2021 Vol. 36, No. 7

Research on Mechanical Properties of Truss String Structure with Spring Rods
Xiaodong Feng, Shengwei Liu, Weijia Yang, Yaozhi Luo
2021, 36(7): 1-8. doi: 10.13206/j.gjgS20042502
Ordinary truss string structures have been widely used in many practical projects and achieved good economic benefits because of its advantages known as reasonable distribution of stresses, light weight and convenient construction. However, the wind uplift damage often occurs in strong wind areas due to the light self-weight of the structure. To avoid this kind of phenomenon, reduce the loss of economic property, give full play to the characteristics of truss string structure, and improve its safety and applicability, a spring rod device suitable for truss string structure system is designed. The device can realize the use of resisting strong wind and bearing normal load, and meet the requirements of truss string structure in strong wind area.
In order to deeply study the influence of the device on the structural performance improvement and further explore the influence law of the spring bar on the mechanical characteristics of structure, a truss string structure with a span of 150 m, a height of 45 m and a length of 160 m was taken as the research object, the influence of stiffness, arrangement position and arrangement angle of the spring rod device on the mechanical performance of the structure was analyzed by using commercial finite element software MIDAS/Gen. Firstly, the design concept and structure of spring rod device were introduced, its components and working principle were described in detail, and the method of simulating the device in finite element software was also described. Then, the spring stiffness of 20,50, 100, 300 kN/mm were taken respectively, and the spring bars with each stiffness were arranged under the first, second and third secondary truss of the string truss in turn, and the maximum displacement, maximum stress and maximum cable force of the structure under the specific load combination were calculated. Finally, the angle between the spring rod and the ground was set as 30°, 45°, 60° and 90° respectively, the maximum displacement of the structure under the load combination controlled by wind load was calculated, and the influence of the spring rod arrangement on the dynamic performance of the structure was analyzed.
The results show that:1) Under the "1. 3 dead+1. 5 live" load combination, the spring bar device stiffness is 100 kN/mm and is arranged under the second truss, the maximum displacement, maximum stress and maximum cable force of the structure decrease by 60. 12%, 34. 68% and 32. 13%, respectively, indicating that the device has obvious effect on the structure against the dead load and live load. The stress and cable force of the structure are greatly reduced, and the burden of the structure system can be reduced; but when the spring stiffness is greater than 50 kN/mm, the device has no obvious effect on the structural performance. 2) Under the load combination of "1. 3dead + 1. 05live + 1. 5wind", the maximum displacement of the structure is reduced by 54. 77% when the angle between the spring rod device and the ground is 30°, which indicates that the spring rod can effectively reduce the impact of wind load on the structure. 3) From the view of the natural frequency of the structure, except for the first period, the vibration period of the truss structure with spring rods is smaller than that of the ordinary one, so the device can increase the stiffness of the structure.
Seismic Performance of Steel Frame-Coupled Steel Plate Shear Wall Structures with Different Coupling Ratio
Zhicheng, Tan Lin, Chen Yi Wu
2021, 36(7): 9-17. doi: 10.13206/j.gjgS20051501
Coupled steel plate shear wall is a new type of lateral force resisting system which is formed by connecting two steel plate shear walls with coupling beams. Through the interaction of coupling beam and wall limb, the capacity of resisting overturning moment and lateral stiffness of the coupled steel plate wall are improved. Coupling beam is the key component that affects the seismic performance of coupled steel plate shear wall, and coupling ratio is an important parameter to measure the wall limb interaction. Therefore, a design method with coupling ratio as control index is proposed. Three 20-story steel frame-coupled steel plate wall structures are designed with coupling ratios of 20%, 40% and 60%, respectively. The finite element model of the structure is established by using ABAQUS software, and the results of mode-decomposition response spectrum analysis show that the difference of each structural performance index of the three models is less than 5%, which indicates that the steel frame-coupled steel plate wall structure can be designed to meet the expected structural performance by using the design method with coupling ratio as the control index under the same design seismic shear force. The coupling ratio will affect the steel consumption of steel plate wall.
In the coupled steel plate shear wall, the steel consumption of frame column is much larger than that of other components. When the design seismic shear force is the same, with the increase of the design target coupling ratio, the design section of coupling beam increases, and the interaction between coupling beam and wall limb is strengthened, which can reduce the thickness of embedded steel plate and reduce the section size of frame column. On the other hand, the shear force and bending moment of coupling beam will increase with the increase of coupling ratio. The increase of coupling beam shear force will reduce the axial force of the frame inner column connected with it, but the increase of coupling beam bending moment will increase the bending moment of the frame inner column. Therefore, when the coupling ratio increases to a certain extent, the cross-section size of the inner column may increase instead of decreasing. Among the three models, the 40% coupling ratio model has the least steel consumption and the best economy.
Seven seismic waves are selected for time-history analysis of the three models. Under frequent earthquakes, the average values of base shear of the three models are basically the same. Under rare earthquake, the average value of base shear of 40% coupling ratio model is the largest, but the average value of interstory drift angle is the smallest, which indicates that the degree of plasticity of the structure is small, and the stiffness degradation is not obvious as the other two models. The smaller the coupling ratio is, the deeper the plastic development of coupling beam is. The larger the coupling ratio is, the deeper the plastic development of steel plate is. However, the maximum equivalent plastic strain of the steel plate is much larger than that of the other two models, while the coupling beam remains elastic. Therefore, the 20% and 40% coupling models are more reasonable than the models.
The horizontal load of inverted triangle distribution mode is applied to the model, and the static elastic-plastic analysis is carried out to obtain the base shear-top drift angle curve and the stiffness-top drift angle curve. According to the yield order of the members, the whole process of pushover curve is divided into eight stages. The failure order of the three models is:steel plate yield→ coupling beam yield→ frame beam end yield → frame column base yield, which indicates that the three steel frame-coupling steel plate wall models have good ductility and meet the structural design performance objectives.
Fracture Prediction and Analysis of Q690D High Strength Steel Based on Cyclic Void Growth Model
Jianzhou Ge, Xuewei Huang, Jun Zhao, Wei Zhao, Chenchen Wei
2021, 36(7): 18-28. doi: 10.13206/j.gjgS20061103
High strength steel has been gradually applied in practical steel structures. The ultra-low cycle fatigue fracture analysis of steel is the basis to evaluate the fracture failure of high strength steel structures under strong earthquake. The traditional fracture mechanics method assumes that the crack already exists, and there is a high strain constraint at the initial crack tip, so it is mainly suitable for the study of brittle fracture problems with extremely limited plastic deformation, but it is not suitable for the ductile fracture problems with no macroscopic initial defects and significant plastic deformation under ultra-low cyclic loading. The ultra-low cycle fatigue fracture model based on void growth mechanism has been gradually applied in the ductile fracture analysis of steel.
In order to study whether the cyclic void growth model (CVGM) is suitable for predicting the fracture failure of domestic Q690D high strength structural steel, 17 rod specimens and 3 dog-bone weakened plate specimens (DB) were designed and processed. The MTS axial servo fatigue test system was used to carry out the fracture test of round bar specimens under monotonic loading and ultra-low cycle loading. The stress-strain relationship, basic mechanical parameters and load-displacement curve of Q690D steel were obtained. The influence of loading system on the bearing capacity and deformation capacity of specimens was analyzed.
The experimental results show that:1) Q690D steel has no obvious yield platform, and there are obvious notch strengthening effect and cyclic softening phenomenon. Due to the damage of material during cyclic loading, the fracture displacement of cyclic loading is less than that of monotonic loading. 2) By analyzing the fracture morphology of the specimen, it is found that the cracks all start at the center of the minimum section of the specimen. The micro morphology of the specimen fracture is observed by scanning electron microscope. The fracture presents the ductile fracture characteristics of dimple, which conforms to the mechanism of void growth fracture mechanics. At the same time, the characteristic length of Q690D steel is about 0. 3 mm by scanning electron microscope.
The finite element model of Q690D high strength steel was established by using ABAQUS software. Based on the test results of Q690D rod specimen and combined with finite element analysis, the parameters of CVGM and DSPS of Q690D high strength steel were calibrated. Under monotonic loading, CVGM and DSPS degenerate into the void growth model (VGM) and the stress modified critical strain model (SMCS). Finally, with the help of user subroutine USDFLD in ABAQUS software, the CVGM model is programmed with Fortran language. In the process of numerical calculation, when the material element meets the requirements of failure criteria, the element failure is determined, the stress of the element is released, and then the finite element calculation is continued according to the new state until the specimen failure. The cyclic void growth model of steel is used to predict the fracture failure of dog-bone weakened plate specimen under different loading systems. The crack initiation position, load-displacement curve and fracture displacement of the specimen are in good agreement with the experimental results, and the prediction errors of ultimate load and fracture displacement are within 2% and 12% respectively.
Finite Element Simulation Analysis of Lifting Point Arrangement and Support Unloading of Long-Span Roof Truss
Yongqiang Qiao, Taiyuan Guo, Qing Hu, Dongze Song
2021, 36(7): 29-34. doi: 10.13206/j.gjgS20031801
A long-span circular truss structure with a steel structure roof span of 66 m in a theater was considered to be divided into four unit structures due to construction conditions, and the unit structure is assembled on the ground and hoisted as a whole. Each unit structure is an irregular steel truss. MIDAS was used to calculate and analyze the changes in the stiffness and strength of the steel structure unit during the lifting process to determine whether it met the construction requirements. In order to find the best position of the lifting point of the shard unit, the center of gravity position of the shard unit was calculated by the definition method of the center of gravity position. The unit structure was selected four ears arrangement points, and the quadrilateral geometric center formed by the arrangement points was close to the center of gravity of the unit structure. According to the setting principle that the average distance between the point and the center of gravity of the unit structure gradually, three kinds of hanging point arrangement schemes were determined and simulated by MIDAS. The lifting lug node was restrained by node elastic support, which imposed small stiffness constraints. The truss was simulated by rod elements, considering the influence of the hoisting process, and the dynamic effect coefficient was 1. 4. The structural element stress, node vertical displacement and stress ratio were considered to determine the rationality of the plan, and to choose the optimal hanging point layout plan.
After 4 unit structures were hoisted in sequence, the welding connection between units was carried out. After the hoisting and installation of connecting beam were completed, the unloading work of the supporting structure started. The long-span spatial structure was complicated. The process of internal force redistribution of structural members required calculation and analysis, so the unloading method of the support system was very critical. Due to the small number of tire frames in this project, the order of unloading individual tire frames was adopted. The unloading plan was divided into four working conditions, and the unit cooling and unloading method was used to simulate the unloading process through MIDAS model. During the unloading process of the supporting structure, the temperature unit was set to be 50 mm long, and the material linear expansion coefficient was 1 mm/℃. A compression-only elastic connection was adopt between temperature unit and supporting point. After the unloading was completed, the temperature unit and supporting point were automatically separated, and the value of stress and vertical displacement of roof members during the unloading process of construction were calculated.
The research results showed:1) the design section of the roof structure met the safety of the construction process; 2) the structure unloading process has sufficient rigidity and strength, and the structural force was stable to meet the construction requirements; 3) the vertical displacement curve of the support point of the removed tire frame showed a linear upward trend. It was judged that the structure was stable under force, and the vertical displacement curve of remained support points was S-shaped. The closer to the position of the first removal point, the more obvious the vertical displacement curve of the observation point was. Therefore, the structural points with the S-shaped vertical displacement curve need to be set up for the limit and anti-overturning of the supporting nodes, and construction monitoring should be carried out. Based on the simulation calculation results, the implementation of the unloading plan was deterrmined.
Study on Equivalent Thermal Resistance of Fire Protection Coating of Tension-String Truss Structure Under Fire
Guoan Yuan, Guozhi Qiu
2021, 36(7): 35-42. doi: 10.13206/j.gjgS21010802
In order to study the safety distance between the fire source and the steel members directly above the tension-string truss structure in a fire and the minimum value of equivalent thermal resistance Ri of fire protection coating, the temperature field under the action of different kinds of fire was simulated based on three truss string structures with the span of 140,188 and 220 meters by the fire dynamic simulation software FDS. The nonlinear analysis on the overall fire resistance of truss string structure with the influence of high temperature on the steel characteristics was conducted through using the finite element software ANSYS, according to the results of calculation, the internal force and displacement of the tensioned truss are analyzed, and the internal reason for the tensioned truss structure to reach the ultimate fire resistance time was revealed.
Based on the analysis of fire resistance of different tensioned truss structures, the specific scope of fire protection coating should be sprayed on tensioned truss structures under fire is concluded. Finally, from the economic point of view, three groups of tension-string truss structures with different spans were taken as the background, and the displacement of representative joints of tension-string truss structures under fire was analyzed under the condition of different equivalent thermal resistance coefficient Ri, and the minimum value of equivalent thermal resistance coefficient Ri was explored.
The research results showed that the reason for the tension-string truss structure reached the fire resistance limit was that the internal force of the rod in the fire danger area reaches the yielding state. The safety distance between the fire source and the steel members directly above the fire source was 15. 0 m. It was feasible to improve the fire resistance performance of tension-string truss structures by spraying fire protection coating, and the minimum value of equivalent thermal resistance Ri of fire protection coating was 0. 02 m2·℃/W.
Study on the Selection of High-Energy Protective Steel Box Shed Against Post Earthquake Collapse
Song Yuan, Lin Shao, Liangpu Li, Peng Zou
2021, 36(7): 43-49. doi: 10.13206/j.gjgS20111203
The terrain and geological conditions in Western China are extremely complex, with high mountains and steep slopes and frequent earthquakes. From the Wenchuan earthquake in 2008, Lushan earthquake in 2013, Jiuzhaigou earthquake in 2017 and other strong earthquakes in mountainous areas, the secondary geological disasters such as high-level collapse are the most destructive, difficult and far-reaching ones. In order to solve the damage of secondary disasters such as high-level collapse after the earthquake to mountain roads and meet the needs of lifeline roads, taking "quick emergency response, small installation risk, low foundation requirements, strong adaptability and strong impact resistance" as the breakthrough point, the paper put forward three concepts of "avoid, slow, resist", "avoid, slow, delay", "avoid, delay, consume" of steel box shed tunnel.
Based on the display algorithm of ANSYS/LS-DYNA, the whole process of steel box shed under rockfall impact load was simulated. According to the dynamic response indexes such as the maximum impact force of rockfall, the energy absorption capacity of each part of shed and the equivalent stress of steel box structure, the rockfall impact resistance performance of three types of steel box shed was analyzed, and the stress mechanism of steel box shed was mastered. The ultimate bearing capacity of steel box shed with three kinds of protection concept was studied.
The results showed that:the structure of three types of steel box shed was reasonable, and the protection energy level could reach 1 000 kJ, which was higher than that of traditional steel shed or reinforced concrete shed; the modified rubber bearing was a compressible hyperelastic material with good self recovery, which could be reused for many times. It could store energy through its large circumferential deformation, prolonged the impact time and reduced the drop. The protection effect of the flexible net protection system was the best, the structure economy and seismic performance were the best, and the ultimate impact energy level could reach 2 000 kJ. The research results provided the basis for the design and optimization of the material and structure of the buffer device of the steel box shed tunnel and the subsequent 1:1 prototype experiment.
Hot Spot Analysis of Steel Structures
2021, 36(7): 50-50.