## 4 Test Results and Analysis
### Confinement Effect by Spiral Rebar Design (A12H-1 and 3-7)
#### 4.2.2 Ultimate Bearing Strength  
Figure 9 shows the load-displacement curves and ultimate bearing strengths according to spiral rebar design. For A12H-1 and -4, the effect of confining pressure by rebar size was investigated. For A12H-3 and -5-7, the effect of spiral diameter was additionally investigated. The net maximum bearing stress (\(P_{u}/A_{b}\)) on the part of the concrete in direct contact with the bearing plate is presented in Figure 5c. The maximum stress of A12H-3 was computed as 84.1 MPa; it increased to 96.8 MPa in A12H-6. This implies that confinement of the larger concrete area made the concrete more effective and resulted in an increase in  
Figure 9: Test results of group 2 specimens: (**a**) load-displacement curve; (**b**) ultimate bearing strength. the net maximum bearing stress under the ultimate state. The maximum stress of 96.8 MPa in A12H-6 was high enough to be achieved by confinement. Therefore, even if the bearing plate had not been fractured in A12H-6, the net maximum bearing stress may not have exceeded approximately 100 MPa. In the comparison between A12H-4 and -1, the ultimate strength increased by 25% when the \(\varnothing\)10 rebars were substituted with \(\varnothing\)16 rebars. Substitution by \(\varnothing\)16 rebars increased the spiral-induced lateral pressure (\(f_{lat,sp}\)) and the total equivalent lateral pressure by both the spiral and stirrup (\(f_{lat,eq}\)) by 89% and 19%, respectively, as shown in Table 3. Considering that the contribution of lateral confinement to bearing strength is generally less than 50% of the ultimate bearing strength, the additional contribution due to the use of \(\varnothing\)16 rebar should be less than 39.5% or 9.5% of the ultimate bearing strength. Therefore, the observed 25% increase in strength indicates that the effect of lateral confinement should be determined considering not only the spiral but also the stirrup for the specimens with minimum anchorage spacing. Between A12H-5 and -3, the increase in strength was 32%, but between A12H-7 and -6, the strength increased by about 46%. Even though the lateral pressure was much greater in A12H-5 and -3 than A12H-7 and -6, as shown in Table 3, the larger confined area in A12H-7 and -6 produced more bearing capacity and more of an increase in strength between A12H-7 and -6. The increase in strength between A12H-5 (\(D=295,\ \varnothing 10\)) and -7 (\(D=745,\ \varnothing 10\)) was 5%, but that between A12H-3 (\(D=295,\ \varnothing 16\)) and -6 (\(D=745,\ \varnothing 16\)) was approximately 16% (= 153%/132%). Thus, the effect of lateral confinement is determined by the combined effect of both lateral pressure and confined area (or spiral diameter). For the sectional efficiency, it seemed that the maximum efficiency for \(A/A_{g}=9.52\) was limited to around 30% regardless of the lateral rebar design, as shown in Figure 5b. The limitation may be because fracture of the bearing plate occurred before fracturing of the concrete and, for the concrete supporting the unfractured part of the bearing plate, the stress dispersion ratio is limited and fracture of all of the specimens with relatively large \(A/A_{g}\) were governed by the local failure of the concrete beneath the anchorage bearing plate. Considering the sectional efficiency of \(A/A_{g}=4.5\) and the net maximum bearing pressure of A12H-6 close to 100 MPa, the maximum efficiency of the anchorage zone with \(A/A_{g}=9.52\) might be expected to not exceed 35%. Considering the results of all specimens with \(A/A_{g}=1.82\), the efficiency of such specimens ranges from 80% to 110%, depending on the lateral reinforcement design. Therefore, the maximum value of \(A/A_{g}\) that produces 100% sectional efficiency is about 2.0 for the anchorage bearing plate used in the test.