2021 Vol. 36, No. 9

Stability Capcity of Circular Steel Tube Beam-Columns Under Uniaxial Bendings
Lei Xu, Genshu Tong
2021, 36(9): 1-9. doi: 10.13206/j.gjgS20111601
In the current Standard for Design of Steel Structures(GB 50017-2017), the clause 8.2.4 is specifically devoted to the buckling strength of beam-columns with circular pipe cross-section, but the only difference from the clause 8.2.1 for the beam-column with H cross-section is in the equivalent uniform moment factor. The cross section of circular steel tube has its unique characteristics compared with the section of the I-beam, therefore, it is necessary to revisit the buckling strength of beam-columns with circular pipe and if possible to propose a design formula that can better reflect the real relationship between the axial force and the bending moment of circular steel tube in unidirectional bending beam-columns.
The paper uses finite element software to calculate the nonlinear buckling capacity of unidirectional bending beam-columns with circular steel tube, and compares the FEM results with the results of the current coded formulas, and discusses whether the current code related formulas are suitable for calculating buckling capacity of circular steel tube beam-columns. Starting from the sectional plastic axial force-bending moment interactive equations of the pipe, substituting the elastic second order bending moment including the geometric imperfection into the cross-sectional interactive equation, the present paper obtained a new equation as upper bounds for the in-plane stability capacity with the codified equivalent moment coefficient, for which a coefficient is modified to make the theoretically derived equation be able to agree with the FEM results.
The following conclusions are drawn:the calculation results of the relevant formulas of the current code are generally conservative compared with the FEM results, especially when the bending moment changes linearly and is in double-curvature bending; when the end bending moment ratio is-1, the axial force-bending moment interactive curve of the relevant formula of the current code and the curve of the FEM result show fully different trends, indicating that the relevant formula of the current code cannot well reflect the actual axial force-bending moment correlation of circular steel tube beam-columns. Comparing the proposed formula of this paper with the FEM results, it is found that the new formula can more accurately calculate the buckling capacity of circular steel tube unidirectional bending beam-columns when the end bending moment ratio is 1, and, after using the equivalent bending moment coefficient of the I-beam members, the buckling capacity of beam-column under different end bending moment ratios can also be calculated more accurately.
Finite Element Simulation and Parametric Analysis of Composite Shear Walls with Steel Plates and Infill Concrete Under Axial Compression
Jing Li, Wucai Lu
2021, 36(9): 10-18. doi: 10.13206/j.gjgS20062202
The previous researches on composite shear walls with steel plates and infill concrete mainly focused on their seismic performance, but few researches on their axial compressive performance and stud stress characteristics. Also, the influences of some key design parameters on the ultimate bearing capacity and ductility of the wall are still unclear, which makes it difficult to determine the values of these parameters reasonably. On the other hand, the studs in the wall are used as shear connectors, which can make the steel plate and concrete work together, but the stress and deformation data of the studs are difficult to measure in the experiment. In order to solve these problems, the ABAQUS software is used to establish a finite element model of the composite wall under axial compression, and the accuracy of the model is verified by experimental data. Further, the aspect ratio, concrete strength, steel plate strength and stud strength are considered, and the parametric analyses are carried out to investigate the influences of these parameters on the mechanical properties of the wall.
In the finite element model, the concrete, steel plates and studs are simulated by C3D8R solid elements. Plastic damage is considered in the constitutive relationship of concrete, and the broken-line model is used as the constitutive relationship for steel plate and stud. A rigid body constraint is created to simulate the loading plate, and two reference points are set at the centers of the top and bottom of the wall model respectively. Then, the boundary conditions are assigned to the two reference points, and the load is applied downward to the top reference point. The accuracy of the basic finite element model is verified by the simulation results of initial stiffness, peak load and local buckling of steel plate, and then the four parameters mentioned above are adjusted to form 13 finite element models, and the load-displacement curves and ultimate loads of the walls with different parameters are obtained and compared.
The numerical results show that the load-displacement curves of the finite element simulation are in good agreement with the experimental results. Also, the local buckling of steel plate observed in the simulation is consistent with the experimental phenomenon. Therefore, the details of the finite element modeling are justified including the constitutive relationships of the materials, contact characteristics, boundary conditions and the parameter settings for the theoretical models, which can be used or referred to for the analysis and design of similar structural components. The three parameters of aspect ratio, concrete strength and steel plate strength have large influences on the ultimate bearing capacity of the wall. Also, the ultimate bearing capacity increases in an approximately linear trend with the increase of each parameter. However, the parameter of stud strength has almost no effect on the ultimate bearing capacity of the wall. Increasing the compressive strength of concrete will increase the ultimate bearing capacity of the composite wall more effectively than increasing the strength of steel plate. The smaller the aspect ratio is, the better the ductility of the wall is. Moreover, the stud stress is concentrated at the root. If the strength of steel plate is equal to that of stud, the root stress of stud will approximate its strength, and in other words, its shear resistance will be fully exerted.
Stress Analysis of Ultra-Wide Orthotropic Steel Box Girder with Large Width-Span Ratio and Single Inclined Pylon
Zhichao Shang, Li Chen, Fan Chen, Tingyu Li, Yong Tian
2021, 36(9): 19-24. doi: 10.13206/j.gjgS21012602
Geometrical asymmetry in cable-stayed bridges with single pylon leads to differences in stresses and deformation in comparison to conventional ones. Thin-walled orthotropic steel girder with big width/span ratio has more complicated loading and deformation performance, which cannot be calculated accurately using conventional approaches. Jialuhe bridge is such an example with even more complicated properties. The 120 m long main span consists of three segments:The first 100 m from the pylon is orthotropic steel girder, followed by 8 m transition segment, and the last 12 m reinforced concrete plate is a cantilever from the abutment on the opposite bank. The width of the steel girder is 54.8 m, not supported on the abutment directly, but connected to the cantilever through a transitional segment with 8×54.8 m side length instead. So there are three materials of different stiffness connected in a small space, leading to two seaming sections, which not only reduces the monolith of the structure, but also increases difficulties in construction controlling.
The investigate shows:The transverse deformation of the steel girder is identical, with a maximal relative difference of-2.8 mm, so that no transverse camber is needed in construction phase. The maximal Mises stress due to gravity is 93.7 MPa, and that due to secondary deadload is about 1/4 of this value. This reveals a big safety reserve of the steel girder. When the height of the U-stiffeners varies between 260~320 mm, the variation of displacements of the bridge deck lies within 5%, variation of maximal stresses of the bridge deck within 8%. In this range the variation of the height of the U-stiffeners has less impact on displacement and stress of the deck. The internal cause is that the height of U-stiffener and the stiffness delivered by the transverse girder are supplementary each other, just demonstrating the salient plate property of this orthotropic girder. Height of U-stiffeners in this range should be chosen when designing.
Construction Technology
Measurement Method of Initial Geometric Deflections of T-Section Steel Struts
Xiaoli Xiong, Meng Ma, Kun Du
2021, 36(9): 25-32. doi: 10.13206/j.gjgS20032402
Initial geometric deflection is one of the important factors impacting on the overall stability bearing capacity of steel struts. The common initial geometric deflections of the steel strut with T-section include the initial bending deformation of the component, the initial eccentricity of the load and the initial torsional deformation of the member. In the process of the study on the overall stability bearing capacity of steel struts, in order to measure the initial geometric deflection more accurately and conveniently, the relationship between the initial geometric deflection of steel struts and the parameters such as load, strain, lateral displacement and torsion deformation of the section are utilized. Based on the traditional theory of elastic stability, the calculation formulas of the initial geometric deflections are derived, and the inverse parameter measuring method is put forward to obtain the initial geometric deflections by using the relationship between the load and deformation of the compression member in the elastic stage.
Using ANSYS, the 3 D model of a T-section steel strut with initial geometric deflections was created, and the geometric nonlinear analysis of the strut was performed. Comparing the data of the initial geometric deflections of the 3 D model with the results calculated by the inverse parameter measuring method, the accuracy of the calculation formula to get the initial geometric deflections was verified. Finally, based on the overall stability capacity test of T-section steel struts, the measured values such as load, strain, lateral displacement and torsion deformation of the section in the elastic stage were obtained. By the inverse parameter measuring method, the actual initial geometric deflections of the steel strut were calculated. The results were compared with those obtained by the traditional optical instrument measuring method, and the correctness of the inverse parameter measuring method was verified.
It is shown that the inverse parameter measuring method of the initial geometric deflections of T-section steel struts was correct and feasible. Since the deflections can be measuring with the equipment in the overall stability capacity test without extra devices, the artificial measurement errors are reduced. Moreover, the influence of the end constraint on the critical force of actual struts is taken into account during the process of the overall stability bearing capacity test. Most of all, the common initial geometric deflections of the steel strut with T-section, not only the initial bending deformation of the component and the initial eccentricity of the load, but also the initial torsional deformation of the member can be obtained. The initial geometric deflections, which are obtained by the inverse parameter measuring method, can be directly used in the subsequent finite element analysis, that it can provide an essential reference for the study of the overall stability bearing capacity of the steel struts with T-section.
Study on Reinforcement of Steel Truss Bridge Based on External Prestressed Load Lifting
Hao Ding, Rui Li, Jing Mo, Xiaozhang Li, Di Liu
2021, 36(9): 33-39. doi: 10.13206/j.gjgS21041601
The concept of in vitro prestressing originated from France at the earliest and is one of the important branches of post-tensioning prestressing system, which usually adopts large-diameter steel bars, steel strands and high-strength steel wires as tension application tools to prestress the beams. This method can effectively reduce the stress level of the structure and can play the roles of reinforcement unloading and changing the internal force distribution of the structure, and at the same time can improve the load bearing capacity, crack resistance and stiffness of the structure. However, the in vitro prestressing method is less studied in reinforced steel truss structures, and the related theory is relatively lacking.
In order to study the effect of in vitro prestressing with different number of strands on the strengthening of steel truss bridges, this paper takes a 128 m span under-bearing simply-supported steel truss bridge over the Omo River as the research object, and proposes three in vitro prestressing strengthening schemes for its load lifting analysis, and uses finite element software to establish the full bridge model with 7, 9, and 11 strands per beam respectively, and analyzes the strengthening of steel trusses in terms of rod strength, structural stiffness, fatigue, overall stability, and nodal plate stress distribution. The result analysis shows that:the stress of longitudinal beam, crossbeam, top chord and bottom chord gradually decreases with the increase of the number of strands, and the stress of the bars gradually improves; the number of strands increases, the deflection of the structure gradually decreases, the fundamental frequency gradually increases, and its stiffness continuously improves; with the increase of the number of strands used for reinforcement, the other strands, except for the longitudinal beam, gradually increase. With the increase in the number of strands used for reinforcement, in addition to the longitudinal beam, the fatigue stress amplitude of the rest of the rod is constantly reduced; the critical load factor is constantly increased, and the stability is increased; the area of the low stress area of the important parts of the node plate is increased, and the area of the high stress area and the secondary high stress area is reduced, and the number of screw holes in the potential tearing area is constantly reduced, and the probability of tearing damage of the node plate is reduced. However, as the number of strands increases, the axial force of the lower chord increases and the stability safety factor of the section decreases, so it is necessary to control the number of strands tensioned and limit the magnitude of load lifting.
The results show that all the three in vitro prestressing reinforcement solutions can achieve the load lifting effect, and with the increase of the number of strands used, the structural strength, stiffness and stability are non-linearly improved, and the proportion of low stress distribution area of the node plate is gradually increased, and the bridge is reinforced by 11 single-steering bending line shaped in vitro prestressing strands, which is the most practical and reasonable reinforcement solution.
Comparison of Chinese and US Code
Design of Members for Torsion
Lijun Wang
2021, 36(9): 40-48. doi: 10.13206/j.gjgS20081802
Design methods of torsion member in AISC 360-16 are introduced. Only closed sections are included for the torsion calculation in AISC 360-16. Pure torsion is considered mainly in torsion calculation with consideration for helpful factors of constrained torsion. Torsion of members is divided into pure torsion and constrained torsion. For open sections, normal stress and shear stress under pure torsion and constrained torsion are both high, while for closed sections, pure torsion plays a controlling role and the stress under constrained torsion is negligible. Hence, the torsion design of closed sections in AISC 360-16 assumes that the torque is all borne by pure torsion and is then modified in terms of the constraints. Pure torsional shear stress is uniformly distributed along the section and is equal to the torque divided by torsion constant C. Considering the buckling effect, torsional capacity is the product of torsion constant C and critical shear stress Fcr.
Design torsion strength of round and rectangular hollow structural sections is φTTn and determined by the critical force of torsional yield and torsional buckling, namely and torsional resistance coefficient φT=0. 9. Fcr is different because of different members.
1) Local buckling critical stress of long tubes under torsion is not affected by the end constraints, in which certain initial imperfections are considered. For medium and short tubes, end constraints will increase the local buckling critical force.
2) The critical stress of rectangular tubes has the same shear buckling coefficient kv=5. 0 as the bending shear stress of Chapter G. The distribution of torsional shear stress on the long sides of rectangular tubes is consistent with the shear stress in the webs of I-shaped beams.
The external torque consists of free torsional resistance and restrained torsional resistance. The two resistances of the open section are very small, so the AISC 360-10 only considers the torsion of the closed section. In practical application, structural measures shall be taken for the open section to prevent torsion.
Hot Spot Analysis of Steel Structures
2021, 36(9): 49-49.