Dimensionamento de vigas de concreto armado em situação de incêndio.

Abstract

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.

Analysis of the assessment methods of reinforced concrete beams in fire conditions proposed by NBR 15200

“…The redistribution of moments in fire conditions was admitted for beams B2. This redistribution reduced moment in the span by 25% and was recalculated based on Equation 6, typifying the criterion practiced by [20]. It was also used at room temperature, although with different safety factors, where "l" is the span of the beam, and are the negative and positive bending moments of calculation redistributed in fire conditions, and p d,fi is the value of the uniformly distributed load in fire conditions.…”

“…The effect is triggered by mechanical degradation of the piece's heated area. The higher heating of the beam's bottom half, directly exposed to the burning environment, in conjunction with the lower heating of the other half, thermally protected along the intersection with the slab, makes the mechanical properties of the less heated part undergo transformations milder than the others [8]. Such fact promotes an uneven redistribution of stresses in the section of the beam, which starts to present different tensile strengths in the multiple layers that compose it.…”

Computational algorithm for the verification of reinforced concrete sections in fire situation

“…The stress-strain diagram of the concrete at high temperatures is obtained by the third-order Popovics equation indicated by ABNT NBR 15200:2012 [36] and by EN 1992-1-2:2004 [29], according to Equation (15).…”

“…Alternatively to the diagram proposed by ABNT NBR 15200:2012 [36], according to previous studies conducted by Albuquerque et al [15] and Moreira et al [17] with satisfactory results, it can be used the parabola-rectangle diagram of ABNT NBR 6118:2014 [35], considering the strength of concrete adapted to high temperatures, as follows: (16) in which ε c2 is the specific compressive strain of the concrete at reaching the maximum strength, and ε cu is the ultimate compressive strain (rupture), both indicated in Table 1.…”

“…To avoid further material losses and especially lives, research in the field of fire has intensified more and more. Experiments with the goal of investigating the behavior of reinforced concrete elements subjected to high temperatures [2,3,4,5], researches for the analysis of thermal and thermomechanical problems of structures [6,7,8,9], studies that have investigated and provided simplified [10,11,12,13,14] and alternative [15,16,17] methods for design and verification of reinforced concrete elements have been performed in recent times. Studies on design up to that time were limited to analyzing rectangular sections of reinforced concrete in fire situation, comprising only normal composite bending.…”