## 6 Parametric FE Investigation
### Mechanical Interactions and CZM Damage Parameters  
A first insight is dedicated to the effects of mechanical interactions, with a special care of the STSs in use. It was shown in Section 3 the key role of the soft layer with the CZM contact interaction, as well as of reliable material property definitions. In this sense, the \"upper limit condition\" for the parametric study is assumed as a \"tie\" rigid constraint that is used to replace the CZM interaction for the soft layer (Figure 6). In other words, any kind of possible damage propagation in the region of screws (with the exception of possible material degradation in the timber and steel components) is fully disregarded. The \"lower limit condition\", at the same time, is set to coincide with the CZM formulation in Section 3 (with \(\delta_{\text{u}}=4\) mm, Table 2). Among these two conditions, further FE analyses are carried out with the CZM input parameters of Section 3 (Table 2), but progressively increasing the reference failure displacement \(\delta_{\text{u}}\) in the range from 4 mm to a maximum of \(10\times 4=40\) mm. From a practical point of view, such a variation in \(\delta_{\text{u}}\) represents a residual capacity of the soft-layer to provide a certain mechanical interaction between each STS and the surrounding timber. Such an input value was in fact magnified so as to reproduce an ideal bonding condition with a weak mechanical degradation for the soft-layer interface, even under large slip amplitudes (Figure 6e). Selected numerical results are proposed in Figure 16 for two different screw arrangements (S#1 joints), in terms of measured vertical (\(F\)) and horizontal (\(H\)) base reaction forces as a function of the measured slip \(s\). Major variations between the comparative plots are represented by the screw inclination, with \(\alpha\) = \(-15^{\circ}\) and \(+45^{\circ}\). The use of CZM interfaces, as also expected, proved to have a key effect on the collected mechanical responses for the selected STS configurations. This was observed especially towards the \"upper limit  
Figure 16: Analysis of mechanical interaction and CZM effects on the PO numerical response of TTC joints with inclined STSs (S#1). Vertical (\(F\)) and horizontal (\(H\)) reaction forces as a function of slip, for (**a,b**) \(\alpha\)= \(-15^{\circ}\) and (**c,d**) \(\alpha\)= \(45^{\circ}\) (ABAQUS/Explicit). condition\" characterized by the use of rigid \"tie\" constraint. In the latter case, the FE outcomes were in fact typically associated to unreliable local and global effects for the examined TTC joints, with a consequent marked increase of both the calculated serviceability stiffness \(K_{\text{s}er}\) and ultimate resistance \(F_{\text{m}ax}\). Such a combination of phenomena, finally, was also found associated to a remarkable modification of the measured reaction forces (see for example Figure 16b-d). Regarding the CZM interaction and failure, on the other side, major issues were represented by the accurate calibration of input parameters for damage initiation and evolution. While the nominal resistance values of timber (Table 1) can be reasonably taken into account for the CZM damage initiation, the characterization of its damage law would in fact necessarily require dedicated studies at the component level, and possibly the support of small-scale experiments. Under the assumption of Table 2, the modification in \(\delta_{\text{u}}\) was commonly associated to a rather constant elastic response for the examined TTC joints, but to a marked decrease of residual resistance and stiffness for most of the tested configurations. Such an effect can be notice in Figure 16. As far as the critical displacement \(\delta_{\text{u}}\) for the CZM interaction increases, a reduced slope can be observed for the descending arm of the collected force-slip curves. Compared to the available experimental data from [6], a reliable fitting of degreasing arms for the comparative force-slip curves was observed in the range of \(\delta_{\text{u}}\) = 6-7 mm.