| Under the earthquake,the building structure should withstand the cyclical reciprocating effect,and the structural members will suffer different degrees of damage.When the earthquake is large enough,it will produce different degrees of plastic deformation or even plastic hinges.From the seismic damage of the grid structure in previous major earthquakes,the damage of the bars is mainly manifested as: buckling or breaking of the bars,plastic hinges in the bars and accompanying fractures,etc.It can be inferred that the damage of these bars has experienced several times.The process of plastic strain accumulation has the characteristics of ultra-low cycle fatigue damage.The performance of the plastic hinge formed by the mesh structure members in the earthquake is very important to the seismic performance of the whole structure.For this reason,the mechanics of the plastic hinge of the rod is studied by the ultra-low cycle fatigue test of the bolt ball and the rod combination test piece.The performanceis to provide a basis for the preliminary establishment of the bolt-ball grid structure in the ultra-low cycle fatigue performance and checking method under earthquake.Based on the project of National Natural Science Foundation of China?Research on ultra-low-cycle fatigue performance and assessment methods of bolt-ball grid structures under strong earthquakes ?(51578358),aiming at the problem of ultra-low cycle fatigue fracture of plastic hinge of bolt-ball grid structure under strong earthquake,a round steel pipe combined test piece model with bolt ball joints at both ends was designed.A large-displacement axial cyclic loading test was carried out on the three ball joint specimens of high-strength bolts.The main research contents are as follows:(1)Designed the loading device of the steel pipe combined test piece model with bolt ball joints,formulated a detailed ultra-low cycle fatigue test plan,determined the specific loading system during the test,and designed the plastic deformation of the round steel pipe by reference.method.Research on the position,extent,shape and formation mechanism of the plastic hinge of the rod.(2)The ultra-low cycle fatigue performance of steel tube hysteretic behavior,skeleton curve,stiffness degradation and energy dissipation performance under three different bolts and three different loading systems were obtained through experiments.(3)Finally,based on the finite element model of the OpenSEES beam-column fiber unit,the damage accumulation is considered,and the test process is numerically simulated and compared with thetest results.The main conclusions of the experimental study are as follows:(1)The failure process and the failure mode of the plastic hinge of the steel tube are almost similar,and all of them undergo the process of bending,depression,crack and opening break.The plastic hinge area presents an elliptical shape when the opening breaks;the plastic hinge area shrinks under the tensile load,becoming elliptical under compressive loading.(2)The closer to the plastic starting section in the plastic hinge region,the greater the amount of necking.The necking amount can be up to 16% of the cross-sectional area of the original member.(3)The position of the plastic hinge is within the axial height ranging from the diameter of the midpoint of the rod.(4)The plastic hinge length of the steel pipe with a diameter of 88.5mm under the three loading systems is in the range of 220mm~380mm,which is2.49~4.29 times of the diameter.(5)Different bolt diameters have little effect on the nonlinear starting point bearing force in the skeleton curve of the combined test piece,and have an influence on the ultimate bearing capacity.The larger the bolt diameter,the greater the rotational rigidity of the bolt ball joint and the greater the ultimate bearing capacity of the rod. |
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