Mechanical Properties And Optimization Design Of Angle Steel Cross Bracing In Transmission Tower

In order to meet the high requirement of electricity and improve the construction of power grid system,China pays more and more attention to the fine design of transmission towers.As the force the structure of transmission tower,the mechanical properties of cross bracing have an important influence on the ultimate bearing capacity of the whole tower.At present,the related researches on cross bracing lacks the verification of true test data,and the calculation values of ultimate bearing capacity of cross bracing in domestic and foreign codes are quite different.The typical transmission tower in China are selected to carry out the true test,and the finite element model of the transmission lines is established by the ANSYS finite element software.The failure mode and the change rule of the ultimate bearing capacity of the cross bracing under different joint forms,material strength,limb width ratio and end constraints are analyzed.The following work is carried out as follows:（1）The stability of cross bracing in different tension-compression ratio is studied by true-type test.The results show that when F_N’/F_N≤-0.2,the long section of the compression bar buckles around the parallel axis after reaching the ultimate bearing capacity,and the tension-compression ratio has little effect on the ultimate bearing capacity.When F_N’/F_N>-0.2,the overall buckling occurs around the parallel axis of the connecting limb,and the tension-compression ratio has a great influence on the ultimate bearing capacity.（2）Three typical Sections finite element models with the cross bracing being equal angle steel and unequal angle steel were established,and the stability performance of the cross bracing under different tension-compression ratios was studied.The failure mode of without auxiliary materials is the minimum axial instability around the section I,and the bearing capacity changes little with the tension-compression ratio.The bearing capacity of equilateral angle steel is higher than that of unequal angle steel.The failure mode of section II and III is instability around parallel axis,and the bearing capacity varies greatly with the tension-compression ratio,and the bearing capacity of unequal angle steel is higher.（3）The finite element model of the cross bracing with unequal angle steel was established,and the material strength,limb width ratio and end constraint parameters of the cross bracing were analyzed.The research shows that with the increase of material strength,the bearing capacity of cross bracing increases significantly under the tension-compression mode.With the increase of limb width ratio,the failure mode of inclined bar transits from instability around parallel axis to weak axis.The optimal limb width ratios are 1.75,1.92,2.17 and 2.48,respectively,when the tension-compression ratios are[-1,-0.5),[-0.5,0),[0,0.4)and[0.4,1].The end constraint has little effect on the failure mode,but the bearing capacity of double-bolted connections at both ends is significantly higher than that of the other connections.（4）With the increase of tension-compression ratio,the calculated value from GB50017-2017 decreases from more larger than the simulated value to less than the simulated value,and the error is large.DL/T5154-2012 can predict the inter-node instability mode,but the ultimate bearing capacity is higher than the finite element value.（5）Based on the finite element results and the bending-torsional column curve of single-limb connection angle steel,the calculation methods of the stability bearing capacity of cross bracing timber with and without auxiliary material support are proposed respectively.The calculation method of inclined timber with auxiliary material support comprehensively considers the influence of limb width ratio,material strength and end constraint,which can provide references for the engineering design of the cross bracing structure.