## 2 Experimental  
### Test Specim and Preparation  
Fourteen RC bridge box piers (B1-B14) at a 1:14 scale representation of the prototype piers were prepared with four heights (\(H\)), four slenderness ratios (\(\lambda\)), three axial load ratios (\(n\)), and three transverse reinforcement ratios or stirrup ratios (\(\rho_{V}\)), as shown in Table 1. Note the axial load was applied on top of the piers to represent their superstructure mass. Figure 1 illustrates the B1 sample as a representation. The constant cross-section of the hollow structure model was 440 \(\times\) 320 mm (Figure 1b), and the wall thickness in \(X\) and \(Y\) directions were 90 and 70 mm, respectively. The configuration of reinforcement details was the same for all specimens, except that the spacing between the transverse bars was different to produce samples with different transverse reinforcement ratios. For the specimens B1-B3, and B6-B14, the transverse bars were spaced with intervals of 50 mm in the 350 mm height zone from the bottom of the pier and at intervals of 100 mm in other zones. For the specimens B4 and B5,  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Specimen No. ** & **Note** & **\(H\) (m)** & **\(n\) (\%)** & \(\lambda\) (\%)** & \(\rho\
u\) (\%) & \(f_{\alpha\alpha}\) (MPa) & \(f_{\alpha\alpha}\) (MPa) & **\(f_{\alpha\alpha}\) (MPa)** & **\(Ec\) (MPa)** \\ \hline B1 & Different axial load & 4.8 & 0.20 & 13.1 & 1.06 & 44.60 & 3.23 & \(3.08\times 10^{4}\) \\ B2 & ratios with H = 4.8 m & 4.8 & 0.10 & 13.1 & 1.06 & 44.60 & 3.23 & \(3.08\times 10^{4}\) \\ B3 & & 4.8 & 0.05 & 13.1 & 1.06 & 49.15 & 3.36 & \(3.14\times 10^{4}\) \\ \hline B4 & Different stirrup ratios & 4.8 & 0.10 & 13.1 & 0.73 & 49.15 & 3.36 & \(3.14\times 10^{4}\) \\ B5 & with H = 4.8 m & 4.8 & 0.10 & 13.1 & 0.56 & 49.15 & 3.36 & \(3.14\times 10^{4}\) \\ \hline B6 & & 5.8 & 0.20 & 16.3 & 1.06 & 49.15 & 3.36 & \(3.14\times 10^{4}\) \\ B7 & Different axial load & 5.8 & 0.10 & 16.3 & 1.06 & 49.15 & 3.36 & \(3.14\times 10^{4}\) \\ B8 & ratios with H = 5.8 m & 5.8 & 0.05 & 16.3 & 1.06 & 49.15 & 3.36 & \(3.14\times 10^{4}\) \\ \hline B9 & & 3.8 & 0.20 & 10.0 & 1.06 & 54.47 & 3.74 & \(3.15\times 10^{4}\) \\ B10 & & 3.8 & 0.10 & 10.0 & 1.06 & 54.47 & 3.74 & \(3.15\times 10^{4}\) \\ B11 & & 3.8 & 0.05 & 10.0 & 1.06 & 49.15 & 3.36 & \(3.14\times 10^{4}\) \\ \hline B12 & & 2.8 & 0.20 & 6.9 & 1.06 & 54.47 & 3.74 & \(3.15\times 10^{4}\) \\ B13 & & 2 the interval spacing in the bottom area were 70 and 100 mm, respectively. Sufficient longitudinal steels are included in pier specimens to present the mid-height plastic response, and sufficient confinements were used to improve the pier ductility and prevent shear failure, as reported by [PERSON] et al. [4]. The top section of a pier was cast to be a solid section with a dimension of 700 \(\times\) 720 \(\times\) 540 mm, as shown in Figure 1a. The load from the load actuator was then applied on this pier top. An RC footing with a dimension of 250 \(\times\) 800 \(\times\) 1300 mm was also cast to strictly fix the bottom of the pier. The RC footing was bolted to the ground through four high-strength screws. The piers were reinforced with 28\(\Phi\)8 longitudinal bars and \(\Phi\)6 transverse reinforcement bars. The measured yield strength of \(\Phi\)6 and \(\Phi\)8 rebars were 375.3 MPa and 326.5 MPa, respectively; the ultimate strengths were 503.8 MPa and 456.6 MPa, respectively; the elastic modulus values were 2.11 \(\times\) 105 MPa and 2.12 \(\times\) 105 MPa, respectively. The normal-strength concrete with fine aggregates and ordinary Portland cement was cast to reach the design strength of C50 (50 MPa at 28 day-curing age). The water-cement ratio was 0.36. Six replicate cube concrete samples with a dimension of 150 \(\times\) 150 \(\times\) 150 mm were cast to measure the compressive strength and cleavage strength; six replicate concrete samples with a dimension of 150 \(\times\) 150 \(\times\) 300 mm were cast to measure the elastic modulus (Ec).