## 1 Introduction
Thus, this approach has been adopted worldwide and is widely used to assess the seismic performance of engineering structures and for after-earthquake management, especially in the case of important and expensive structures [14; 15]. Bridges are key elements that play significant roles in the modern transportation network [16; 17]. In the event that bridges are damaged during earthquakes, this can cause substantial economic losses and delay the speed of disaster relief and reconstruction of the hazard area [18]. Studies into earthquake-induced damage have revealed that bridges are one of the components that are most susceptible to damage in the transportation network [17]. Therefore, more and more researchers all over the world are focusing on and employing the seismic fragility analysis approach to assess the seismic risk to and the damage probability of bridges. For example, [PERSON] and [PERSON] [19] used the seismic fragility analysis approach to analyze the seismic performance of a typical bridge structure in Japan and obtained the fragility curves for the bridge; [PERSON] et al. [20] developed seismic fragility curves for a retrofitted multicolumn bridge in Salt Lake City and used them to investigate the effectiveness of different retrofitting methods; [PERSON] et al. [21] generated the fragility curves of highway bridges in Tehran, Iran, which can be used to identify vulnerable bridges throughout the Tehran city highways; [PERSON] et al. [22] chose a typical cable-stayed bridge in China as their case study and generated fragility curves to evaluate the damage characteristics of cable-stayed bridges. Elsewhere, the seismic fragility analysis approach has also been applied to high-pier bridges. Using the probability fragility analysis approach, [PERSON] [23] investigated the seismic behavior of a high-pier bridge undergoing near-fault ground motion. [PERSON] et al. [24] used the seismic fragility analysis approach to research the damage probability of a long-span continuous rigid frame bridge with a pier of 154 m in height. [PERSON] et al. [25] proposed a novel seismic fragility analysis approach, which they used to perform a fragility estimation of deep-water high piers located in reservoirs that were undergoing near-field ground motion. As demonstrated above, the seismic fragility analysis approach has been widely used to assess the dynamic performance of bridges. Moreover, this approach has also been utilized to evaluate the fragility characteristics of high-pier bridges, which has achieved significant progress in our understanding of the subject. According to the seismic fragility analysis approach, it is first necessary to select a series of ground motions that directly affect the accuracy of the result. However, there are currently various methods for selecting ground motions and there is no unified standard on how to choose the appropriate ground motion variables in seismic fragility analysis, which remains a long-term and unresolved problem [26]. Through a review of the relevant research literature, it is clear that certain methods are available for selecting ground motion variables. (1) According to the seismic design specifications or geological survey data at the bridge site, a site response spectrum of the structure is generated, then the monitored ground motions are selected from ground motion databases, such as the Pacific Earthquake Engineering Research Center (PEER) ground motions database, which is based on the site response spectrum. (2) Using the theoretical formula for ground motion synthesis, the ground motions are synthesized according to the relevant parameters for engineering structures. (3) A series of ground motion parameters, such as peak ground acceleration amplitude and period, which have a wide range of values are selected to reflect the characteristics of ground motions and represent the uncertainty of ground motions.