## 5 Mechanical Characterization of TTC Joints  
### Experimental Assessment of Maximum Force Predictions (\(F_{max}\))  
In Figure 10, some comparisons are proposed in terms of maximum resistance values for the S#1-to-S#4 TTC joints, grouped by series of specimens, with the support of experimental data from [6]. As a general outcome of the overall parametric numerical simulations, the FE models generally gave evidence of a mostly stable variation of the estimated \(F_{max}\) values with. Despite such a stable numerical dependency of \(F_{max}\) estimations on \(\alpha\), however, in some cases the scatter between numerical and past experimental predictions was found to be in the order of \(\pm 30\%\). The numerical results were in fact found to either underestimate or overestimate the corresponding experiments, depending on the number and inclination of STSs. For the majority of the examined TTC joints, the FE results proved to be non-conservative especially for the specimens under shear-tensile loads (\(0<\alpha\leq 45^{\circ}\)).  
A possible motivation of such a kind of comparative outcomes could lie in localized numerical issues (i.e., numerical singularities, local damage phenomena), and this is especially the case of TTC joints with STSs characterized by high inclination values. In any case, given also the lack of a detailed experimental characterization for the mechanical properties of the material in use, the FE modelling approach herein discussed proves to offer reasonable estimations for the expected maximum force of TTC joints with inclined STSs, and thus to represent a valid support for design.  
Worth of interest in Figure 11, in this regard, is the general trend of the calculated percentage scatter for the so-derived maximum force values \(F_{max}\). In the figure, the scatter \(F_{max}\) is calculated as:  
\[\Delta=100.\frac{x_{FE}-x}{x} \tag{5}\]  
where \(x_{FE}\) denotes the numerical force peak for each \(FE\) analysis and \(x\) the corresponding experimental average value (for each test series), as derived from [6].  
Figure 9: Typical damage propagation in the selected TTC joints with inclined STSs, under a standard PO setup (ABAQUS/Explicit). Example for the S#1 specimen with \(\alpha=-15^{\circ}\), with evidence of: (**a**) local damage of timber (stress values in Pa), (**b**) yielding of screws (stress values in Pa) and (**c**) CZM damage parameter. Reproduced from [21] with permission from Elsevier®, Copyright license number 4895820420991, August 2020.  
It is thus possible to notice that as far as the number and arrangement of the STSs in use modifies, the calculated \(F_{max}\) is mostly regular, for all the examined series of TTC specimens. This can be also perceived by the linear fitting curve that is proposed in Figure 11, as a function of the screw inclination \(\alpha\).  
Figure 11: Percentage scatter of maximum force values for TTC joints with inclined STSs (Equation (5)), as obtained from the FE numerical analyses (ABAQUS/Explicit) and by the experiments in [6].  
Figure 10: Comparison of numerical (ABAQUS/Explicit) and experimental [6] maximum force estimates for TTC joints with inclined STSs: (**a**) 5#1, (**b**) 5#2, (**c**) 5#3 and (**d**) 5#4 joints.