###### Abstract  
Self-tapping screws (STSs) can be efficiently used in various fastening solutions for timber constructions and are notoriously able to offer high stiffness and load-carrying capacity, compared to other timber-to-timber composite (TTC) joint typologies. The geometrical and mechanical characterization of TTC joints, however, is often hard and uncertain, due to a combination of various influencing parameters and mechanical aspects. Among others, the effects of friction phenomena between the system components and their reciprocal interaction under the imposed design loads can remarkably influence the final estimates on structural capacity, in the same way of possible variations in the boundary conditions. The use of Finite Element (FE) numerical models is well-known to represent a robust tool and a valid alternative to costly and time consuming experiments and allows one to further explore the selected load-bearing components at a more refined level. Based on previous research efforts, this paper presents an extended FE investigation based on full three-dimensional (3D) brick models and surface-based cohesive zone modelling (CZM) techniques. The attention is focused on the mechanical characterization of small-scale TTC specimens with inclined STSs having variable configurations, under a standard push-out (PO) setup. Based on experimental data and analytical models of literature, an extended parametric investigation is presented and correlation formulae are proposed for the analysis of maximum resistance and stiffness variations. The attention is then focused on the load-bearing role of the steel screws, as an active component of TTC joints, based on the analysis of sustained resultant force contributions. The sensitivity of PO numerical estimates to few key input parameters of technical interest, including boundaries, friction and basic damage parameters, is thus discussed in the paper. timber-to-timber composite (TCC) joints; push-out (PO) test setup; inclined self-tapping screws (STSs); finite-element (FE) method; cohesive zone modelling (CZM) method; boundaries; friction; sensitivity study +
Footnote †: journal: Appliedmechanism of a given TTC joint, including the bending capacity of screws, the embedment strength of wood, the withdrawal capacity of fasteners, the amount of friction phenomena between the involved components. Appropriate assessment methods and tools are thus required for their accurate mechanical characterization (Figure 1). In the last years, analytical formulations have been proposed for the prediction of the expected stiffness and load-carrying capacity of TTC screwed connections [1; 2; 3; 4; 5; 6; 7]. However, it is generally recognized that both the joint features and the loading conditions can strongly affect the overall mechanical performance. This is in contrast with most of the design applications, that are commonly developed on simplified estimates of serviceability (or ultimate) stiffness and ultimate resistance values, and generally use a constant stiffness value for joints with variable spacing. In this regard, the more refined analytical methods of literature (i.e., [6; 7]) are still partially capable to capture the actual performance of inclined STSs configurations. In the years, several research studies have been thus focused on more refined but cost/time consuming experimental investigations for the assessment of TTC joints, aiming at overcoming the actual gaps of design knowledge [8; 9; 10; 11; 12], and including also several timber-concrete composite (TCC) solutions [13; 14], or novel hybrid techniques for TTC beams with inclined STSs [15; 16; 17]. This paper presents an extended Finite Element (FE) numerical study (ABAQUS/Explicit [18; 19]) that takes into account a wide set of configurations for TTC joints with inclined STSs, under a standard push-out (PO) setup.