## 1 Introduction  
Y-shaped irregular bridges are widely used in three-dimensional traffic hubs, separated ramps, and upper and lower approaches to divert traffic and adapt to the terrain and road alignment. As shown in Figure 1, Y-shaped bridges have prominent curves, and they are generally located along major road traffic sections. Their importance is self-evident, but events in the past 50 years have shown that irregular and curved bridges are especially vulnerable to earthquakes [1; 2; 3; 4; 5; 6]. The most destructive case was the San Fernando earthquake in 1971, which caused severe damage to a multi-span beam bridge between two large interchange projects. In response, researchers have focused on evaluating the seismic performance of bridges through various methods. In 1979, [PERSON] and [PERSON] [7] published their experimental results for a shaking-table model of a curved bridge that collapsed during the San Fernando earthquake and the corresponding theoretical results from finite element analysis. [PERSON] and [PERSON] [8] proposed that the stress direction of the structural response is excited by multidirectional seismic inputs, but later studies have proven that this is only applicable to unidirectional input for single-degree-of-freedom structures.  
Experimental research is needed to test and verify numerical results derived from theoretical analysis. Shaking-table tests are a common approach to studying bridge structures. [PERSON] et al. [9] performed shaking-table tests on a \(1/20\) scale model of a three-span continuous girder bridge with longitudinal viscous dampers to investigate the seismic response and damping effect at different seismic intensities. [PERSON] et al. [10] carried out a shaking-table test to study the nonlinear seismic response characteristics of a continuous-beam bridge with laminated rubber bearing (LRB) isolators. [PERSON] et al. [11] conducted shaking-table tests on a 1/12 scale continuous girder bridge for a high-speed railway. [PERSON] et al. [12] designed a 1/62.5 scale model of a three-span curved bridge and performed a multi-angle shaking-table test to analyze the linear response history. [PERSON] et al. [13] used a structural model of a continuous girder bridge with LRB isolators to perform shaking-table tests with multidirectional seismic waves as the input. [PERSON] et al., studied the responses of scaled models of a three-segment bridge [14] and a single segment with abutments [15]. Both of their studies showed that spatial variation in the ground motion significantly increased the relative displacement between adjacent bridge structures. [PERSON] et al. [16] concluded that the maximum seismic response of a curved bridge is closely related to the input angle. They developed a series of finite element models to determine the most unfavorable angles for the seismic input. [PERSON] et al. [17] studied the effects of spatial variations in ground motions on curved bridges by carrying out shaking-table tests on a 1/10 scale curved bridge. [PERSON] et al. [18] carried out a bidirectional shaking-table test of reinforced concrete (RC) columns retrofitted and repaired with shape memory alloy spirals.  
The above studies mostly focused on straight bridges, and only a few considered irregular bridges, especially Y-shaped bridges. In addition, shaking-table tests generally simulate the unidirectional seismic excitation of a regular bridge, which normally comes under less stress than an irregular bridge. The direction of seismic excitation also has a significant effect on the seismic response of a structure [19], but has rarely been considered. In this study, a 1/20 scale model of a Y-shaped irregular bridge, with a prototype designed by an engineer, was constructed, and shaking-table tests were carried out to evaluate the dynamic response of the structure when excited by strong and multidirectional seismic waves. The results of this study are expected to provide an experimental basis for the seismic design and research of Y-shaped irregular bridges.