## 4 Test Results and Analysis
### Effect of A/A\({}_{g}\) Ratio (A12h-1-5)
#### 4.1.2 Ultimate Bearing Strength  
Figure 7 shows the load-displacement curves and ultimate bearing strengths with respect to \(A/A_{g}\) ratio. A12H-4 and -5 have the same dimensions as A12H-1 and 3, respectively, but they are reinforced with \(\Phi\)10 mm spiral rebars, resulting in reduced lateral confining pressure. A12H-2 and -3 have identical spiral rebars and lateral confining pressure as A12H-1, but they have larger supporting concrete surface area or \(A/A_{g}\). The load-displacement curves of specimens with high lateral confinement, A12H-1 to -3, show hardening behavior after the proportional limit. Sudden drops after the proportional limit at around 4500 kN are observed in the curve of A12H-3, which is attributable to fracture of the anchorage bearing plate. However, the hardening of the curve is still evident, meaning that the concrete beneath the bearing plate remained unfractured with the aid of sufficient lateral confinement. It would be reasonable to conclude that a specimen with \(A/A_{g}=9.52\) could have greater strength than the test result if a stronger cast iron is used. Figure 6: Fracture mode of group 1 specimens: (**a**) A12H-1; (**b**) A12H-2; (**c**) A12H-3; (**d**) A12H-4; (**e**) A12H-5. A12H-4 and -5 have identical spiral rebar designs, but the larger specimen, A12H-5, showed 21% greater strength, and the curves of A12H-4 and -5 in Figure 7a showed similar softening of their slopes after the proportional limit. This reveals that the concrete surrounding the spiral played a role in the increase in the peak load, but the post-peak behaviors of both specimens were governed by the same factors. These factors may include crushing of the concrete under weak confinement because splitting failure and vertical crack formation were not observed in A12H-5, as shown in Figure 6e. In addition, the few narrow cracks on the side surface of A12H-5 may have been caused by the spiral-controlled transfer of splitting stress to the outer concrete. The sudden change but almost linear slope after the proportional limit of A12H-5, as seen in Figure 7a, may be attributable to the yielding of the spiral rebar because the yield of the rebar shows similar behavior. Additionally, the local concrete beneath the bearing plate may have reached peak stress right after the rebar yielded because the softening curve was almost linear. Considering the fracture mode and crack patterns of the two specimens leads to the following conclusions: (1) The peak load of A12H-4 was determined mostly by wedge action, and the post-peak softening curve was due to lateral reinforcement; and (2) The peak load of A12H-5 was determined mostly by yielding of the spiral rebar, and the post-peak softening curve was due to lateral reinforcement. Under identical spiral-induced lateral confinement, an increase in the concrete block area from \(A/A_{g}=1.82\) to \(A/A_{g}=9.52\) increased the ultimate strength, as shown in Figure 7b. However, the ultimate strength increased by only 4% between A12H-1 and A12H-2 even though the \(A/A_{g}\) of A12H-2 is 2.47 times that of A12H-1. This may be because the effective lateral confinement pressures were the same in the two specimens, as shown in Table 2, and the side surface cracking of A12H-2, as shown in Figure 6b, lessens the additional confinement by the surrounding concrete. A12H-3, unlike A12H-1 and 2, exhibited only negligible cracking, so the surrounding concrete should contribute to confinement of the local zone; indeed, the strength of this specimen increased by 28%. \begin{table} \begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{\(f_{lat,sp}\) (MPa)} & \multirow{2}{*}{\(\begin{subarray}{c}\text{Effective}\,f_{lat,sp}\) (\textless{}8.3 MPa) \\ \end{tabular}