# Numerical Analysis on Spanwise Correlation of Vortex-Induced Force of Split Double-Box Beam
## 1 Introduction  
As a relatively new type of box beam, the split double-box beams are used in long-span bridges, the structure of which relies more and more widely on their superior flutter stability and higher critical flutter wind speed than the traditional box beams [1]. However, the existence of the middle groove causes the complex flow field around the main beam. As a result, the vortex-induced vibration performance becomes much worse. The vortex-induced vibration of split double-box beam has often been observed in engineering practices in recent years, such as the Akashi Strait Bridge [2], the Denmark Dahai Bridge [3], and the Xiboumen Bridge [4, 5]. The vortex-induced vibration generally occurs at a low wind speed. Although vortex-induced vibration will not cause destructive damage, the impact on driving comfort and safety should not be ignored. Therefore, it is of vital importance to study the mechanism of vortex-induced vibration. There is much research on the vortex-induced vibration of split double-box beams. Taking a long-span suspension bridge as the research object, [PERSON] studied the flow field characteristics of split double-box beam through the wind tunnel and particle imaging technology [6]. [PERSON] conducted a wind tunnel test on the segment model of the split double-box beam and analyzed the fluid flow law of the flow field around the split double-box beam at low Reynolds number conditions [7]. [PERSON] pointed out that the aerodynamic coefficients, the Storahar number, and the distribution of surface pressure present the Reynolds number effect obviously through a static test, and because of a dynamic test, the initial reduced wind speed, the width of the lock-in region and the maximum amplitude present Reynolds number effects [8]. Numerical simulation methods have the advantages of convenience, low cost, and the ability to impose some conditions that cannot be achieved experimentally, and with the development of CFD (computational fluid dynamics), the numerical simulation research method of bridge section vortex excitation force spreading correlation has been gradually recognized and adopted [9; 10]. The research on the split double-box beam mainly focuses on the two-dimensional flow field characteristics of vortex-induced vibration, the response characteristics of vibration, and the Reynolds number effect, while there are few types of research on the three-dimensional characteristics. In fact, due to the influence of the three-dimensional characteristics of the actual wind field, the vortex-induced forces are not completely synchronized along the spanwise direction but also present certain three-dimensional characteristics instead. The spatial correlation of the vortex excitation force is more obvious, especially for large-span bridges. The spanwise correlation of vortex-induced forces of bridge structures has attracted the attention of many scholars. [PERSON] found that the amplitude of structural vibration has a certain influence on the spanwise correlation of surface pressure on cylinder through the pressure test of the rigid segment model [11]. [PERSON] studied the spatial correlation of wind load on a typical streamlined closed box beam through a segmental model test and found that the spanwise correlation of wind load has a great impact on average velocities, turbulence, wind attack angle, and spanwise spacing [12]. [PERSON] studied the spanwise correlation of the vortex-induced force of rectangular sections. The results showed that the spanwise correlation of vortex-induced force in the lock-in region is stronger than that outside the lock-in region. The spanwise correlation of wake wind speed was weaker than that of aerodynamic force [13]. [PERSON] found that the spanwise correlation of the vortex-induced force of a rectangular model was better than that of a trapezoidal model through thae segmental model test [14]. [PERSON] explored the mechanism of vortex-induced vibration (VIV) of a streamlined box girder from the perspective of the flow field and pressure distribution.