Abstract
A significant portion of rural building’s roof is supported by timber trusses. Increasing distance between the trusses can be achieved by using composite cross sections (T and I) for beams. With this, it is possible to reduce the number of columns and trusses, bringing significant savings in material and the desired cost reduction, mainly using wood from planted forests. The Brazilian Code ABNT NBR 7190:1997 establishes a coefficient (αr) for the reduction of the theoretical moment of inertia (Iteo) of 0.95 and 0.85, for beams with T and I composite profile, respectively. However, no specification is showed about connections responsible for conferring such coefficients. This research aimed to investigate, based on static bending tests, analysis of variance and polynomial regression models (linear, quadratic, cubic), the precision of αr coefficients set by the Brazilian Code for Pinus sp. beams. We considered to evaluate influence of factors such as: nails number (3, 5, 9, 17, 33), nails size (19×27 mm, 19×33 mm), profile types (I, T) and calculation form to obtain the equivalent modulus of elasticity (Edeq) used to determine the cited coefficients. The highest values of αr were derived from beams with 17 and 33 nails, which provided similar results. The analysis also allowed admitting the adequacy, only to the beams with T section, of αr stipulated by the Brazilian Code. For I section, αr overestimates beams performance. Nails size and form of calculating Edeq were not significant in obtaining αr. The quadratic model showed the best results, indicating that the amount of 25 nails can provides the highest values of αr coefficients.
Highlights
Searches for production models that adopt sustainability standards have gained strength in recent years in the Brazilian civil construction
In order to collaborate with more information about the reduction of theoretical moment of inertia of composite cross section beam design, we aimed to investigate, in this study, αr coefficients indicated by ABNT NBR 7190:1997 Brazilian Code, evaluating the following factors: geometry of the cross section profile; nails amount and size; and the method of calculating the equivalent elastic modulus of the parts number for determining the αr coefficients
Timber beams with "T" profile and ten other with "I" profile (Figure 1) were manufactured, all of them tested in three-points static bending, same structural model adopted by ABNT NBR 7190:1997 in obtaining modulus of elasticity (E) and strength modulus in static bending, in small clear specimens
Summary
Searches for production models that adopt sustainability standards have gained strength in recent years in the Brazilian civil construction. Defects resulting from fiber inclination, nodes, shrinkage due to drying, among others, require from the construction industry professionals to develop alternatives to minimize these suppressive effects and bring to consumer market products with quality and durability (FIORELLI, SORIANO & LAHR, 2012) In this scenario the concept “engineered wood” emerged, terminology that defines its industrialization process. An example of this material is the composite beams formed by joining smaller wood pieces, allowing obtaining larger cross section of commercial dimensions (GÓES, 2002; GÓES & DIAS, 2005). Such parts may be applied as purlins in roof structures, especially in rural buildings, which allow to significantly increasing distance between build ing columns, reducing the final cost of framework set (CALIL et al, 2003; CHRISTOFORO et al, 2013; GONÇALVES et al, 2013)
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