## 1 Introduction  
Detailed seismic design standards have been introduced recently in countries in low to moderate seismicity regions. In the case of Korea, detailed standards for seismic design were established after recognizing the earthquake damage from the Northridge earthquake in the US and the Kobe earthquake in Japan in the 1990s. Therefore, structures constructed before that time cannot be considered to have been secured against earthquakes. In the case of bridges, the mass is concentrated in the superstructure, therefore when a lateral load such as an earthquake is applied, the bridge should withstand the seismic force through the bending behavior of piers. In the case of a single-column pier, a plastic hinge is generated at the bottom of the pier where the bending moment is at its maximum. This plastic hinge must have sufficient ductile capacity and energy absorption capacity to be safe against earthquake loading. Reinforcement details in the plastic hinge section have a great influence on this flexural behavior. In order to have sufficient ductility capacity, the core concrete must be sufficiently confined by transverse reinforcing bars, and the main reinforcing bar should be connected continuously without a lap-splice. However, when the detailed seismic design criteria are not applied, the longitudinal reinforcing bars at the plastic hinge region were often connected by a lap-splice due to the convenience of construction. Therefore, the behavioral characteristics of such lap-spliced longitudinal bars has emerged as a very important issue in the seismic performance evaluation of the existing bridges. Most studies about the seismic behavior of bridge piers are about the single-column piers because the number of existing single-column pier bridges is much higher than that of multi-column pier bridges. The multi-column piers may show similar behavior in the longitudinal direction of the superstructure to the single-column piers, but it can be expected that the behavior of the multi-column piers in the transverse direction is different from the single-column piers. Therefore, in this study, one of the aims is to verify the seismic performance of multi-column piers in each direction, as well as the reinforcement details. Since the superstructure of the bridge has a large mass and is located at the same height, it acts as an inertial force concentrated at the top of the pier when an earthquake load is applied. Therefore, the pier behaves as a deformable body that transmits the inertial force of the superstructure. Most of the piers are reinforced concrete, and how much seismic force these piers can withstand in the horizontal direction is a key consideration in seismic design. If the aspect ratio of the pier, that is, the height compared to the cross-sectional dimension, is low, the shear behavior dominates, and if this ratio is large, the bending behavior occurs [1]. In order to avoid such behavior, reinforcement may be necessary to prevent damage, for example, by seismic isolation, in bridges [2]. The pier can withstand the seismic force with its horizontal strength, but when excessive seismic force is applied, sufficient ductile capacity is required. In order to increase the ductility of reinforced concrete piers that behave in flexure, it is necessary to prevent compressive failure of the concrete in the cross section by bending. For this purpose, core concrete is confined by a transverse reinforcement bar. The confinement effect at the plastic hinge region of the pier column during earthquakes increases seismic capacity due to ductile behavior [3; 4]. In the case of installing sufficient transverse reinforcement, considerable flexural ductility is exhibited while maintaining the load resistance capacity even after the plastic hinge occurs [5; 6]. Even if the transverse reinforcement bar ratio is the same, if the spacing is small, the confinement effect increases [7]. Therefore, the horizontal reinforcement ratio is an important variable in seismic detail.