**1. Introduction**

Timber-to-timber composite joints are widely used in novel or existing buildings, with variable detailing (i.e., type of fasteners, detailing, spacing, arrangement, etc.). Among others, self-tapping screws (STSs) are particularly efficient due to their continuous thread, and their high withdrawal capacity allows one to realize connections with increased stiffness and load-carrying capacity. The benefit of STSs, compared to traditional TTC joints, can be clearly perceived, particularly when the screws are used with an inclined configuration with respect to the timber grain. On the other side, the arrangement of screws, requires the designer to account for several aspects that could directly affect the load transfer mechanism 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).

**Figure 1.** Available tools for the mechanical characterization and analysis of design parameters of timber-to-timber composite joints.

In the last years, analytical formulations have been proposed for the prediction of the expected stiffness and load-carrying capacity of TTC screwed connections [1–7]. 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 [8–12], and including also several timber-concrete composite (TCC) solutions [13,14], or novel hybrid techniques for TTC beams with inclined STSs [15–17].

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. The numerical investigation takes inspiration from past experimental results reported in [6] by Tomasi et al., where various joint prototypes of technical interest have been explored, as well as from [20,21], where an enhanced cohesive zone modelling (CZM) approach has been proposed and validated in support of an enhanced mechanical characterization of the fasteners. The reference modelling strategy is described in Section 3.

The numerically predicted resistance and stiffness parameters are thus briefly compared in Section 4, based on the available experimental and analytical data from [5,6]. As a reference, standard test procedures for timber joints are taken into account from [22,23]. Accordingly, the correlation of collected data is assessed with the derivation of empirical fitting curves. Successively, the load-bearing performance and sensitivity of TTC-PO specimens to some key input parameters is explored (Section 5). The attention is focused on the effects of the base restraint, as well as friction phenomena and timber contact interfaces, and further on some basic CZM damage parameters for failure detection (Section 6). In doing so, major advantage is taken from the numerical derivation of the resultant forces that are sustained separately by the STSs or transferred by the timber components.
