Influência do valor médio do módulo de elasticidade à compressão paralela às fibras no projeto de treliças em madeira

Abstract

In the evaluation of the service limit state, the Brazilian standard ABNT NBR 7190 (1997) recommends the adoption of the mean value of the elasticity module to compression in the direction parallel to grain (Ec0,m) for all the members of the structure. Due to the variability of the wood, this methodology can generate vertical displacements higher than the limit established by the regulations (L (span) / 200), which can cause pathological manifestations in the structure. In order to evaluate the influence of the adoption of Ec0, m for all the truss bars in the calculation of displacements, mechanical properties of the species Angelim Ferro (Hymenolobium sp) and Angelim Pedra (Hymenolobium petraeum Ducke) were obtained through experimental tests, according to the normative specifications of ABNT NBR 7190 (1997). With the aid of numerical models developed in ABAQUS® and the experimental values of the mechanical properties, 10 typologies were analyzed (Porch, Inclined Banzos, Upper Inclined Banzo, Parallel Banzos, Bowstring, Fink, Howe, K Truss, Pratt, Scissor), 3 spans (5, 10 and 20 meters) and 2 species (Angelim Pedra and Angelim Ferro). Initially, adopting the Ec0, experimental for all bars and performing a linear analysis, the loads to generate a displacement of L/200 for each combination of parameters were determined. In the next step, using the loads previously defined, random experimental values of stiffness were assigned to each element of the truss and, through geometric nonlinear analyzes, the maximum displacements of each model were obtained. Based on the results of 1260 numerical simulations, displacements greater than the reference value (L/200) were obtained in 17.32%, 30.18% and 34.80% for spans of 5, 10 and 20 meters, respectively, suggesting that the methodology proposed by the standard may generate excessive displacements. These excessive displacements can cause pathological manifestations in the structure, reducing its useful life. This problem can be overcome with the use of non-destructive methods in the pre-classification of the structure bars, positioning the elements rationally in the regions of greatest demand.