Analysis Of Response And Continuous Collapse Risk Of Loaded Welded Steel Tube Truss During The Whole Process Of Reinforcement

Steel-tube truss structure has the advantages of simple structure,clear force and low cost,and is widely used in various infrastructure and long-span structural engineering.In the truss structure,the force of each member is mainly axial force,but the force of the joint is more complex,so the whole truss can be strengthened by strengthening the joint.Welding ring plate at the key joint position is a common reinforcement method,but the influence of welding plate strengthening process on the truss itself and the reinforcement effect of ring plate under load are not clear.This thesis takes the three-span scale truss as the research object,studies the influence of the ring plate on the stiffness of the truss through the method of combining experiment,finite element and theoretical analysis,reveals the change rule of temperature and the force change of the truss in the process of welding plate reinforcement,and explores the influence of welding process on the reinforcement effect of the ring plate.At the same time,a calculation formula of joint risk coefficient based on stiffness is proposed to analyze the progressive collapse risk of truss through the change of joint risk coefficient.First of all,Two truss in-plane flexural tests of reinforced truss and welded truss under load were completed.The reinforcement effect of ring plate on truss in elastic stage was explored,the influence of welding process on truss under load was investigated,and the influence of welding on bearing performance of truss under load was compared.The results show that:(1)the influence of the ring plate on the initial stiffness of the truss is not obvious.When the load is about 20 k N,the deflection of the truss strengthened by the ring plate basically does not change;(2)The strain changes in the welding process mainly occur around the welding joints,mainly because the steel is "softened" at high temperature,so the strain changes around the measuring points to a large extent;(3)In the process of welding,the truss expands under heat,and the truss appears a slight "head up" phenomenon;With the cooling of welding,the "cold shrinkage" of steel is gradually obvious,and welding brings large residual deformation,so that the truss has obvious deformation in the welding cooling stage.(4)The temperature changes in the welding process are concentrated,mainly in the area around the weld,and changes quickly,with the highest temperature exceeding 800℃.Then,the ANSYS finite element model of the experimental truss was established,and the welding process was simulated by death and death element technology,the coupling of temperature field and strain field was simulated by indirect heat-structure coupling method,and the welding heat was simulated by death and death element selfgenerating heat model.The changes of steel properties with temperature were considered by referring to existing literature.The accuracy of the model is verified by comparing with existing literatures and text experiments.After that,the parameters of135 groups of models were analyzed by changing the load class n,whether to consider the effect of welding heat,the height span ratio α of the truss,the diameter to thickness ratio of the lower chord 2γ,the thickness to τ and the diameter to beta.The results show that:(1)the existence of annular plate increases the stiffness of the truss,but does not change the failure mode of the truss,and the truss still has bending failure;(2)With the progress of welding,the ring orifice plate gradually plays a strengthening role.With the progressive welding of each weld,the stress on the left and right sides of the ring orifice plate also changes constantly.However,the right ring orifice plate basically does not play a strengthening role between the right weld and the right one.(3)All dimension parameters have obvious influence on the deflection of the truss,and the key parameters are the height span ratio of the truss and the diameter thickness ratio of the lower chord.(4)Welding heat reduces the stiffness of the truss,and the higher the welding load grade is,the greater the influence of welding heat is.The influence of welding heat on the stiffness can be reduced by adjusting the dimension parameters.Finally,based on the importance analysis method of truss members,a formula of joint risk coefficient based on strength is proposed.The average value of maximum stress ratio of each member is used to judge the risk of progressive collapse of the truss.When the risk factor of the joint exceeds 1.0,it is considered that all the rods yield and the truss collapses progressively.When the risk factor is less than 1.0,progressive collapse may also occur.In this case,the larger the risk factor is,the higher the possibility of progressive collapse will be affected by the size parameters and the state of the truss.At the same time,the change of joint risk coefficient in the welding process is analyzed.In the welding process,the joint risk coefficient continues to increase,and the change is mainly concentrated in the first four stages of welding.The influence of different dimension parameters on the joint hazard coefficient is analyzed.Among all dimension parameters,the influence of the height span ratio of the truss is the greatest.When the load class is 0.65 and the height span ratio is reduced from 1/6 to 1/10,the risk coefficient of node 1 is reduced by 0.266.This thesis includes 88 figures,31 tables and 99 references.