Mechanical Behavior Analysis And Fire Safeguard Study On Cylindrical Mega-grid Structure In Fire

In recent years,a large number of space structures have been used in long-span public buildings,among which the giant grid structure has become one of the forms of space structure due to its superior mechanical performance.However,since the main load-bearing members of this structure are mostly steel,the fire high temperature has a significant impact on the properties of steel.Therefore,it is very important to analyze the mechanical behavior of the giant grid structure under fire and to reinforce the structure.In this paper,cylindrical mega-grid structure is selected as the research object.Based on the finite element theory,ANSYS finite element software is used to analyze the displacement characteristics,internal force variation rules and protective measures of the mega-grid structure under fire.The main contents are as follows:Firstly,the distribution of temperature field in the cylindrical giant grid structure is analyzed by using the air heating curve of large space fire.Then,the whole fire process of the structure is analyzed with uniform temperature field and non-uniform temperature field.The conclusion is that the deformation of the whole structure expands upward in the early stage of fire,and the structure softens in the later stage due to the reduction of the elastic modulus and yield strength of the fire-heated steel.Under the action of uniform temperature field,the upper chord and lower chord of the arch lower chord located at the center of the structure,the longitudinal truss support and the upper chord may become the control members of the fire-resistant bearing capacity.Under the action of non-uniform temperature field,the control rod may appear in the arch upper chord and longitudinal upper chord and lower chord at the support.By investigating the different parameters of high temperature structure fire(rise-span ratio,large span,grid size,bar section,the boundary constraint and substructure are involved in collaborative bearing)for refractory time limit and ultimate displacement influence law,get cylinder when giant grid structure fire resistance design,should pay attention to the choice of rise-span ratio,the span of control right,choose suitable for large grid size,control of steel tube wall thickness,can effectively reduce the structure reasonable decorate boundary constraints in large deformation under high temperature,fire so as to improve the fire resistance bearing capacity of the structure.The influence of different fire source models on the elastic-plastic ultimate bearing capacity of the cylindrical mega-grid structure under the action of non-uniform temperature field and the influence of initial defects on the elastic-plastic bearing capacity of the structure are analyzed.When the fire source is close to the edge of the structure,the ultimate bearing capacity of the structure decreases slowly and the fire resistance time increases.When the applied load is slightly higher than the design load,the bearing capacity of the structure decreases by 25% when the initial defect reaches S/300.According to the buckling mode of the structure at the limit time,the distribution position of the failure bars and the weak anti-fire position of the structure are summarized as the area directly above the fire source.Based on the stress performance of the huge grid structure under fire,three fire retardant coating schemes were developed and their fire resistance was evaluated.From the point of view of construction and economy,the number of three painted rods is small and the safety tolerance of the structure is maximized.