# The Current Development of Structural Health Monitoring for Bridges: A Review
## 1 Introduction  
As the core pillar of economic development, the bridge transportation system has been highly valued by countries around the world in recent decades. With the rise of bridge engineering, bridge construction technology has been continuously upgraded, and large-span bridges, such as Hong Kong's Tsing Ma Bridge, the Hangzhou Bay Sea Crossing Bridge, Italy's Messina Strait Bridge, and Canada's Golden Ears Bridge, have been put into use successively. However, as the service lives of bridges increase, changes in external environmental conditions, such as wind loads, geology [1], temperature, and humidity [2], will gradually reduce the durability and safety of bridge structures. Therefore, in order to understand the current health status of bridges in a timely manner, many large-span bridges are equipped with health monitoring systems to assess and decrease the potential bridge health risks, and extend the service life of bridges. Unlike traditional manual visual inspection with carried devices, modern bridge health monitoring (BHM) systems reduce a significant amount of manpower and material costs used for inspections, and achieve the real-time monitoring of bridge strain, deflection, vibration, and other characteristics by installing sensors in various parts of the bridge. The monitoring system integrates functions such as data collection, health diagnosis, and damage warning, making the entire bridge structural testing process dynamic and convenient. The operation process of the traditional BHM system is shown in Figure 1. Considering the complexity and importance of BHM, scholars from various countries have paid extensive attention to it and have constantly proposed new improvement and development schemes. Yokohama-Bay Bridge is one of the bridges in Japan with the most installed monitoring equipment. During the Great East Japan Earthquake (Mw 9.0) in 2011, the BHM system on this bridge collected comprehensive response data sets from the earthquake's foreshocks, main shock, and aftershocks, providing important evidence for scientific research [3]. The significance of BHM lies in its ability to predict various events that may occur on bridges, apart from force majeure, to protect life and property safety. In 2007, due to design defects and neglect of stress conditions, the I-35W Mississippi River bridge in Minneapolis collapsed during rush hour, causing serious casualties [4]. In the same year, the Jiujiang Bridge in Guangdong, China, collapsed after being hit by a sand-hauling ship [5], indicating that bridges should be continuously monitored for important waterways and warnings should be issued when danger approaches. Reviewing past bridge collapse accidents can explain the importance of BHM in bridge engineering from another perspective. In the early development of BHM, due to immature analysis techniques and the scarcity and inaccuracy of monitoring equipment, it often led to mismatched predictions and actual results, which posed serious safety hazards to bridges. [PERSON] and [PERSON] [6] reviewed the research undertaken in the past 20 years on bridges with BHM installed in the United States. Among these bridges, the North Halawa Valley Viaduct had installed over 200 sensors at various locations on the bridge during the construction period for monitoring multiple parameters. A subsequent study [7] found that the long-term deformation results obtained using general prediction methods for the bridge differed significantly from the actual deformation. This difference was confirmed to be due to the neglect of concrete shrinkage and creep. [PERSON] et al. [8] studied creep and shrinkage prediction models such as those from the American Concrete Institute, Japan Society of Civil Engineers, and CEB-FIP Model Code. The results showed that these widely used models at the time generally underestimated bridge deflection. In addition, BHM faces many challenges, such as fatigue  
Figure 1: Flow chart of bridge health monitoring system. and corrosion evaluation, scour effects, etc.