welding process of DRJ was simulated by thermal-stress sequence analysis to obtain the distribution of temperature fields and stress fields; then the obtained WRS was imported into the finite element model of UIT as its initial analysis state by stress-strain initialization techniques; finally, the UIT process was simulated based on the dynamic explicit analysis, and the numerical oscillations of the dynamic explicit analysis were reduced by mass proportional damping technique.
To demonstrate the reasonability of the multi-step sequential analysis finite element model, it was validated from the following aspects:the analyzed melting zone shape was compared with the real weld morphology to validate the correctness of the analyzed temperature fields; the analyzed longitudinal and transverse WRS was compared with the test data to validate the correctness of the analyzed stress fields; the analyzed impact pit and impact residual stress were respectively compared with the real impact pit and the test data to validate the correctness of UIT simulation. On basis of fully validating the correctness of the finite element model, study on the effect of UIT on WRS in DRJ was executed and the residual stress around the weld was analyzed under different impact times and velocities.
The study showed that:the multi-step sequential analysis finite element model could accurately simulate the effect of UIT on WRS in DRJ; the residual tensile stress around the weld toe of DRJ was transformed into residual compressive stress due to UIT, and the decreased stress amplitude was 427.2 MPa; the residual compressive stress existed from the bottom surface of the deck to a certain depth, and the maximum of the residual compressive stress appeared on the subsurface of the deck; the residual compressive stress amplitude and impact affected depth around the weld toe of DRJ gradually increased as the impact times of ultrasonic impact pin increased; when the impact velocity of ultrasonic impact pin increased, the depth of the peak value of residual compressive stress increased gradually, and the depth was up to 0.8 mm when the impact velocity was 7 m/s.
Research on the Fatigue Performance of Rib-to-Deck Welded Joints of Wujiagang Yangtze River Bridge
Kening Hu, Houdeng Zhang, Fan Wu, Zhixing Zhou
2021, 36(10): 34-41.   doi: 10.13206/j.gjgs20092801
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Abstract:
The fatigue cracks in the rib-to-deck welded joints are typical fatigue diseases in orthotropic steel deck. The fatigue crack penetrating through the deck will cause secondary diseases such as pavement damage, water seepage and corrosion, which will endanger the durability and safety of steel box girder seriously. The natural "crack-like" structure is formed at the deck root of the traditional rib-to-deck single-side welded joints because the traditional welding technology can only weld at the outside of the U-rib, which causes the critical fatigue cracking problems at the deck root. In order to solve the fatigue problem of crack initiating from the deck root for traditional rib-to-deck single-side welded joints in the Wujiagang Yangtze River Bridge project, the innovative rib-to-deck both-side welded joints were proposed to improve its fatigue resistance. The rib-to-deck welded joints were analyzed systematically by the equivalent structural stress method. The influence surface of each fatigue cracking mode of the rib-to-deck single-side and both-side welded joints was determined respectively. The predominant fatigue cracking mode of the two welded joints was determined considering the lateral distribution probability of the wheel load, and then their fatigue life was evaluated.
The results indicated that the length of the longitudinal influence line of each fatigue cracking mode of the rib-to-deck single-side and both-side welded joints were mainly between the two diaphragms adjacent to the specific construction details. The fatigue cracking mode of deck root and deck toe of the traditional rib-to-deck single-side welded joints were mainly in tensile and compressive cyclic stress under the longitudinal movement of wheel load. The location(e=-150 mm) where the wheel load acts directly above the traditional rib-to-deck single-side welded joints was the most unfavorable transverse loading position. The crack initiating from the deck root was the predominant fatigue cracking mode of the traditional rib-to-deck single-side welded joints. The maximum equivalent structure stress amplitude is 70.4 MPa under the action of standard fatigue vehicle. The fatigue cracking mode of deck inner and outer toe of the innovative rib-to-deck both-side welded joints were mainly in tensile and compressive cyclic stress under the longitudinal movement of wheel load. The most unfavorable transverse loading position of the rib-to-deck both-side welded joints was same to the traditional rib-to-deck single-side welded joints. The crack initiating from the deck outer toe was the predominant fatigue cracking mode of the innovative rib-to-deck both-side welded joints. The maximum equivalent structure stress amplitude is 63.2 MPa. The predominant fatigue cracking mode of rib-to-deck welded joints was changed from the crack initiating from the deck root of the traditional rib-to-deck single-side welded joints to the crack initiating from the deck outer toe of the innovative rib-to-deck both-side welded joints by introducing the innovative rib-to-deck both-side welded joints. The fatigue life of the innovative rib-to-deck both-side welded joints was increased by about 42.4%. The introduction of the innovative rib-to-deck both-side welded joints can effectively improve the fatigue performance of rib-to-deck welded joints.
Effect of Slab-Truss Combination Pattern on Fatigue Damage of Integral Joint in Steel Truss Bridge
Zhao Xu, Huaifeng Li, Chaoqiang Zhang, Hongjiao Zhao, Xueyuan Ma
2021, 36(10): 42-49.   doi: 10.13206/j.gjgS21051301
Abstract(0) PDF (10943KB)(0)
Abstract:
The interaction effect between the deck system and the main truss of the steel truss bridge will significantly affect the internal force of the longitudinal and transverse beams and the stiffness of the main bridge, thus affecting the stress level of the connection details between the overall joints and the corresponding stressed members, and then affecting the fatigue damage effect of the relevant structural details. Semi-combined and fully combined slab-truss combination are the two most commonly used forms of steel-concrete composite bridge deck system. Different combination methods have a significant impact on the interaction effect between bridge deck system and main truss. In order to explore the effect of slab-truss combination on the fatigue damage characteristics of the integral joints of a typical steel truss bridge, taking a wide and long-span steel truss bridge as an example, the most unfavorable integral joint positions corresponding to the two types of structural details are determined by analyzing the internal force distribution law which controlling the fatigue damage characteristics of the connection details between the beam and the joint as well as the butt joint details between the lower chord of the main truss and the joint. Based on the equivalent structural stress method and the linear damage accumulation theory, the multi-scale fatigue damage analysis is carried out, and the stress history of the main cracking modes of the two types of structural details under the standard fatigue load is determined, so as to analyze the fatigue damage characteristics of the two types of structural details. Based on the series failure model, the controlled fatigue cracking mode of the integral joint is determined, so as to quantify the effect of the slab-truss combination on the fatigue damage of the integral joint.
The results show that:compared with the semi-combined method, the full combined method of steel truss bridge will significantly improve the stiffness of the main bridge, so as to reduce the out of plane bending moment of the beam and the internal force of the main truss chord, but it has no obvious effect on the in-plane bending moment of the beam; the most disadvantageous position of the integral joint is the joint near the support, and the most disadvantageous position of the butt joint detail between the lower chord of the main truss and the joint is the joint near the mid span. There is no difference in the most disadvantageous joint position between the two types of slab-truss combination; compared with the semi-combination method, the full combination method reduces the equivalent structural stress amplitude of the connection details between the beam and the joint by more than 60%, and the equivalent structural stress amplitude of the butt joint details between the lower chord of the main truss and the joint by 80%; the controlled fatigue cracking mode of the integral joint is that the crack starts at the weld toe of the joint plate connecting the upper flange of the transverse beam and the joint plate. The fatigue damage degree of the integral joint under the full combination mode is 64% lower than that under the semi-combination mode. The slab-truss combination mode will have a significant impact on the deck interaction effect of steel truss bridge, so as to change the stress state of longitudinal and transverse beams and main truss members of the deck system, and then have a significant impact on the fatigue damage of the integral joint. Therefore, the impact of the interaction effect of bridge deck and main truss should be fully considered when evaluating the fatigue performance of the integral joint.
Fatigue Assessment and Fatigue Test of the Full-Scale Model of Tensile Anchor Plate of Cable-Stayed Bridges
Bida Pei, Lianhua Wang, Lifeng Li
2021, 36(10): 50-59.   doi: 10.13206/j.gjgs20112801
Abstract(0) PDF (9961KB)(0)
Abstract:
In order to study the fatigue performance and fatigue assessment method of the key fatigue categories of the gusset plate anchorage device of cable-stayed bridges, with the Wujiang Bridge as the engineering background, according the cable force amplitude under the traffic load of the Midas full bridge model, the last cable anchor plate of mid-span of this bridge was selected as the research object. This paper establishes a three-dimensional finite element analysis model of the gusset plate anchorage device for fatigue assessment. The nominal stress method and the hot spot stress method was used to assessment the key fatigue details of the gusset plate anchorage device with JTG D64-2015, ASSHTO LRFD Bridge Design Specifications and Eurocode 3. The assessment methods of each code were compared and analyzed. At the same time, the fatigue test of the full-scale model of the gusset plate anchorage device was carried out.
The theoretical analysis results showed that:the most unfavorable position of tensile anchor plate structure formed by the butt welding of was tensile anchor plate and the side web was the excessive arc of the upper welding seam of the tensile anchor plate and the anchor cylinder(detail A), which required special attention; the key fatigue details of the tensile anchor plate structure met the infinite life design requirements by nominal stress methods with those specifications, and the results were more secure when used Eurocode 3. From the perspective of structural safety, it was recommended to adopt Eurocode 3 to infinite life fatigue design of anchor plates by nominal stress.
The analysis results of the hot-spot stress method showed that the tensile anchor plate of the Wujiang Bridge met the requirements of the specification. The results of the hot-spot stress method were more objective than the nominal stress method, and its fatigue strength-life(S-N) curve was relatively uniform and was less affected by subjective factors. Therefore, it was recommended to use the hot-spot stress method to evaluate the fatigue of the weld toe of the tensile anchor plate.
The full-scale fatigue test results showed that the structure was still in the elastic stage after the fatigue loading 2 million times, the stiffness had not been significantly reduced, and no visible cracks were found on the surface of the structure. The fatigue load amplitude increased to 640 kN and the fatigue load was continued to 3 million times. It was equivalent to loading approximately 5 million times according to the initial fatigue load amplitude, the structure was still not cracked, and the gusset plate anchorage devices met infinite life design. Both theoretical analysis and full-scale model fatigue test showed that the design of the tensile anchor plate of the Wujiang Bridge had reasonable design, and its fatigue properties met requriement of actual engineering.
Hot Spot Analysis of Steel Structures
2021, 36(10): 60-60.  
Abstract(0) PDF (438KB)(0)
Abstract: