Research On Nonlinear Cellular Automata-finite Element Model Of Reinforced Concrete Structure Under Fire

In practical structural analysis,most of them are non-linear analysis,and non-linear theory is also widely used in large-span and space structures.Therefore,it is necessary to study the non-linear process of space structures.During the service life of the structure,the material properties deteriorate constantly.The basic laws of stress and strain must be taken into account in the prediction of the structure life,and the damage of the material caused by fire is particularly sudden.In fire analysis,reinforced concrete structure suffers damage and its performance is decreasing.In durability analysis,it is necessary not only to consider the deterioration of materials,but also to analyze the temperature distribution of structures under different working conditions.In the analysis of temperature field,the internal force distribution and deformation state can generally be neglected.Independent structural performance analysis can be carried out,and temperature diffusion analysis can be carried out first.In this paper,the process of temperature diffusion in reinforced concrete structures subjected to fire is studied.Based on the theory of cellular automata,a temperature diffusion model is established.Based on the temperature distribution,the section bearing capacity and load-displacement law of members are analyzed.The main research contents are as follows:1.The thermal properties of the material,such as thermal conductivity,mass heat capacity and mass density,and the corresponding high temperature stress-strain relationship were studied.The decline of compressive strength of concrete is an important index to judge the fire damage of concrete,and the yield strength of steel bar is the main factor to determine the bearing capacity and ductility of concrete structure.2.The basic theory of cellular automata is summarized,including one-dimensional and two-dimensional neighborhood types,local rules and boundary conditions.In the temperature diffusion model,the ISO834 heating curve,the temperature diffusion equation,the definition of local rules and the solution of evolution coefficients are given,and Fick's second diffusion law is reproduced by cellular automata model.Without considering the influence of reinforcing bar on the temperature field of cross section,the diffusion model of cellular automata is established,the calculation program of MATLAB is compiled,and two three-sided fire sections are checked.The correctness of the cellular automata theory is proved by comparing the numerical calculation value with the experimental value.3.The basic theory of fiber beam-column element is sorted out,and the model of space fiber beam-column element is established without considering torsion and shear.Based on the incremental constitutive relation of cross-section,the cross-section deformation,cross-section force and cross-section stiffness matrix of material are obtained.The cross-section deformation is simplified to linear strain and non-linear strain.Based on the virtual work principle,the corresponding elastic stiffness matrix and the elastic stiffness matrix are obtained.The geometric stiffness matrix is solved iteratively by Newton-Lafferson,and the geometric nonlinearity analysis program of MATLAB is compiled to analyze the geometric nonlinearity of cantilever beam.4.By introducing the stress-strain relationship of material,based on the balance of cross-section force produced by internal force and external load,the equivalent cross-section stiffness is obtained by using the layered method and Cranstone iteration.By calculating the average cross-section stiffness of the left and right sections of the element,the element stiffness matrix is further calculated,and the MATLAB cross-section non-linear analysis program is compiled to count the pure bending section,the compression-bending section and the simply supported beam.Value analysis.5.Based on material nonlinearity analysis,material nonlinearity analysis is nested into geometric nonlinearity analysis,and Newton-Lafferson method is used for iteration calculation.The analysis program of MATLAB is compiled to carry out numerical analysis of long-span simply-supported beams.6.Based on the cellular automaton temperature diffusion model and step method,the formulas for calculating the bearing capacity of high temperature section in tension zone and compression zone are given.According to the damage model of materials,the expression of material damage factor is given,and the bearing capacity of pure bending section and compression bending section is analyzed.Finally,the basic law of bearing capacity of simply supported beams and continuous beams under fire is studied and analyzed,and the durability life of structures is evaluated.