Influence of structural timber fibre orientation on the strength capacity of dowel-type joints

Threaded-dowel type timber joints with free ends and with nut-washer end fixings were experimentally investigated, modifying the orientation of the timber beams and the evolution in the deformation of their fibres as the structure was loaded, with the objective of quantifying their contribution to the strength in relation to displacement. The work is based on an analysis of the equations for the dimensioning of dowels, compiling the original models developed by Johansen in comparison with the superimposition extensions of Eurocode 5. This provides the basis for carrying out the experimental tests to compare the load capacity established in the dimensioning of structures with the experimental results. This work focuses on dowels-type joints that fail in plastic mode, which improves the distribution of stresses and optimizes the resistant capacity of the joint while analysing the crushing regions in the contact areas of the timber dowel which contain the dowel and redefine the angle of the hinge joint in successive flat planes perpendicular to the axis of the hole where the dowel is housed, which allows analysing the dominant stresses related to the repositioning of the timber fibres exposed to crushing. The tests with nut-washer fastening show a behaviour that notably improves the resistant capacity when the degree of displacement increases. This research shows how the capacity of the structural joints is linked to the effects of crushing, provoking a reorientation of the fibres. This work raises the need to study in detail the resistance of the structural joint, limiting the displacement, when comparing the analytical results against the displacement curves of the experimental tests. It can be seen that increases in the angle of orientation of the fibre of the beams are penalized by the analytical equations with load capacity losses greater than 10% for fibre angles located at 90 degrees, however the tests show an improvement in the progressive resistance capacity with the displacement. The load capacity in the joint improves by 7% and 11% for fibre orientations at 45 degrees and 90 degrees, if no fixing is established at the ends of the joint. These values reach improvements of 13% and 28% when there is fixing at the ends of the dowel.