Analysis Of Plastic Mechanical Characteristics Of Portal Frame Structure At High Temperature

There are many advantages of portal frame structure,such as good seismic performance,fast installation speed,short construction period,high strength and low cost.It is widely used in industrial buildings.But there are also many defects,and some of them are deadly,such as rapid damage by fire.Under the action of fire,the stiffness of steel members decreases rapidly,which will lead to the collapse of the whole structure,resulting in casualties.Therefore,it is of great significance to study the whole mechanical response of portal frame at early high temperature.At present,the research of portal frame is mainly based on the research of elastic state,for the mechanical characteristic plastic of portal frame under high temperature(fire)In this paper,the portal frame is taken as the research object,through the theoretical analysis and the finite element numerical simulation,the mechanical properties of the portal frame are studied,and some research results are obtained.The main research contents and results are as follows:1.The thermal and mechanical properties of steel under fire are summarized,and the material characteristic parameters suitable for portal frame structure under high temperature(fire)are obtained.It lays a foundation for the thermal internal force and stability analysis of portal frame under high temperature(fire)conditions.2.The theoretical calculation methods and corresponding steps of critical temperature,ultimate combustion time and critical moment of portal frame under high temperature(fire)are summarized.The main steps to calculate the boundary moment of portal frame are to calculate the critical moment of portal frame at room temperature,to determine the critical section of critical moment,and then to calculate the critical moment and stress at the critical section of portal frame under high temperature(fire),and then to calculate the critical moment and stress at the critical section of portal frame under high temperature(fire).Through an example,the critical moment and stress at the critical section are calculated,and the development of plastic hinge of portal rigid frame under high temperature(fire)is obtained.3.The changes of temperature field,deformation and internal force of portal frame structure under high temperature(fire)are analyzed with finite element software.The research shows that when the fire time reaches 880 s,the portal frame has entered the plastic state,and through the analysis of the stress value at the end of the beam and the ridge,the development of the plastic hinge after the portal frame has entered the plasticstate is determined,that is,there are three plastic hinges in the portal frame structure.At the roof of the ridge and at the beam end,the plastic hinge was first appeared at the ridge,and the plastic hinge appeared at the end of the beam and compared with the calculated results.4.The changes of temperature field,deformation and internal force of portal frame structure under high temperature(fire)are analyzed considering the effect of horizontal load and different column foot stiffness.The displacement and stress values at the end of the beam and the ridge are analyzed,and the development of the plastic hinge after the entry of the portal frame into the plastic state is determined.5.The variation of temperature field,deformation and internal force of two-span portal frame structure model under high temperature(fire)is analyzed by finite element software.The results show that: without considering the influence of temperature with distance,the change of temperature field,deformation and internal force of two-span portal frame structure model under high temperature(fire).It is difficult to change the temperature of the cross-portal rigid frame structure on the right side by the heat conduction inside the steel structure material alone.Through deformation and stress analysis,the development of the plastic hinge of the structure is obtained,that is,after entering the plastic state,the plastic hinge appears first at the left span ridge,then at the left beam end,and the plastic hinge at the left beam end at the right span.A plastic hinge then appears at the end of the right beam.