## 2 Experimental Programs
### Rotational Deformation at the Base of Columns  
As shown in Figure 11, a steel rod was installed at the height of 200 mm from the pier base, penetrating the pier cross-section in the loading direction, and displacement sensors were installed at both ends of the steel rod to measure vertical displacement. Displacement gauges were installed at both ends of a steel rod penetrating the cross-section of the bridge pier to measure the vertical displacement. From these measurements, the rotational angle at the base of the RC column was calculated. Furthermore, based on the calculated rotation angle, the displacement induced by the load position can be determined in relation to the rotational deformation at the column base as follows:  
\[u_{fd}=\theta\cdot h_{ef} \tag{2}\]  
Figure 11: Measurement of Rotational Deformation. Figure 10: Damage to Axial Reinforcements at 100 mm & 200 mm of Columns. (**a**) LS History on South Side (100 mm). (**b**) LS History on North Side (100 mm). (**c**) LS History on South Side (200 mm). (**d**) LS History on South Side (200 mm). Here, \(h_{ef}\) represents the effective height of the bridge pier. The rotational angle \(\theta\) at the lowest level of the bridge pier is used to approximate the rotational angle induced in the pier body due to the longitudinal reinforcement elongation caused by bond failure inside the footing. Therefore, it is believed that the horizontal displacement \(u_{fd}\) in Equation (2) can approximate the horizontal displacement caused by the bridge pier rotation to some extent. However, it should be noted that the influence of the bending deformation and shear misalignment deformation occurring in the range below the displacement gauge is included in Equation (2). As a result, the actual rotation displacement \(u_{fd}\) at the loading point caused by the deformation of the longitudinal reinforcement elongation inside the footing, may be overestimated. Figure 12 shows the rotational deformation history of both specimens. The horizontal axis represents the loading Drift, and the vertical axis is the rotational angle at the base of the column. The rotation deformation history of Case-1 is approximately linear, and when the rotation deformation reaches its maximum value, with approximately 60% of the total deformation \(u_{total}\) being caused by the rotational deformation \(u_{fd}\). As mentioned earlier since the bending deformation and shear deformation occurring below the displacement meter are included, the actual rotation deformation is smaller than 60%. In contrast, the red triangular markers representing Case-2 exhibit the rotational deformation at the column base for each Drift level. In the initial stage of the experiment (before 2.0% Drift), Case-2 and Case-1 were same, with the rotational deformation at the base increasing linearly with the load. However, from 2.5% Drift onwards, the positive rotational deformation in Case-2 experienced a drastic increase (approximately doubling), while the negative direction saw a slight decline from \(-\)2.5% Drift due to the abrupt increase in the positive rotational deformation. Overall, the decrease was less than 20%. Simultaneously, linear growth was restored in the positive direction from 2.5% Drift to 4.0% Drift. At 4.0% Drift, while the negative rotational deformation in Case-2 remained consistent with Case-1, the positive rotational deformation reached 2.5 times that of Case-1. Figure 13 illustrates the bond failure mechanism of Case-2 and its subsequent impact. In the initial stage of the load test, the south side reinforcement experienced bond failure due to concrete cracking, preventing stress from being effectively transferred from the concrete to the reinforcement. This is the reason why the south-side reinforcement remained in an elastic state. Simultaneously, because of the bond failure of the south side reinforcement, the stress was concentrated on the north side reinforcement, leading to stress concentration and significant plastic deformation.