Research On Bearing Capacity Of Cross-bracing Influenced By Transmission Tower Node Constraints

The cross-bracing is the main component of the transmission tower.At present,the calculation results of this kind of components in the codes of various countries are quite different,and most of them use the slenderness ratio correction coefficient K to consider factors such as the structural eccentricity and end constraints.In Chinese 《Technical code for the design of tower and pole structures of overhead transmission line》,only the constraint effect of two bolts is considered.However,in the actual design,the influence of constraint is not considered due to the difference of cross-bracing.Therefore,in order to quantitatively analyze the influence of transmission tower node constraints on the outof-plane stability bearing capacity of cross-bracing,this paper studies the out-of-plane stability performance of cross-bracing of common transmission towers through test,finite element analysis and theoretical derivation.The following works are mainly completed:(1)The torsional stiffness of eight groups of main materials of three different types of tower were studied by using the method of outer covering and inner pasting.The results showed that the change of parameters such as the length of the intersection,the main material,the inclined material and the presence of auxiliary material will only lead to a slight linear change of torsional stiffness of main materials,and will not produce a difference in magnitude.For the test piece,the difference is within the range of 1.2 ～ 2.1times.(2)Thirty-three groups of spatial finite element models of transmission tower intersections were established,and the calculated results were verified by comparing with the torsional stiffness tests of main materials.The influences of main materials specifications,cross-bracing specifications and joint plates on torsional stiffness of main material were analyzed,and the approximate range of torsional constraint stiffness of main materials to cross-bracing was obtained.The results show that the torsional stiffness of main material can be increased by about 10% with the slight increases of the size of the main material or cross-bracing.The existence of joint plate can increase the torsional stiffness of main material by 3～5 times.For bars(e.g,auxiliary materials)outside 0.4m of the joint,the torsional constraints are in the range of 20-50kN·m/rad.For bars(e.g,cross-bracing)within 0.2m of the joint,the torsional constraints are in excess of 200kN·m/rad.(3)Based on the potential energy equation of the thin-walled member,the eccentric compression equilateral single angle steel model with spring constraints at both ends was established,and its stability bearing capacity was theoretically deduced.Forty groups of angle steel data were calculated by MATLAB,and ninety groups of finite element models were established by Workbench software for comparative analysis.The influence laws of stability bearing capacity and parameters such as slenderness ratio,end torsional constraint stiffness,width-thickness ratio were studied.The results show that the stability bearing capacity of equal angle steel components increases linearly with the increase of the end torsional restraint stiffness,but does not increase significantly when the torsional stiffness exceeds 200kN·m/rad.And the increase rate of bearing capacity of equal angle steel components is related to slenderness ratio.When the slenderness ratio is less than 105,the increase rate can reach 13% ～ 32%,and when the slenderness ratio is greater than 140,the increase rate can even reach 13% ～ 59%.When the end torsional constraint stiffness is less than 30kN·m/rad,the difference of width-thickness ratio has little effect on the stability coefficient.(4)A cross-bracing member-tension-compression system was established.Based on the zero out-of-plane lateral stiffness of the intersection point,the calculated length coefficients of the cross-inclined member compression bar under the conditions of eccentricity,end torsion constraint and tension-compression ratio are derived.Based on the fact that the lateral stiffness outside the intersection surface is zero,the calculated length coefficient of the cross-inclined material compression bar is derived under the combined action of eccentricity,end torsion and tension-compression ratio.