# The Current Development of Structural Health Monitoring for Bridges: A Review
## 4 Early Warning Systems  
In the previous section, we summarized the research on the data processing methods of BHM in recent years from three aspects: data preprocessing, noise reduction, and loss reconstruction. In this section, we will review the recent research progress in early warning systems in BHM. Figure 7: Conditional adversarial net [102]. Aiming to save on the cost of system operation, a command from the BHM system is usually required before implementing SDI. Therefore, in BHM, the study of early warning systems is also crucial. It needs to be emphasized that, in the actual monitoring process, issuing warnings and identifying anomalous structural states are closely related but are not necessarily at the same level of operation. The warning system may not need to precisely locate and quantify damage like the damage identification system does. Instead, it often provides warnings of different levels by setting one or more warning thresholds. [PERSON] et al. [108] studied the vortex-induced vibration (VIV) phenomenon of long-span bridges in a special wind environment. A method based on multisource load response data was proposed for the early warning of VIV. [PERSON] et al. [109] conducted a numerical and case study on the early warning method of the Hong Kong-Zhuhai-Macao Bridge's immersed tunnel. Based on monitoring data with wavelet threshold denoising, an autoregressive integrated moving average (ARIMA) was used for future data prediction. They developed a hierarchical early warning system that proved to be effective at grading the detected anomalies. [PERSON] et al. [110] developed a platform based on Revit to realize visual warnings and the integrated management of monitoring information. They associated monitoring data with a BIM model through a virtual sensor system and then imported it into Revit. Based on monitoring data during typhoon Haikui, [PERSON] et al. [111] developed a machine learning model, called the integrated girder vibration indicator. The indicator takes wind data as the input and girder and tower vibrations as the output. When the prediction results exceed the normal threshold, it will give an early warning. Other scholars have considered temperature conditions. [PERSON] et al. [112] studied the early warning method of deflection change in railway bridges caused by temperature and train load. They used the monitoring data and the mutual update of the train-bridge dynamic model to monitor the bridge deflection behavior and, at the same time, determined the early warning threshold of it under the combined action of the two factors. [PERSON] et al. [113] introduced the temperature-displacement relationship (TDR) model to predict the performance degradation of bridge bearings for the first time. They used SBL for model parameter identification to solve the problem of sparse model parameters caused by the insensitivity of bearing displacement response to temperature. [PERSON] et al. [114] applied TDR to the abnormal boundary condition warning of the bridge. The high anomaly detection rate and low false detection rate of the proposed method were verified by using the actual girder end displacements from a large-span suspension bridge in China. [PERSON] et al. [115] studied the early warning method of cable force anomalies in cable-stayed bridges, considering the change in structural temperature. A baseline model of the frequency-temperature relationship (FTR) was established to give early warning when the predicted cable force error occurred. [PERSON] et al. [116] studied anomaly warning methods under the influence of various environmental factors and proposed an environment-frequency relation model based on local linear regression. The effectiveness of the proposed method was verified on a cable-stayed bridge. [PERSON] et al. [117] investigated the application of an interferometric synthetic aperture radar (InSAR) for visualizing and alerting unexpected bridge displacement. The validation was conducted on Jacques Cartier Bridge and Victoria Bridge, located in Canada.