PSI - Issue 36

Svyatoslav S. Gomon et al. / Procedia Structural Integrity 36 (2022) 217–222 Gomon S. et al./ Structural Integrity Procedia 00 (2021) 000 – 000

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The GRB-12A and GRB-12B beams were reinforced in the compressed zone with steel reinforcement in the form of two class A500C rods with a diameter of 12 mm and composite tape Sika CarboDur S-512, which was glued to the externally stretched zone according to Patent № 135229 (2019) as shown in Fig. 3. The GRB-12C beam was reinforced in the compressed zone with only two A500C class rods with a diameter of 12 mm. Glued wood beams GB-A and GB-B made of 25 mm boards were used as the reference ones.

Fig. 3. Cross-section of GRB-12A and GRB-12B beams.

Reinforced glued beams GRB-16A and GRB-16B were made similarly to the beams GRB-12A and GRB-12B with the only difference that in the compressed zone 2 rods of A500C with a diameter of 16 mm were used. For layer-by-layer determination of relative deformations of wood along the perimeter of the cross-section of the beam with a certain step and in the middle of the span, in the zone of pure bending, deformation gauges with a base of 20 mm with a resistance of 201 ± 0.7 Ohm were glued. All sensors were attached with BF -2 glue. Deformation gauges were also glued to the metal reinforcement before gluing it into the grooves. They were placed in the middle of the span on mechanically ground rod ribs. On the composite tape, the deformation gauges were glued in the middle of the span after the tape was firmly sticked to the wooden beam. The obtained experimental data were recorded using a deformation gauge measuring system SIIT and recorded on a PC. 3. Results and discussion In the calculation model of the normal cross-section, when determining the strength of solid and glued wood structures according to current standards of different countries (Eurocode 5 1995; DBN B.2.6-161: 2017; SP 64.13330: 2011), a triangular diagram of stresses in compressed and tensed areas of wood under the action of one-time short-term loads is adopted. In such diagrams, the height of the compressed area of wood and the position of the neutral plane (neutral line) are considered constant from the beginning of loading and throughout the time of increasing loads until fracture. The current regulations do not take into account the main factor of wood anisotropy, that the strength of wood in axial compression in almost all species is twice less than in tension. These regulations do not consider also the fact that with the onset of loads on the axial tension and compression of wood with the same layer-by-layer relative deformations, we obtain different compressive and tensile stresses, even to the limit of conditional proportionality under compression. According to the results of experimental tests, the deformation distributions along the cross-sectional height of the GB-A beam in the middle of the span under different loading modes from the beginning of the application of force to the failure (Fig. 4) are constructed. When constructing the deformation diagram of different layers (Fig. 4), the distribution of deformations on one of the sides of the cross-section of the beam is indicated. From Fig. 4 it is seen that at initial loads the neutral line does not coincide with the main axes of the cross-section of the beam. It shifts towards a stronger extended area, increasing, to ensure the balance of internal forces, the least strong compressed zone of the bending element, which ultimately creates the conditions for the formation of folds in the most stressed wood layers, as demonstrated by Gomon et al. (2019).