Research On Mechanical Properties Of Lining Joints With Different Initial Defects Under Fire And High Temperature

The construction of infrastructure in my country’s major cities is proceeding swiftly,and the development of underground transportation projects has reached an unprecedented stage.Fire is one of the main disasters acted on the tunnel.Once a fire occurs in tunnels,it will not only cause serious casualties and huge property losses,but also induce irreversible damage to tunnel lining structures.Shield tunnel segment joints are the weakness of the tunnel structure.In this aspect,the research on the fire resistance performance of shield tunnel segment joints is particularly important.Scholars at home and abroad cannot consider the influence of different initial defects on the mechanical properties of lining structures when studying the fire resistance of lining structures.This article will follow the Xuzhou Metro as a reference to conduct fire tests on the joints of lining structures with installation errors,and establish a thermal-mechanical coupling three-dimensional numerical analysis model of tunnel lining structures with the help of large-scale finite element software.Research on the mechanical properties and damage laws.The main research results are as follows:(1)There will be cracking sound about 6-10 minutes after the ignition temperature rises,the tube bursting is concentrated in 15-20 minutes,and the bursting temperature is between 600-700℃,after which the bursting gradually slows down,and finally disappears.The distance between the concrete of the lining segment joint and the fire surface is 25 mm,and there will be a 100℃ heating platform within 10-22 min.The inside of the joints of the lining segments and the bolt holes burst and collapse obviously.The lining segments without initial defects have a large burst area and shallow depth,and the lining segments with initial defects have a large peeling depth.Among them,there are lining segments with staggered initial defects(JQ-1-0)The maximum peeling depth is 72 mm,and the joint damage rate is 10.52%.(2)The initial defect indirectly affects the temperature distribution of the joint bolts.The bursting degree of the lining segment with the initial defect is more serious.The lower edge of the joint is damaged first,and the hole is seriously damaged.In the absence of concrete protection,the joint bolts directly Exposing to flames causes its temperature to rise sharply.(3)Under the action of fire,the bending moment of the segment joints with initial defects changes faster and there is a larger residual deformation after cooling.Among them,the bending moment of the joints with initial defects of misalignment increases at a rate of 61.05% faster than that of the non-defective joints.Its rate is 53.7% slower than that without defects in the cooling stage,which shows that the bending and shearing properties of the joint are sharply reduced,which is manifested by the increase in the value of the joint opening angle and the opening amount,and further misalignment occurs in the joint part.(4)The stiffness of the segment joints with initial defects will decay faster and the stiffness loss will be greater than that without the initial defects.After the test,the remaining stiffness will be 69.7% less than that without defects.After the disaster,the stiffness of the defective segment joints will be greatly reduced.(5)In the finite element analysis,the temperature of the concrete 25 mm from the inner side of the segment is 238℃,and the temperature of the concrete 90 mm from the inner side of the segment is below 88 ℃.Under the protection of concrete,the temperature of the bolt in the hole is the temperature at the hand hole.45.58% of the total,concrete has a good thermal insulation effect.How to avoid concrete bursting is the key to improving the fire resistance of the joint.(6)The stiffness of the segment joint without initial defects decays faster at the initial stage of the fire.The attenuation rate of the joint stiffness within 20 minutes is2.23 times that of the subsequent attenuation rate,and the stiffness is finally attenuated by 29.72%;the segment with the initial defect is misplaced.The sudden increase in the shear force of the contact surface causes the bending moment and opening angle of the joint to increase.The greater the degree of misalignment,the greater the sudden increase in the bending moment,resulting in greater defects in the joint stiffness will attenuate more,and the joint stiffness will slowly recover 28.96% After the disaster,the stiffness of the joint attenuated by 57.69%;the joint stiffness of the segment joints with misalignment and initial opening defects has a much larger rate of change in the opening angle of the joint than its bending moment,resulting in a sharp initial attenuation of the joint stiffness,and the degree of opening defects has changed.The attenuation of the ambassador’s stiffness has increased by 58.92%,but the stiffness of the more splayed joint has recovered 7.19%,and the stiffness of the joint after the disaster has decreased by 40.73%.