criaram-se gráficos que associam o parâmetro µ, relação entre o momento fletor solicitante em situação de incêndio e o momento fletor resistente à temperatura ambiente, ao tempo de resistência ao fogo (TRF), para cada situação de interesse. Esses gráficos, que envolvem tanto armaduras positivas quanto negativas, também permitem levar em conta a redistribuição de momentos, o que conduzirá à otimização na solução encontrada. Nos exemplos de aplicação realizados, os resultados obtidos a partir do método gráfico se mostraram, em geral, mais econômicos, quando comparados aos do método tabular. Palavras-chave: Concreto armado. Vigas. Dimensionamento. Incêndio. Análise térmica. Método gráfico. ABSTRACTReinforced concrete beams lose their load capacity when subjected to fire. The most practical tool for their design is the tabular method, presented in ABNT NBR 15200:2012, in which, from the time required for fire resistance (TRRF), the minimum width of the transversal section and the smallest admissible distance between the centroid of the rebar and the heated face can be found. Yet, albeit simple, it limits the calculation to a few fixed values, not allowing the engineer to seek different solutions.Hence, an alternative method was developed in this Dissertation, using the Swedish Super Tempcalc software. Firstly, the temperature field of beams subjected to ISO 834 (1999) standard fire situation was defined, as a function of time. Next, considering the reduction in concrete and steel strength, the resistance bending moment was calculated for the different cases studied. Beams with different widths, heights, covers, diameters and reinforcing bars dispositions were thermally and structurally analyzed. The resistance moments in fire situation obtained from the software were compared to values deriving from simplified methods, proposed by Brazilian and European (Eurocode 2 part 1-2, 2004) standards, besides a more advanced method. After data validation, graphs were created, associating the parameter µ, ratio between the applied bending moment in fire and the resistance bending moment at ambient temperature, to time for fire resistance (TRF), for each situation of interest. These graphs, involving both positive and negative reinforcement, also allow consideration of the redistribution of moments, which will lead to the optimization of the solution found. In the examples analyzed, the results deriving from the graphic method were generally more economic compared to the tabular method.
The most expeditious method for the design of concrete beams under fire situation is the tabular method, presented by the Brazilian standard ABNT NBR 15200:2012. Albeit simple, this method constrains the engineer's work, as it prevents him to seek alternative solutions to the few tabulated values. Yet, the Brazilian standard allows employing more advanced methods. Hence, the purpose of this work was to perform a thermal and structural analysis of beams with several widths, heights, covers and diameters/layouts of steel reinforcement (upper and lower). From those results, graphs were constructed, associating the ratio between the applied bending moment in fire over the resistance bending moment at ambient temperature, for the fire resisting time of each situation. These graphs also allow taking into account the redistribution of moments from positive to negative, which will lead to savings in the solution found.
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