Abstract This article presents a numerical formulation based on finite element procedures for application in nonlinear thermo-mechanical analyses of steel planar structures under fire condition. The mechanical properties of structural elements degrade when subjected to high temperatures, resulting in significant reductions in strength and stiffness. Under these conditions, the structures present complex behaviors associated with nonlinear models, requiring an advanced mathematical analysis. As such, a computer program called NASEN has been developed to investigate the behavior of steel structures subjected to fire, considering the effects of geometric and material nonlinearity, as well as the thermal gradients acting on the cross-section. The solution strategy is based on sequential coupling of numerical processes. Initially, the two-dimensional thermal field is determined, followed by an assessment of structural behavior. In each solution step, corrective processes are implemented to ensure convergence of the temperature and displacement nodal vectors. Numerical experiments are performed in order to evaluate the accuracy and capacity of the computer program. Results are compared with experimental tests and computer simulations found in pertinent literature. The program shows good agreement with reference solutions, indicating its accuracy and applicability for the cases studied.

resumo: O presente artigo tem como objetivo o desenvolvimento de um modelo computacional com base nos procedimentos do método dos elementos finitos. A análise avançada é capaz de estimar o comportamento de estruturas planas de concreto armado e, mistas de aço e concreto em situação de incêndio. O programa computacional implementado é denominado de NASEN e, utiliza um módulo termo-estrutural específico para análise de estruturas em condição de incêndio. Incorpora-se ao modelo os efeitos da não linearidade geométrica, não linearidade do material e gradientes térmicos não lineares, bem como a degradação das propriedades dos materiais com aumento da temperatura. A metodologia aplicada para solução tem como base o acoplamento unidirecional das soluções térmicas e mecânicas. As seções transversais dos elementos estruturais são discretizadas com malhas bidimensionais para análise térmica, enquanto a análise estrutural utiliza elemento unidimensional de viga-coluna. Exemplos numéricos são apresentados para demonstrar a precisão do código computacional desenvolvido em relação as soluções numéricas e experimentais encontradas na literatura. O programa computacional prevê adequadamente a resposta das estruturas estudadas.

abstract: This paper is concerned on the development of a computational model based on finite element procedures for advanced analysis capable of estimating the behavior of reinforced concrete and composite steel-concrete plane structures exposed to fire. The program implemented is called NASEN, the specific thermo-structural module is used to analyze structures under fire conditions. The effects of geometric nonlinearity, material nonlinearity and nonlinear thermal gradients are incorporated into the model, as well as the degradation of material properties with increased temperature. The methodology applied for the solution is based on the unidirectional coupling of the thermal and mechanical solutions. The cross-sections of the structural elements are discretized with two-dimensional meshes for thermal analysis, while the structural analysis uses a one-dimensional beam-column element. Numerical examples are presented to demonstrate the accuracy of the computational code developed in relation to the numerical and experimental solutions found in the literature. In summary, the program adequately predicted the response of the studied structures.