## 3 Results and Discussion
### FEM--Validation
#### 3.1.2 Bar-Shaped Investigations
The minimum initiation height from the beginning of the filled of the mortise to the lower edge of the girder is only capable of ensuring a connection depth \(t_{\rm eff}\) of 50 mm. The widely spreading values are due to local defects, which lead to different transverse tension crack propagation. This results in premature failure below the tenon base of the load-bearing main girder without redistribution possibilities. In contrast, the stepped tenon base geometry of the STZ series exhibits a high dispersion in terms of its stiffness. This is due to the form closure on several levels and the associated tolerances, i.e., until the closing of the multi-stage overall system. However, the load transfer to the multiple levels results in a significant increase in the ultimate load, which has a positive effect on the characteristic calculation value \(F_{\rm v,k}\) in particular (see Figure 20). The initial stiffness values \(K_{\rm ser}\) spread with a covariance (COV) of up to 32% (see Table 5). However, if deformations of up to 15 mm are allowed, the connection force-locks at all step levels. The mean load curve of the STZ series thus approaches that of the reference form REF 01 at 15 mm deformation. By stepping the tenon base, a height distribution of the load application can be achieved, which is also reflected in the fracture patterns. Failure is thus not determined by a defect in the transverse tension volume. Ultimately, all test specimens reach a similar ultimate load level after deforming to a higher or lesser degree. The individual levels of the stepped tenon base communicate with each other, so that in the event of an individual level yielding, the load can be redistributed to other, intact levels. In the reference form REF 03, the entire force is applied via the single \"step\" located far below, with a low connection depth \(t_{\rm eff}\). As a result, only a small transverse tension volume of the main girder is activated in the transverse connection. If the load introduction is distributed over several levels according to the geometry of the STZ series, an overall higher force introduction is achieved. In this way, approximately 75% of the width of the 120 \(\times\) 160 mm main beam \(b_{\rm HT}\) can be activated as the resisting connection depth \(t_{\rm eff}\). The stepped tenon base is designed so that the step with the lowest connection depth also introduces the lowest load component. The two higher steps introduce higher load components with greater effective connection depths. This means that the widest step at the top transfers more than half of the resultant force to the load-bearing main beam. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Series** & **Tests** & **\(\omega\)mean** & \(\rho\)mean** & \(F_{\rm vmean}\) & \(F_{\rm v,k}\) & **COV** & \(K_{\rm ser,mean}\) & \(K_{\rm ser,k}\) & **COV** \\
**[-]** & **[-]** & **[\%]** & **[kg/m\({}^{3}\)]** & **[kN]** & **[kN]** & **[\%]** & **[N/mm]** & **[N/mm]** & **[\%]** \\ \hline REF 01 & 5 & 13.1 & 446 & 22.1 & 16.5 & 12.0 & 9246 & 8884 & 11.8 \\ STZ & 5 & 12.3 & 439 & 18.7 & 13.8 & 12.0 & 7102 & 6338 & 32.4 \\ REF 03 & 5 & 12.7 & 450 & 17.1 & 10.8 & 17.7 & 9562 & 8927 & 20.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Evaluation of the tests—loads and stiffnesses according to EN 14358 [21]. Figure 20: Weighted strengths of Series 03 (graphs). FEM--Validation  
The positive effect of the stepping can also be seen in the course of numerical simulations (see Figure 21). Stiffness values for solid spruce wood C24 according to EN 338 [17] and yield stresses according to Sandhaas and [PERSON] [18] are applied for the bilinear material model with Hill yield criterion and isotropic hardening in the course of the simulations (Table 1). All results of the test runs of the reference geometry REF 03, as well as the stepped tenon base geometry STZ, can thus be validated in numbers (see Figure 22).