## 3 Results and Discussion
### Coupling Element for CLT Wall-ceiling Connections  
_Double Dovetail_  
In principle, the failure modes shown in Figure 25 are possible:  
* Mode 1: Failure of the cover lamella of the CLT wall element (transverse tension, or transverse compression). * Mode 2: Failure of the LVL system connector (combined shear, bending tension failure)
* Mode 3: Failure of the cover lamella of the CLT ceiling element (transverse tension with rolling shear). In the present studies of the variants of the wall-ceiling connection, the failure is due to the nothing of the cover lamella from the CLT (mode 3) or shearing of the system connector (mode 2). Calculation models for notched CLT-elements are described in [PERSON] et al. (2018); [PERSON] (2018) and [PERSON] et al. (2019). Values of tests conducted with the \"double dovetail tenon\" on CLT 5 s 160 mm elements, indicate much higher load-bearing capacities of the rounded dovetail joint geometry as for the fully notched CLT-elements with width \(b_{\text{z}}\) and height \(h_{\text{z}}\). Test values for fully notched bar-shaped CLT 5 s 160 mm cross sections can be found in [PERSON] (2018). The load bearing capacities of mode 3 of the system connector can therefore be best compared with fully notched elements with comparable heights (\(h_{\text{z}}\)-r). The reasoning is that the load transmission starts from the beginning of the rounding of the rounded dovetail joint down to the tenon base contrary to notched elements, as all the load transmission has been transferred to the same level support. Shear strength values for edgewise and flatwise veneer arrangements of Kerto\({}^{\copyright\) LVL-Q (see ([PERSON], 2018))  
Figure 24: (**a**) Load-bearing components of the steps (graphs) (**b**) Tabular listing. Figure 25: Failure modes of the system connection. (**a**) Cover lamella wall side (**b**) System connector (**c**) Cover lamella ceiling side. can be used for the calculation of mode 2 load bearing capacities. The reason being that nearly the same strength value was reached in shear tests of the connector with flatwise and edgewise arranged dowels. Failure on the wall side (mode 1) is not decisive for any of the variants due to the low support torsion that occurs. The eccentricity limit center is also maintained by means of a stiff bearing with regard to torsion of the wall element. In the course of optimization measures, which have the objective of delaying the failure on the ceiling side, a significant load increase is achieved for V2 and V3 compared to V1. The connection is optimized in such a way that the failure of the cover lamella of the CLT ceiling element is delayed in V2 by stepping the tenon base on the ceiling side, or it even occurs on the side of the system connector in V3. V2 and V3 fail at the same load level, which corresponds to an optimization of these two failure modes. The variants investigated are characterized by a ductile failure of the connection, whereby variant V2 and V3 do not fail up to a 10 mm displacement (see Figure 26). In variant V2, this is due to a deeper force transmission introduced over several levels, which leads to a delay in failure of the CLT on the ceiling side as compared to variant V1. In variant V3, the connector fails on shear, whereby the doweling by means of 12 \(\phi\)10 grooved beech dowels ensures the necessary ductility with a flatwise arrangement of the veneers. Damage patterns of Variant V2 and Variant V3 are shown in Figure 27.  
The V2 and V3 variants are optimized geometries with the same load-bearing capacity level at different failure modes for the present combination of the same-height connection of a CLT 160 5 s ceiling element to a CLT 100 5 s wall element by means of a pure wood-wood connection, which is ensured by system connectors of the wood-based material Kerto(r)Q. In the tests, variants V2 and V3 show a load-bearing capacity that is 50% higher than that of variant V1. Their initial stiffness values \(K_{\text{ser}}\) are also 4% and 15% higher than those of variant V1.