## 4 Results
### Laboratory Tests  
Curves showing the engineering stress-strain relationship for samples cut from the GFRP tubes are presented in Figure 11. To determine the stresses of individual samples, the cross-sectional area determined on the basis of the average width and thickness of the sample was taken into account, which is presented in Table 1. When analysing the  
Figure 10: Boundary conditions of the foundation numerical model: (**a**) Constraints: translational TX, TY, and TZ (red marked); rotational RX and RZ (blue marked). (**b**) Loads: G—gravity, FX—point load. stress-strain curves in the initial phase, straight lines parallel to the horizontal axis can be observed. This results from the assumed initial force for the tensile test. The initial force was used to pre-tension the sample to avoid slippage between the sample and the grip. In the range of the initial force load, displacements were not measured. In the tests carried out, an identical initial force was applied to all samples, but due to differences in the geometry of the rings, the strains of rings W1-W5 shown in Figure 11 are not identical. Additionally, in the case of ring W5, it turned out that the assumed initial force was too small, and a small slip occurred. After reaching the initial force, an increase in stress was observed with increasing strain. The strain-stress relationship appears to be a straight line until failure of the samples. This pattern of graphs indicates brittle fracture of the samples. Figure 12 shows photos of the samples after destruction. The destruction of the samples occurred as a result of shearing of the resin strands, which caused the glass fibres to be separated from the composite. A summary of the tensile strength of samples cut from the GFRP tubes is presented in Table 6. The average tensile strength was 489 MPa. The relative standard deviation was equal to 3%, so repeatability of the results was assessed as being high. Sample No 3 reached the tensile strength closest to the average. Therefore, sample No 3 was used as a representative for further analysis. Figure 11: Engineering strain–stress graphs from static tensile tests of samples cut from the tubes. Figure 12: The sample after failure: (**a**) separation of glass fibres from the composite and (**b**) shearing of resin strands. The tensile tests of the samples were carried out to determine [PERSON]'s modulus of the analysed composite material and used to build an elastic numerical model of the material, allowing for a simulation of foundations made of GFRP. Figure 13 shows the graphs of sample No W3. The chart was limited to the straight section for which a trend line was defined. [PERSON]'s modulus was taken as the tangent of the angle between the straight line and the horizontal axis. Based on the equation of the straight line (Figure 13), it was determined that [PERSON]'s modulus of the tested sample is 7020 MPa. Therefore, for numerical simulations of the analysed GFRP composite, [PERSON]'s modulus was assumed to be 7 GPa. Curves showing the equilibrium paths of rings cut from the GFRP tubes are presented in Figure 14. In all rings, an almost uniform increase in force with respect to displacement was observed, up to brittle failure. In specimens R2 and R5, an initial plateau (horizontal line) was observed. This results from the slippage between the sample and the testing machine. Because the slippage was about 2% of elongation, this effect was assessed as being irrelevant. The load-bearing capacities of the rings are shown in Table 7. The load-bearing distribution is presented in Figure 15 in the form of a box plot. The average force transferred by the rings was 1.5 kN, and the standard deviation was only 0.13 kN. Repeatability of the results was also observed in the box plot. No outliers were observed in the graph, and the mean almost coincides with the median. Thanks to this, the repeatability of the results was considered satisfactory.