## 4 Discussion of Experimental Observations
### Service Class and Load-Bearing Capacity  
The above quantitative comparisons were further supported by the qualitative analysis of experimental outcomes. In general terms, the analysis of BiR performances under different service conditions can be summarized as follows. For service class 1:  
* test results confirmed the assumption of similar load-bearing capacity for both the epoxy and the polyurethane adhesive types;
* specimens with polyurethane showed mild bilinear behaviour and 10% higher capacity then specimens with epoxy, which proved to offer a pure linear behaviour with brittle fracture. For service class 2:
* specimens showed a \(\approx\)20% drop in the measured average load capacity;
* the load-bearing behaviour and failure modes were found to closely agree with the experimental observations of specimens in service class 1. Finally, for service class 3:
* specimens showed large drop of load-bearing capacity. A huge drop was found especially for epoxy bonded specimens, where failure happened on \(-\)50% of maximum average force of corresponding specimens in service class 1;
* in any case, the failure modes were still observed in agreement with the previous specimens. In this regard, the analysis of test results can take advantage of existing empirical formulations that have been proposed for glued-in-rods with parallel or perpendicular rod-to-grain orientation. Among the literature efforts for the analytical analysis and design of BiR connections, the failure load of parallel rod-to-grain arrangement can be for example estimated as [15]:  
\[F_{ax,0}=\pi\;L\;\big{(}f_{v}d_{equ}+k(d+e)e\big{)} \tag{6}\]  
The empirical equation has been proposed by [PERSON] et al. [15] to predict the pull-out strength at failure (in N), where \(L\) is the joint length (mm); \(f_{v}\) is the shear strength of wood (MPa); \(d\) is the rod diameter (mm); \(d_{equ}\) the smaller between the hole diameter \(d_{h}\) and the rod diameter \(d\) multiplied by 1.25 (mm); \(e\) is the joint thickness (mm); \(k\) is a parameter proposed in 0.086 or 1.213 (based on experimental fitting), for adhesives with brittle or ductile behaviour respectively. For comparative studies with the current experimental results, the above parameters are set:  
\[d_{equ}=min(14;1.25\times 10)=\;12.5\;\text{mm} \tag{7}\]  
\[f_{v,k}=1.2\times 10^{-3}d_{eq}^{-0.2}\rho^{1.5}=10.65\;\text{MPa} \tag{8}\]  
with \(\rho=600\;\text{kg/m}^{3}\) the average density of wood specimens (\(\pm 25\)); \(e=2\;\text{mm}\), \(L=60\;\text{mm}\). From Equation (6), the comparative analysis is carried out towards the average (mean) experimental failure loads earlier discussed, for various configurations of specimens. The findings are summarized in Figure 11, as a function of the analytical vs. experimental failure load, the adhesive type and the moisture/service class. As far as the service class 1 is taken into account, it is possible to see that the analytical to experimental ratio is in the order of the unit. This suggests a certain correlation of literature model with the current experiments. Besides, the analysis of higher moisture content tends to progressively overestimate the analytical failure force for the examined specimens, as shown in Figure 11 for both the adhesive types. Furthermore, the high moisture content reveals also pronounced effects due to the input \(k\) coefficients calibrated from [15]. For specimens with perpendicular rod-to-glue arrangement, the analytical model proposed by [PERSON] et al. is considered [PERSON] et al. (2018). The model, in particular, assumes that the load-bearing capacity is given by:  
\[F_{ax,90,mean}=f_{y,90,mean}\pi\ d_{h}L \tag{9}\]  
with the limit applicability condition of \(L<15d_{h}\). The empirical model of Equation (9) agrees with the experimental trends earlier discussed in Figure 8. As far as the moisture content increases and the adhesive degradation of mechanical properties increases, Equation (9) itself severely overestimates the expected axial force at failure for the tested joints.