Long-Term Behavior of Timber to Timber Shear Connections made by Inclined Self-Tapping Screws

Self-tapping screws are an interesting solution for timber connections due to their performance, their economic advantages and their ease of use. These connections are particularly useful for the bending reinforcement of existing timber floors with the overlaying of timber or timber-based elements. The composite section so created is able to increase the bending strength and stiffness of the floor and to reduce vibrations under service loads. The screws are preferably used inclined with respect to the shear flow direction as, in this case, the stiffness and the strength of the connection are strongly improved. The shear transfer mechanism is so influenced also by the withdrawal capacity of the fastener and by the friction between the timber elements. The design of this timber-to-timber composite section requires the knowledge of the connection behavior. Several short-term tests have been performed and formulations have been proposed to calculate the load-bearing capacity and the stiffness under shear/compression or shear/tension load. However, very few data are available on the long-term behavior. An experimental campaign to follow the connection behavior during time has been planned and it is now going on. Twenty-four identic specimens were prepared at the same time to be tested in groups of four. Specimens are stored in a dry place, but not in a controlled environment, to simulate the real weathering. Air temperature, humidity and timber moisture content are regularly recorded. The first push-out test (as soon as built, after 3 months, 6 months and 12 months) have been performed and the experimental results are here presented. The main mechanical parameters of the connections as strength, stiffness, and ductility at different ages are compared. The other groups will be tested after 18 and 24 months.