## 2 Materials and Methods
### Laboratory Experiment  
A simplified method for designing timber-concrete composite roof and floor structures with posi-joisted beams underwent validation through a laboratory experiment [34]. To conduct this experiment, two groups of specimens were prepared, each comprising four specimens. The specimens in the first group consisted of two posi-joisted beams, each with a length of 1390 mm, joined by an oriented strand board (OSB/3) measuring 400 mm in width and 18 mm in thickness. The depth of the beams' cross-sections was set at 253 mm, aligning with beams of grade PS10 [30]. The top and bottom chords of the beams had equal cross-sections made from solid timber, measuring 95 mm \(\times\) 65 mm. Additionally, three bracing members with cross-sections of 100 mm \(\times\) 45 mm connected to the bottom chords of the beams. The second group of specimens differed in terms of the addition of a 50 mm concrete layer, comprising self-levelling mass SakretBAM produced by SAKRET (Riga, Latvia). This layer was bonded with OSB/3 sheets using epoxy glue SicaDur 31 produced by Sika (Baar, Switzerland) and granite chips with a fraction size of 16-32 mm were incorporated. The dimensions of both types of the laboratory specimens are summarized in Figure 2. The production process of the specimens in the second group is illustrated in Figure 4.  
The concrete underwent hardening over the course of one month from the placement moment. To prevent the development of shrinkage cracks, the surface of the concrete was covered with polyethylene foil. Subsequently, the moulds were removed, and both groups of specimens were subjected to testing until collapse under laboratory conditions. Before the laboratory experiment, the load-carrying capacities and expected maximum vertical displacements for both groups of specimens were determined using the previously described simplified method for designing timber-concrete composite roof and floor structures with posi-joisted beams, specifically for the case of three-point bending. The load-carrying capacities were found to be 24 kN for the first group and 46 kN for the second group. The experimental setup is illustrated in Figure 5 [34]. Each specimen underwent three loading cycles until the load-carrying capacity of specimens without a concrete layer was reached, followed by loading until collapse. At each stage, the load increment was set at 3 kN, and the loading velocity did not exceed 2 kN/min. The results obtained for the first group of specimens are depicted in Figure 6.  
Figure 4: The process of the second groups specimens’ development: (**a**) joining of the posi-joisted beams with the bracing members; (**b**) joining of the posi-joisted beams with the OSB/3 sheet and steel screws; (**c**) placement of the self-levelling mass, Sakret-BAM, on the OSB/3 surface, covered by the epoxy glue Sicadur 31 and granite chips. Figure 5: Scheme of laboratory experiment: (**a**) scheme of the specimen loading; (**b**) general view of the laboratory equipment during the testing of the specimen from the second group (with concrete). The experimentally obtained load-carrying capacities of the first group of specimens were 32.95 kN for the S1-1 specimen, 34.17 kN for specimen S1-2, 35.92 kN for specimen S1-3, and 35.47 kN for specimen S1-4. The mean value of the experimentally obtained load-carrying capacity was 34.63 kN. The maximum vertical displacements, when subjected to a design load of 24 kN, varied between 5.54 mm and 6.07 mm. When the load reached critical values, the maximum vertical displacements ranged from 8.45 mm to 10.63 mm. The results for the second group of specimens are presented in Figure 7. Notably, the fourth specimen in the second group had a defect, and consequently, only three specimens from the second group were subjected to testing [34].