## 1 Introduction  
To meet the stricter requirement for girder rigidity in railways and noninterference between rail and road traffic in China, truss griders are becoming more commonly applied in the design of long-span cable-stayed bridges, combining roadways and railways with double decks over 30 m in width [1]. When the truss girder length exceeds 1,000 m, the girder stiffness becomes lower, and the bridge flexibility becomes higher. Additionally, the first natural frequency is approximately 0.1 Hz or even lower. When built in wind-prone regions, these bridges suffer considerable buffeting-induced vibration. The buffeting-induced vibration occurs over a wide range of wind speeds and persists throughout the entire design life of the bridge. As a result, buffeting plays the most important role in the wind-resistant design of long-span cable-stayed bridges with double-deck truss griders (DDTGs). The interaction of buffeting and wind is regarded as a significant excitation in flexible long-span bridges due to the low frequencies that characterize both the turbulence input and the structure. The aerodynamic admittance function (AAF) is one of the most important factors in evaluating the buffeting responses of slender, link-like structures, such as long-span cable-stayed bridges, in turbulent flows. The AAF is a transfer function from wind turbulence energy to fluctuating buffeting forces on structures. The AAF can take into account the unsteadiness of gust loading in a linear time-invariant system. In the 1900s, the AAF was derived from the aerofoil lift force studied by some scholars. Based on the strip assumption, the spanwise correlation of the buffeting force on structures could be displaced by the lateral correlation of turbulence. In 1941, [PERSON] [2] was the first to propose a complete 2-dimensional lift AAF of a thin aerofoil immersed in a transversely fully coherent sinusoidal gust. As the milestone of AAF, this transfer function has been named the Sears function and applied in many studies as the 2D AAF of streamlined objects in turbulence. Following [PERSON] approach, [PERSON] [3; 4] used statistical concepts to develop a buffeting analysis method for aerofoils in the frequency domain and obtained an approximate expression for the Sears function. [PERSON] [5; 6] gave the aerodynamic transfer function for the lift force on a thin aerofoil with infinite span by a numerical method. Moreover, some scholars ([PERSON] [7; 8] and [PERSON] [9]) put forward an approximate expression of the admittance for an aerofoil. However, all of these theorized admittance expressions were not derived for bluff bodies, and their universality is questionable. In recent years, some new developments have been made regarding the aerofoil admittance in parameter analysis, but most of these developments have been limited to streamline bodies [10; 11]. For bluff bodies, such as a bridge girder section, the exact expression of the admittance cannot be theoretically derived like that for aerofoils due to their much more complex aerodynamic force caused by the separation and reattachment of flow. [PERSON] and [PERSON] [12] measured the AAFs of 2D rectangular sections in smooth flow using a gust generation technique. They found that the Sears function underestimates the 2D AAF of these simplest bluff bodies. Therefore, the aerodynamic admittance of bluff bodies has mainly been determined based on wind tunnel tests. The experimental methods used to identify the AAF mainly consist of random response measurements on aeroelastic models and direct buffeting force/pressure measurements on rigid segment models. Based on the assumption that the Sears function could be expressed by the Wagner function or Theodorsen functions, some studies [13; 14; 15; 16] obtained the admittance functions of bluff bodies with non-strict adaptability using the former methods. Considering convenience and practicability, a widely used experimental method is the wind tunnel test, which consists of surface pressure tube measurements and direct force measurements.