Strength and displacement under tension and compression of wood joints fastened with nails and Screws for use in trusses in Costa Rica

The objective of the present study is to determine the behaviour of two typical types of fastener (nails and screws) used in trusses made of Gmelina arborea and Hieronyma alchorneoides timber. Wood joints with metal fasteners (nails and screws) and five angles (0°, 30°, 45°, 60° and 90°) were subjected to tension and compression loads in order to establish values of displacement in relation to applied loads, strength, stiffness values, mode of failure and a model for prediction of stiffness for intermediate orientations. Results indicate that the differences in loads and displacements appear among species in the compression test, whereas those differences appear among fasteners in the tension test. The results obtained for stiffness indicate that joints of H. alchorneoides wood present the highest values. Models for prediction of stiffness for truss joints of intermediate orientations were: k_θ = (k_∥ ^∗k_‰) / (k_∥ ^∗ sin^{(n = a ∗ θ +c) ∗} θ + k_‰^∗cos^{(n = a ∗ θ + c) ∗} θ) in compression, while for tension the model was (k_∥ ^∗k_‰) / (k_∥ ^∗ sin^{(n = a ∗ θ2 + b∗ θ + c) ∗} θ + k_‰^∗cos^{(n = a ∗ θ + b∗ θ + c) ∗} θ).