| Building fire is one of the high-frequency disasters,and the fire safety of widely used reinforced concrete(RC)frame structures is particularly important.However,research on the characteristics and rules of time-varying fire behavior based on the structural integrity of these frames remains insufficient,which limits the fire safety assessment of such structures.This paper proposes a three-dimensional nonlinear finite element analysis method for the time-varying fire behavior of structures,based on the theoretical research of degenerated three-dimensional solid virtual laminate element,this paper proposed a three-dimensional nonlinear finite element analytical method for time-varying fire behavior of structures.The time-varying fire behavior of RC frame structures considering the spatial coordinated interaction of beam-slab-column was investigated through programming numerical simulation,calculation and analysis.The main research work and results are as follows:(1)A method to deal with the fire temperature field of degenerated three-dimensional solid virtual laminated element was proposed.This method considers the change in thermal performance of materials with temperature,reasonably simplifies the boundary conditions of temperature field calculation,and solves the heat conduction equation of the temperature field.Consequently,a nonlinear analysis program for the temperature field was developed to simulate the time-varying temperature inside RC spatial frame structural members under fire conditions.(2)A time-varying fire behavior analytical method for RC spatial frame structures was proposed,based on the theory of degenerated three-dimensional solid virtual laminate element,considering structural high temperature double non-linearity.This includes an iterative method for finite element analysis of structural time-varying fire behavior,a construction method for the three-dimensional elastoplastic incremental constitutive relation of concrete at high temperatures,and an analytical method for stress release during material softening,cracking,and failure at high temperatures.A mechanical model was established to analyze the time-varying fire behavior of structures,taking into account stirrup constraints.A three-dimensional nonlinear analysis program for time-varying fire behavior was developed to perform parametric calculations and analyses of RC spatial frame structures.(3)The correctness and applicability of the proposed time-varying fire behavior analytical method and program for RC spatial frame structures were verified.By analyzing existing typical fire tests,material-sensitive parameters and commonalities in component analysis for fire behavior research were identified.The influence factors of structural fire behavior were analyzed based on the boundary conditions of the components and nodes within the structure,determining the degree of influence of each factor.This facilitated the selection of typical components for overall fire behavior research on RC spatial frame structures.(4)By applying the proposed time-varying fire behavior analytical method and program,a finite element analysis model of a typical RC spatial frame structure was established.Comparative analyses of the fire behavior of single-layer RC spatial frame structures under different fire conditions were conducted,investigating the fire behavior characteristics of typical components within the entire structure.This analysis were extended to high-rise structures,examining the time-varying fire behavior of the structure.Based on the characteristics and rules of time-varying fire behavior,the spatial coordinated interaction of beam-slab-columns was studied.The entire process of structural time-varying fire behavior and the factors affecting the redistribution of internal forces were identified,revealing the spatial cooperative working mechanism of column top pushing,beam-plate flat pushing,and the limiting action of the space arch membrane in the time-varying fire behavior of RC frame structures. |
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