Theoretical And Experimental Study On Bearing Capacity Of Unequal Angle Steel In Transmission Tower

The effective length of tower cross bracing out-of-plane was far more than that of in-plane,which results a great gap between the stiffness of the two directions.It is unfavorable for structure design.When the short leg of unequal angle steel is bolted,out-plane stiffness of cross bracing increases and in-plane stiffness decreases simultaneously.It is an effective measure for narrowing the bearing capacity gap between inside and outside plane,improving material usage and ensuring economy and security.In general,the length ratio of the two legs for hot-rolled unequal angle is from 1.5 to 1.64.The behavior of unequal angle is more complex than that of equal angle because of the unsymmetrical section.Nowadays,most of the studies focus on the equal angle but pay less attention to the unequal angle.In order to study the stability of unequal angle members and effectively apply to power transmission tower,a large amount of experimental and theoretical investigations were carried out.The main research works covered in this dissertation include:(1)A total of 218 compression equal and unequal angle members were tested under knife-edge bearing to investigate failure process,bearing capacity and deformation performance.The influences of joint parameter,section type,eccentricity and slenderness ratio were analyzed.The results show the most of the small slenderness members controlled by torsional buckling or local buckling of connected leg,and the large ones were controlled by flexural buckling or flexural-torsional buckling.As the slenderness ratio increases,the bending axis moves from parallel axis to minor axis.When the members were connected by one leg,the bearing capacity of unequal angle is larger than equal angle with the same length,and the difference decreases with increasing length.(2)The finite element models with different end supports were established,which taken account of the geometric nonlinear property,initial geometric imperfection and residual stress.FEM results show that the stability coefficient decreases with increasing width-thickness ratio for small slenderness member,and decreases with increasing material strength for all slenderness member.The effect of material strength on stability bearing capacity is negligible while the slenderness is more than 120.(3)Two types of common lattice sections were tested to research the bearing capacity and failure model of cross bracing under tension/compression system.The failure mode is out-of-plane flexural buckling for equal angle cross bracing,while both in-plane and out-of-plane buckling for unequal angle cross bracing.The bearing capacity of cross bracing with unequal angle is higher than the ones with equal angle in the same conditions.It indicates that unequal angle is a good alternative to equal angle in the design of cross bracing in lattice tower section.(4)Through the numerical simulation of tower section finite element model,the stability analysis of unequal angle cross bracing was carried out at the conditions with different force ratio between the cross member.When one member is in tension and the other is in compression,both the stability capacity of cross bracing and the effect of eccentricity increases with tension.When the cross members are both in compression,the stability capacity of cross bracing decreases with compression,and the effect of eccentricity is negligible.(5)The element flexural-torsion stiffness matrix of single angle is deduced based on the principle of minimum potential energy and Vlasov’s displacement mode,which considered the interaction of local buckling,torsional buckling and flexural buckling.The flexural-torsion nonlinear buckling analysis of equal and unequal angle under knife-edge bearing is accomplished combined with finite-element method.The calculate method of stability coefficient and slenderness ratio correction factor,and the range of width-thickness ratio were proposed.The stability design method of cross bracing with equal or unequal angle members was presented.This method takes account of the interaction of in-plane and out-of-plane bucking,and offered reliable theoretical basis for application of unequal angle steel components in transmission towers.