## 2 Materials and Methods
### Beam and, Column Specimens  
Table 1 presents the details of the beam specimens and column specimens subjected to concentric and eccentric loads using GFRP tubes. BE 1-1, BE 1-2, and BE 1-3 beam specimens are made of the three types of the GFRP tubes, T-1, T-2, and T-3, as their specifications are shown in Table 2. The geometry of the three full-scale test beams was identical. The span of the test beams was 4.5 m and the distance between the applied loads was 1.2 m. Figure 2 shows the arrangement of the installed instrumentation such as for the full-scale piles reinforced with the GFRP composite tubes. Constant moment region instrumentation such as hoop and longitudinal strain gauges were used to measure the circumferential (hoop) and longitudinal displacements of these three beam specimens. A hydraulic actuator was used to measure the compressive strength of the specimens at a fixed deflection rate of 1.5 mm/min. The displacement of the beam at the midspan and the amount of the load application is recorded during the tests. The ultimate bending capacity for each beam is recorded. The results are shown as moment load in failure in the Table 1. The goal here was to measure responses of the geometrically identical beams reinforced with three different types of the GFRP tubes, as is shown in the Table 2. The load-displacement behavior, load-deflection response, of the beam specimens at midspan is illustrated in Figure 3. The general responses of the beams are curvilinear before reaching the failing points. Moreover, in all three beam specimens, the ultimate load capacity was considerably greater than the calculated load for cracking the samples. Some minor data fluctuations occurred; however, they were insignificant due to the sample's calibration and initial measuring conditions [24]. Table 1 also provides specifications of the columns under a concentric compression load, which are mentioned as AX-1-1, AX-1-2 and AX-1-3. All these three full-scale column specimens have comparable geometry specifications. The height of these columns was almost 1 m. Ultimate axial and bending capacities of these full-scale column specimens under concentric loads are reported in Table 1. The average accumulated axial load-strain response of the columns is presented in Figure 4. The results were based on the mean value of three installed strain gauges to control the displacements in these columns. Figure 4 also desalinated the comparison of the results with the idealistic axial-strain behavior following the lamination theory. Linear stress-strain behavior of the GFRP piles is simulated following a linear regression based on the results of this experimental study. The distribution of the forces in the GFRP tube along with concrete material was measured using numerical integration. The internal moment capacity of the pile was predicted at various strain levels using these calculated forces. The verification of the material is explained in the following section. Figure 2: Instrumentation and beam test setup. Furthermore, the specifications of three full-scale columns under the combination of axial compression load and bending moments are presented in Table 1 as BE-AX 1,2,3,4,5. Three types of GFRP reinforce tubes are implemented in these columns. For instance, BE-AX 2-1 is the second column under eccentric loads and reinforced with T-1 type of the GFRP tubes. The eccentricity of the applied loads and height of the test specimens are shown in Table 1. Rigid steel plates are installed at both ends of the column specimens and utilized to provide the test columns with five variations of eccentric loads during the test. The axial and moment load were applied on the bottom and top of the column  
Figure 4: Comparison of axial stress–strain based on laboratory data, the lamination theory, and an ideal hypothetical line. The GFRP material properties are as shown in Table 2, compressive strength of the concrete in the pile: 30 MPa. Figure 3: Axial compression load-deflection response of the test beam specimens. specimens as coupled forces.