Study On Failure Of Top And Seat With Web Angle Connections And Its Influence On Structural Collapse Under Fire

The failure of beam-column joints of steel structures is one of the main causes of structural collapse.It is great of significance to study the mechanical properties and failure mechanism of beam-column joints under fire to evaluate the fire resistance of steel structures.The top and seat with web angle joint is one of the most commonly used beam-column joints.The mechanical properties and failure modes under the whole process of fire(ambient temperature,heating,heating-cooling,and after-fire)were studied.Previous studies have focused on the basic mechanical properties of the joint at ambient and heating stages,but rarely on the failure mode during the entire fire process.Therefore,this study conducted experimental research and numerical simulation on the failure mode and mechanical properties of the top and seat with web angle joint during the entire fire process.By introducing the joint into the frame model through a hybrid model,the influence of joint failure on the progressive collapse resistance of structures under fire was investigated.The main research contents and conclusions are as follows :(1)A three-column-two-beam joint model test(scale ratio 1/2)was carried out on the top and seat with web angle joint under normal temperature and the whole process of fire(heating,heating-cooling,and after-fire).The test results show that:(1)The failure mode of the top and seat with web angle joint during the entire fire process is related to the temperature scenario,loading,and relative strength of the angle steel and bolts.At ambient temperature and heating stages,the joint failure mode is the failure of the lower angle steel bolt hole under tension,and the joint failure after fire is caused by the shear failure of the lower angle steel bolt.(2)The load under fire has a great influence on the residual ultimate bearing capacity of the joint after fire.Ignoring the influence of load will lead to unsafe results(error up to 60 %).(3)As the temperature increasing,the load-bearing capacity of the structure significantly decreases.When the temperature rises to 650 ℃,the load-bearing capacity is less than 50% of the loadbearing capacity at room temperature.(4)The bearing capacity of the joints decreased significantly after fire,and the bearing capacity decreased to 77 % of the normal temperature value after 1000 °C high temperature.It is necessary to check the residual bearing capacity of the steel structure after fire.(2)A three-column-two-beam finite element model of top and seat with web angle joint is established.The model is verified by experimental data,and then a full-scale joint model is established.The parameter analysis is carried out to study the influence of scale ratio,bolt preload,distal constraint condition,material model,angle steel thickness,high temperature load ratio and stiffener on the mechanical properties and failure mode of the joint.Numerical results show that:(1)During the heating stage and after the fire,the failure mode of the full-scale joint model was tension failure of the lower angle steel bolt,which was different from the bolt hole failure and bolt shear failure of the scaled model.The main reason was that the relative strength of the angle steel and bolt changed under the coupling effect of the temperature field and the scaling ratio,leading to the change of the failure mode.(2)The bolt preload has no effect on the bearing capacity of the joint and the axial force inside the beam,and different farend restraints had a significant effect on the bearing capacity of the joint.The bearing capacity of the hinged restraint was only 76.9% of that of the fully fixed restraint,but it did not change the failure mode of the joint.(3)The ultimate bearing capacity of the joint using the real material model was 63% higher than that using the standard material model,indicating a large safety margin in the design according to the standards.(4)The failure modes of the top and seat with web angle joint were mainly lower angle steel root tearing failure,lower angle steel bolt hole tension failure on the column side,and lower angle steel bolt tension failure on the column side.The ultimate bearing capacity of the lower angle steel root tearing was the lowest,and it was necessary to increase the thickness of the angle steel or add stiffeners to improve the performance of the joint.(3)A hybrid finite element model of four-span five-story plane steel frame structure is established,in which the solid element is used in the joint area and the beam element is used in the beam and column.The influence of load ratio,joint form and fire scene on the progressive collapse resistance of the structure is studied by parameter analysis.The results show that :(1)Compared with the solid model,the hybrid model can greatly reduce the calculation time of the frame model considering the nodes(at least 4 times).(2)Under the fire condition of the middle column in the first floor,the load ratio,the joint form and the number of non-rigid joints have a great influence on the collapse temperature and collapse of the structure.The larger the joint stiffness,the higher the collapse temperature of the steel frame,or even no collapse;the more the number of non-rigid joints,the lower the collapse temperature of the steel frame.(3)The fire scenario(fire area)has an important influence on the collapse mode of the frame.Only the fire condition of the middle column is significantly different from the fire condition of the middle column-joint-beam,depending on the failure sequence of the column and the joint.The innovations of this thesis are as follows:(1)Pioneered the experimental study of top and seat with web angle joint under the whole process of fire,revealing the mechanical properties and failure modes of top and seat with web angle joint in different stages of fire(2)The hybrid model was used to introduce the joint into the collapse analysis of the planar steel frame,and the influence of the joint on the continuous collapse of the steel frame was explored.