## 2 Bridge Structural Health Monitoring (SHM) System  
### The Importance of Using the SHM System  
In terms of infrastructure, bridges are among the costliest options. Bridges may serve their communities for decades or longer, depending on factors like location and construction materials. The traffic loads carried by many bridges now far exceed what they were originally designed to handle. Because operating condition stresses are getting higher, structural fatigue is now a threat to the whole transportation system, not just to a single structure [27; 28]. Therefore, monitoring, analysis of failures that occurred in the past, and the knowledge of the structure's condition in the future can play a key role in increasing the lifespan and performance of bridges. There are both static and dynamic modal variations brought on by traffic conditions. Changes in static pressure are directly related to mass [29]. On the other hand, changes caused by traffic have been shown to be nonlinear and may go down as load effects go up. Modal properties may also be altered when a healthy bridge interacts with a moving vehicle. Because of this, it becomes far more challenging to do vibration monitoring on bridges that are currently in service. Therefore, it is crucial to recognize any structural degeneration. The optimal system for monitoring, managing, and assessing conditions must be determined. One of the most effective systems for controlling structure performance is the structural health monitoring (SHM) of bridges. SHM is a deliberate procedure in which, firstly, the required static or dynamic reactions of the structure are recorded and gathered by employing sensors and  
Figure 1: Flow chart of study. electrical equipment. After this, computer tools and algorithms are used to process the data. Data processing is followed by interpretation to provide decision-useful insights about the state of the structure. In fact, the term \"structural health monitoring\" is used to describe a wide range of methods that give accurate information about a structure's current state and how well it works, data which can then be studied in the near future. For accurate diagnosis and ongoing monitoring of bridge degradation, it is important to look at both how the bridges look and how they work. Innumerable beneficial studies have been carried out to explore the grounds for using SHM systems to monitor bridges as well as the manner in which these systems are used, and these studies have revealed the reasons [30; 31; 32; 33]. [PERSON] and [PERSON] [34] reviewed recent technology developments in the field of SHM and their potential use in large-scale bridge projects. Their review revealed that new methods for inspecting and monitoring bridge safety are being made possible by advances in sensing systems, signal processing, communications, and data-mining technologies. In another study, [PERSON] et al. [35] created a new SHM technique by combining the Precise Point Positioning GNSS-Global Navigation Satellite System (PPP-GNSS) measurement technology with a different probabilistic strategy. They used the El Carrizo Bridge as a case study, and their results showed the precision of PPP-GNSS measurement technology as well as the potential advantages of the alternative probabilistic SHM technique. [PERSON] et al. [36] provided an overview of the vision and digital image correlation (DIC)-based SHM techniques for bridges. In their study, they considered different kinds of bridges, such as concrete, suspension, masonry, and steel bridges. Their study showed that the DIC could identify structural factors including vibration, deflection, and rotation as well as damage, like fractures and spalling. [PERSON], and [PERSON] [37] reviewed the most modern techniques and technologies for building large structures. They also talked about problems and limits, which helped them figure out some future research needs and directions.