## 3 Experiments
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \(f_{ci}\) & \(c\) & \(a\) & \(D\) & \multirow{2}{*}{**Spiral Rebar**} & \multirow{2}{*}{**Stirrup Rebar**} \\  & **(MPa)** & **(mm)** & **(mm)** & & & & \\ \hline A12H-1 & 32.6 & 350 & 260 & 1.82 & 295 & \(\Phi\)16 mm–8@50 mm & \(\Phi\)16 mm–8@50 mm \\ A12H-2 & 32.6 & 550 & 260 & 4.50 & 295 & \(\Phi\)16 mm–11@50 mm & \(\Phi\)16 mm–11@50 mm \\ A12H-3 & 32.6 & 800 & 260 & 9.52 & 295 & \(\Phi\)16 mm–16@50 mm & \(\Phi\)16 mm–16@50 mm \\ A12H-4 & 32.6 & 350 & 260 & 1.82 & 295 & \(\Phi\)10 mm–8@50 mm & \(\Phi\)10 mm–8@50 mm \\ A12H-5 & 32.6 & 800 & 260 & 9.52 & 295 & \(\Phi\)10 mm–16@50 mm & \(\Phi\)10 mm–16@50 mm \\ A12H-6 & 32.6 & 800 & 260 & 9.52 & 745 & \(\Phi\)16 mm–16@50 mm & \(\Phi\)16 mm–16@50 mm \\ A12H-7 & 32.6 & 800 & 260 & 9.52 & 745 & \(\Phi\)10 mm–16@50 mm & \(\Phi\)10 mm–16@50 mm \\ A12H-8 & 32.6 & 350 & 260 & 1.82 & 295 & \(\Phi\)16 mm–8@50 mm & \(\Phi\)13 mm–8@50 mm \\ A12H-9 & 32.6 & 350 & 260 & 1.82 & 295 & \(\Phi\)16 mm–8@50 mm & \(\Phi\)10 mm–8@50 mm \\ A12H-10 & 32.6 & 800 & 260 & 9.52 & 295 & \(\Phi\)16 mm–16@50 mm & \(\Phi\)13 mm–16@50 mm \\ A12H-11 & 32.6 & 800 & 260 & 9.52 & 295 & \(\Phi\)16 mm–16@50 mm & \(\Phi\)10 mm–16@50 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Test specimens. Figure 3: Representative drawings of test specimens: (**a**) A12H-1; (**b**) A12H-3. The second group, A12H-1 and A12H-3-7 was created to investigate the confinement effect induced by the spiral and the concrete surrounding the spiral. Two different spiral diameters (\(D\) = 295 and 745 mm) and two different sizes of spiral and stirrup rebar (\(\Phi\)10 and \(\Phi\)16 mm) were chosen under a constant \(A/A_{g}\) of 1.82 or 9.52. Theoretically, as shown in Figure 2, the concrete surrounding the spiral may contribute additional lateral confinement with a pressure of \(f_{lat,conc}=\frac{f_{r}(L-D)}{D}\), where \(f_{r}\)(=0.63\(\sqrt{f_{ci}^{\prime}}\)) is the modulus of rupture of concrete. However, this is only possible in the absence of bursting cracks, so the additional confinement effect of the surrounding concrete should be carefully considered. In the absence of cracking in the 550 and 800 mm specimens with spiral diameters of 295 mm, \(f_{lat,conc}\) was calculated as 3.11 and 6.16 MPa, respectively. The third group, A12H-1 and -3 and A12H-8-11, was made to investigate the additional confinement effect of stirrup rebar, which is mainly intended to resist the bursting force due to wedge action of the anchorage. According to [PERSON] et al. [14], placement of the stirrup such that it resists the bursting force more than necessary could contribute to the additional confinement. However, the maximum confining pressure is limited to 13 MPa. The additional confinement of the stirrup (\(f_{lat,st}\)) can be expressed as shown in Equation (11), and it confines the concrete as shown in Figure 2b. The lateral confinement contribution on the bearing strength by stirrup can be expressed by Equation (16) or Equation (17). When the spacing between the spiral and stirrup is negligibly small, Equation (16) may be used. Otherwise, Equation (17) may be used. \[f_{lat,st}=\frac{2A_{s}f_{y,st}^{*}}{L\cdot s} \tag{11}\]  
\[f_{y,st}^{*}=f_{y}-f_{st,bursting} \tag{12}\]  
\[f_{st,bursting}=\frac{T}{A_{st}\eta} \tag{13}\]  
\[T=kP\Big{(}1-\frac{a}{b}\Big{)} \tag{14}\]  
\[f_{lat,eq}=f_{lat,sp}+\beta f_{lat,st} \tag{15}\]  
\[P_{s}=4.1f_{lat,eq}A_{core,sp} \tag{16}\]  
Figure 4: Test setup and loading procedure: (**a**) specimen under loading by UTM; (**b**) load-transfer test loading procedure. where \(f_{y,st}^{*}\) is the remaining strength of the stirrup up to the yield strength \(f_{st,bursting}\) is the stress acting on the stirrup rebar by the bursting force, \(T\). \(P\) is the tendon force. To compare the test results and the estimations, the maximum load in the load-transfer test is used. a and b are the widths of the bearing plate and concrete block, respectively.