# Evaluation of Ride Comfort under Vortex-Induced Vibration of Long-Span Bridge
## 3 Ride Comfort Criteria  
According to the sources of discomfort, comfort can be categorized into six types: static, vibration, noise, heat, hearing pressure, and visual [32; 33]. Generally, it is primarily assessed through collecting physiological data such as eye movement, EEG, and electrodermal activity, along with environmental vibration data in biological and medical fields [34; 35]. Although this method can obtain relatively accurate results, it requires numerous personnel, extensive experimental equipment, and repeated experiments over extended periods, which is unsuitable for implementation in the field of engineering, owing to its time-consuming and labor-intensive nature. Currently, numerous objective evaluation methods exist globally for assessing ride comfort. These methods include the average absorbed power [36], VDI2057 [37], BS684:1987 [38], ISO2631-1 [14], Sperling stability index [39], UIC531 [40], EN12229:2009 [20], etc. Notably, the initial three methods find extensive application within the automotive industry, while the latter three are commonly employed in the railway industry. The ISO 2631-1 [14] standard serves as a universally applicable evaluation method. They provide comprehensive assessments that take into account factors such as resonance intensity, the human body's sensitive frequency range, and the applicable comfort index range. These methods primarily rely on frequency weighting functions for their calculations and evaluations. Among these approaches, the ISO2631-1 standard [14] evaluation method enjoys the broadest usage. Consequently, this paper utilizes the OVTV and MSI indices from the ISO2631-1997 standard [14] to evaluate driving comfort, which mainly involves measuring physical attributes like frequency, amplitude, and the direction of a human body's vibrational response or the corresponding environmental vibrational response. Methods for measuring periodic, random, and transient whole-body vibrations are defined in the ISO2631-1997 standard [14], considering three human vibration models of sitting, standing, and lying. In the evaluation of driving comfort in this paper, the sitting posture model is used. As shown in Figure 6, the vehicle vibration is mainly transmitted to the human body through the seat, backrest, and floor. In view of the complex modeling of the human body model, the vehicle vibration acceleration can be used to replace the vibration acceleration of each axis of the human body, which is used in some papers [24; 25]. \(x_{i}\), \(y_{i}\), \(z_{i}\) (\(i=b\), \(s,f\)) denote the line vibrations in three directions at the support surface, indicated by red arrows, where \(i=b\) denotes the seat back support surface, \(i=s\) denotes the seat support surface, and \(i=f\) denotes the foot support surface. The blue dashed line indicates the reference system established through the seat support surface, mainly to  
Figure 4: Vortex-induced vibration mode at 0.328 Hz [31]. Figure 5: Vertical displacement of wheels under VIV. (**a**) Vehicle speed is 10 m/s. (**b**) Vehicle speed is 20 m/s. facilitate the representation of the 3 directions of rotation of the human body. The black arrows indicate the 3 directions of rotation, which is annotated in the Figure 6.  
The vibration model for human sitting posture considers linear and angular vibrations in three directions at the seat surface input point. Additionally, it accounts for linear vibrations in three directions at both the seat backrest and foot support surface input points, resulting in a total of twelve axial vibrations across three input points. According to the ISO 2631-1997 standard [14], the human body exhibits varying sensitivities to vibrations at different frequencies. Frequency weighting functions are provided for each axis, as displayed in Figure 7. The axis weighting coefficients of each axial vibration are also specified in order to consider the differences in the impact of different input points and different axial vibrations on the human body.