# A Field Investigation on Vortex-Induced Vibrations of Stay Cables in a Cable-Stayed Bridge
## 1 Introduction  
Long-span cable-stayed bridges with excellent structural performance and elegant appearance have been frequently built worldwide in recent years. Inclined cables serve as key components of the long-span cable-stayed bridges, and their increasing lengths make them prone to oscillations under the actions of wind, rain, traffic, and seismic loads. The vortex-induced vibration (VIV) of an inclined cable easily occurs when it is subjected to a wind field [1; 2]. Generally, uniform incoming airflow generates a mono process of vortex shedding. By contrast, nonuniform flow, e.g., a shear flow, could possibly excite responses with multiple modal frequencies. This is mainly because the vortex-shedding process changes with the flow speed passing by and multimodal frequencies will be covered. Because all of the long-span bridges are constructed in the atmospheric boundary layer, the sheared oncoming airflow acting on the inclined cables increases with height and can easily excite multimodal VIVs of cables [3; 4]. [PERSON] and [PERSON] [1] conducted a field study on the Fred Hartman Bridge (Texas, USA) and investigated the wind effects of the approaching speed, direction, and rainfall on the vibration amplitude of cables. [PERSON] et al. [5] and [PERSON] and [PERSON] [6] then conducted field investigations on the wind-induced cable vibrations on the same bridge. These studies found both wind-rain inducedvibrations under rain condition and large amplitude vibrations of dry-cable at high reduced velocities. It was suggested that the large-amplitude vibration with or without rain might be due to a new type of vortex shedding at high reduced velocities that is different from the classical VIV induced by von Karman vortex shedding. Based on numerical simulations, [PERSON] et al. [3] found that a long flexible cable (length L > 100 m) would experience mono-modal and multimodal VIVs when subject to varying velocity profiles. More recently, [PERSON] et al. [4] tested the VIV of a flexible cable with a length of 6.08 m, under a sheared flow in the wind tunnel. The speed difference between two ends of the cable was found to determine the vibration type: single-modal or multimodal VIV. As the speed difference increased, a single-modal VIV of the testing model would shift to a multimode. The VIVs of oceanic risers take place more frequently compared to stay cables. Under shear flows, the experimental and field tests were performed on long, flexible marine risers, which had multiple frequencies of response (e.g., [7; 8; 9; 10]). [PERSON] et al. [7] conducted an experimental investigation on VIVs of flexible marine risers, with an aspect ratio of 1400. The results indicated that the in-line (IL) displacement is less than the cross-flow (CF) displacement for each velocity case. Moreover, the IL vibration modes are nearly twice as great as those of the CF vibration modes. The band of dominant vibration modes is relatively narrow at a smaller flow velocity, and the band extends at a higher flow velocity, which is most evident for the CF response. [PERSON] and [PERSON] [8] performed a modal analysis on the large-scale VIV in a linearly sheared flow by employing a riser model with a diameter of 3 cm and a length of 90 m. At a high velocity, the riser experienced multimode response, and no single mode (lock-in) was observed. [PERSON] et al. [9] performed an investigation on VIV for linear and exponential shear flows by using long and flexible circular cylinders, which usually have larger aspect ratios than 500. These researchers found that the bandwidth of a large vibration under an exponential shear flow was larger than that in a linearly sheared flow. [PERSON] and [PERSON] [10] found that the long flexible cylinder response might change from classic lock-in to non-lock-in multimode responses in the sheared flow, which was a simplified two-slab flow, with each slab having uniform flow. [PERSON] et al.