## 4 Results
### Experimental Analysis
#### 4.1.1 TGIB-fw  
Figure 19 shows the force versus absolute value of average vertical beam displacement for TGIB with annealed float glass. A multistage failure mechanism is observed here. In stage I, the relationship between force and displacement is almost perfectly linear elastic until the first crack in glass occurs. Thereafter, there is a sudden drop in bending stiffness and an increase in vertical displacement. After the first crack in the glass, which occurs directly under the load point, the bottom edge of the timber flanges acts as a bridge between the cracks, which, together with the uncracked zone at the middle and top edges, ensures that the beam can continue to carry the load. In the next phase, the existing crack is enlarged and a new crack forms below the second load point. As the process continues, more and more cracks form, usually between the two load points. The tensile forces are thus transferred from the glass web to the timber flange. This distribution of forces prevents brittle collapse and ensures the ductility of the whole system. This is called postcritical strength [21], which thus defines the increased value of the load from the first crack in the glass to the final failure. The results for annealed float glass from all three test groups, summarised in Figure 20, show that the failure pattern is the same for all three adhesives, with a multistage failure mechanism occurring in all test specimens. The bending stiffness of the TGIB-fw is constant  
Figure 19: Force versus vertical beam displacement for TGIB with annealed float glass. until the first crack occurs. Thus, the theory described above that the beam continues to carry the load until final failure despite cracks in the glass is confirmed here by a practical example. The first crack implies a reduction in bending stiffness, an increase in vertical displacements and a transfer of tensile stresses through the adhesive to the lower wooden flange, which contributes to an increased ductility of the beam. The crack formed continues to grow and a new crack form on the other side where the force was applied. Later, more cracks form along the entire length of the beam. Specimens with stiffer adhesives (acrylate and epoxy) have higher bending stiffness. The first cracks appear earlier in silicone specimens than in acrylate or epoxy specimens, which is due to the stiffness of the adhesives, silicone being the most elastic in this case. The graph shows that for epoxy the first cracks occur at a force of about 10 kN, for acrylate it is about 6 kN and for silicone these forces are lower, about 4 kN. The exact results can be found below in Table 3. The decrease in stiffness is also most noticeable and visible with the most elastic adhesive--silicone. Table 7 shows the numerical values recorded during the test. For silicone, the first crack (Fcr) occurs on average at a force of 4.7 kN, while for acrylate, a 1.2 kN higher average force value was recorded at the first crack compared to silicone. For epoxy, this force was measured at 10.6 kN. The average maximum force for the acrylate specimens was 17.6 kN, followed by silicone at 16.4 kN. The lowest values were recorded for the epoxy adhesive specimens at 12.1 kN. The highest deviation was recorded for the silicone bonded test specimens. For epoxy adhesives, these displacements were on average three times smaller. From the obtained results, it can be concluded that Fcr strongly depends on the stiffness and on the thickness of the adhesive and the displacements also depend on the stiffness of the adhesives. The results of load increases from first crack to failure also show similar results. Silicone generally provides higher ductility than epoxy or acrylate. The results in Table 7 show that the maximum displacements (u\({}_{\text{max}}\)) are expected for the most elastic adhesive, i.e., silicone, with an average value of 61.1 mm, followed by the acrylate specimens with an average value of 39.5 mm and finally the specimens with the most rigid adhesive epoxy, with an average value of 23.2 mm.