## 3 Results and Discussion
### FEM--Validation
#### 3.1.2 Bar-Shaped Investigations  
Stepped Tenon Base  
Girder combinations with low pre-timber heights \(h_{\text{VH}}\) at the main girder fail by exceeding the transverse tension strength. The main reason is that there is too little transverse tension volume available due to insufficient connection depth \(t_{\text{eff}}\) below the fillet of the main girder. Moreover, the combination of a low transverse tension strength of the wood and load introductions with a low pre-timber height, lead to premature transverse tension failure resulting from defects in the wood. The total dimensions of the pre-timber of the main beam is composed of:  
\[h_{\text{VH}}=(h_{\text{HT}}-h_{\text{NT}})+(h_{\text{NT}}-h_{\text{Z}}) \tag{1}\]  
As already mentioned, the unfavorable case of load distribution occurs with beam combinations of the same height, as frequently applied in practice. Z-9.1-649 [1] does not limit this dimension parameter and allows \(t_{\text{eff}}=100\) mm in any case in the transverse tension check for mode 1 failure (transverse connection--failure of the main girder side). In order to investigate the bearing behavior in such a case, a test series of 120 \(\times\) 160 mm GLT-girder combinations with minimum pre-timber heights and permissible geometry parameters was developed according to [1]. The REF 03 series was thus tested in five tests and on the same test setup as in the previous investigations with respect to mode 1 failure according to [1]. Furthermore, a series is developed following the measure of a \"double dovetail tenon\" set by [PERSON] in [3]. Step-shaping the depth of the tenon forms a three-row shape of the stepped tenon base, starting from the fillet of the tenon. The individual levels of the steps are optimized in terms of their widths and heights so that the uppermost step \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Series** & \multicolumn{2}{c}{\(K_{\text{seer,mean}}\)} & \multicolumn{2}{c}{\(K_{\text{seer,FEM}}\)} \\
**[-]** & **[N/mm]** & **[\%]** & **[N/mm]** & **[\%]** \\ \hline REF 01 & 9246 & 100\% & 10,145 & 100\% \\ ZGR 01 & 8031 & 87\% & 8506 & 84\% \\ FLA 01 & 1419 & 15\% & 1639 & 16\% \\ \hline REF 02 & 12,688 & 100\% & 13,257 & 100\% \\ ZGR 02 & 7905 & 62\% & 8081 & 61\% \\ FLA 02 & 5107 & 40\% & 5223 & 39\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: FE—Comparison of stiffnesses with test series. Figure 17: FEM—Comparison of the load-bearing behavior of reference shapes REF 01 and REF 02.  
has the maximum width and can thus take the most load. The aim here is to distribute the load application over several levels. This results in a more favorable distribution of the transverse tension stresses in the area of the fillet of the mortise of the load-bearing side and a delayed failure of the main girder. By introducing part of the forces at the higher level (uppermost step), a greater connection depth \(t_{\text{eff}}\) is achieved and thus a greater transverse tension volume is attained. As a result, even imperfections in the wood have hardly any influence on the load transfer. Due to the stepping of the tenon base, the connection depth \(t_{\text{eff}}\) of series STZ increases by approximately 50% (see Figure 18). The loads of the mean value curve of series STZ can almost reach those of the reference geometry REF 01 at 15 mm deformation (see Figure 19). Therefore, REF 01 is taken into account in all comparisons. The high spread of the ultimate loads in the REF 03 series with minimum pre-timber height is striking, which is due to the effects of local defects and thus premature failure. The  
Figure 19: Load-deformation mean value curves (thick colored lines, listed in legend) of the related single tests (thin colored lines) for 120 mm \(\times\) 160 mm beam combinations. Figure 18: Damage patterns and cracks for (**a**) REF 03 series (**b**) STZ series. tests of this reference geometry spread with respect to the ultimate loads \(F_{\rm v,k}\) at a covariance COV of 18% (see Table 5).