Influence of nails size and layout to obtain the reduction coefficient of moment of inertia for timber beams with composite cross section

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 (α r ) for the reduction of the theoretical moment of inertia (I teo ) 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 α r 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 (E deq ) used to determine the cited coefficients. The highest values of α r 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 α r stipulated by the Brazilian Code. For "I" section, α r overestimates beams performance. Nails size and form of calculating E deq were not significant in obtaining α r . The quadratic model showed the best results, indicating that the amount of 25 nails can provides the highest values of α r coefficients.

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