Study On Deformation Characteristics Of Combined Test Piece In Ultra Low Cycle Test Of Bolt Ball And Steel Tube(?88.5×4.0)

With the development of society,large-span steel grid structures have been widely used in a large number of public buildings such as stadiums,airports,and stations.In addition to the function of daily public activities,such buildings should also be used as shelters and disaster relief sites in the event of a disaster or other disasters.For this reason,the safety performance of such buildings in the event of a disaster is directly related to the safety of a large number of victims and the effectiveness of emergency rescue.Earthquake damage and related research show that the collapse of such buildings under strong earthquakes is characterized by ultra-low cycle fatigue damage.Therefore,the research on the ultra-low cycle fatigue failure mode and the anti-collapse design method of the space grid structure under strong earthquake has become an important research topic in the field of disaster prevention and disaster reduction and construction of resilient cities.This paper is one of the contents of the National Natural Science Foundation of China: Ultra-low cycle fatigue performance and evaluation method of bolt-ball grid structure under strong earthquakes(51578358).The research team designed a special one-way reciprocating loading test device based on the laboratory facilities and equipment conditions,and carried out the ultra-low cycle fatigue test of the bolt ball and steel pipe combined test piece.Inthe test,the ultra-low cycle fatigue test was carried out on the combined test pieces of the round steel pipe ?88.5×4.0 and the corresponding bolt ball joints by different loading systems.This paper focuses on the deformation characteristics of the combined specimens,including the deformation of the mid-point space,the ellipticity of the midpoint,the fracture section,the distribution of the fracture wall thickness,the deformation characteristics of the bearing capacity and the energy dissipation law.Based on the bearing capacity degradation and energy dissipation characteristics of the combined specimens,two mathematical expressions for evaluating the damage degree of the combined specimens are summarized.The main research results are as follows:(1)The ellipticity and spatial deflection of the midpoint of the specimen can be used to characterize the degradation process of the bearing capacity of the specimen.This kind of index can be used as a reference for the evaluation of the residual bearing capacity of the bolt in the grid structure of the bolt ball after the disaster.(2)The amplitude of the cyclic loading increases,and the degree of plasticity of the combined test piece increases,resulting in degradation of the bearing capacity of the test piece and a decrease in the ultra-low cycle fatigue life;(3)The loading system is different.The spatial deflection,ellipticity,fracture cross-section shape and fracture wall thickness variation of the midpoint of the specimen under ultra-low cycle fatigue failure show different characteristics.(4)Based on the cumulative energy consumption and bearing capacity degradation,the evaluation criteria of the damage degree of the two components and their corresponding indexes are summarized.The evaluation results of the two evaluation criteria are basically the same,which correspond to the damage index of the test piece when the cycle is reciprocating to the opening fracture.(5)The hysteresis curves of the combined specimens were simulated by OpenSees software.The results were basically consistent with the experimental ones.The OpenSees software could be used to simulate the ultra-low cycle fatigue damage performance of spatial grid structural members and nodes.