**1. Introduction**

Civil engineering structures are usually designed to serve for 50 years to 100 years, during which they are expected to maintain structural integrity. Unpredicted and unexpected structure failure due to accumulated damages during design life may cause significant life and economic losses; therefore, structural health monitoring (SHM) is very important since it can monitor the structural response, evaluate the structural safety in real time and maintain structure safety and symmetry in nature.

SHM techniques have been developed for many years [1–10]. Generally, the vibration based SHM methods are the most widely adopted. The objectives of these SHM approaches are determining the existence of structural damages, identifying the location and severity of structural damages, evaluating structure safety, predicting the remaining service life of the structure, and making decision of the maintenance strategy, if possible. In fact, the vibration characteristics of a structure are a function of its physical parameters. Structural damage causes change in physical parameters of the structure, and change in physical parameters therefore can be used as an indicator of structure health condition. Through the signal monitored by the sensors installed on the structure, the vibration characteristics can be extracted and the corresponding change can be detected and analyzed. In addition, from the change of vibration characteristics, one can further obtain the change in the physical parameters of the structure to diagnose the structure health condition. Recently, with the rapid development of modern computer technology and the progress of sensor technology and signal processing technology, test signals can be accurately and quickly analyzed and processed. Therefore, vibration-based structural health monitoring technology has become a research hotspot at home and abroad.

**Citation:** Yang, Y.; Zhang, Y.; Tan, X. Review on Vibration-Based Structural Health Monitoring Techniques and Technical Codes. *Symmetry* **2021**, *13*, 1998. https://doi.org/10.3390/ sym13111998

Academic Editor: Jan Awrejcewicz

Received: 19 September 2021 Accepted: 18 October 2021 Published: 22 October 2021

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Based on these techniques and recently developed IT and sensing techniques, many technical standards and codes on vibration-based SHM have been developed and implemented for engineering applications. For a SHM system that can be used in practice, it usually contains four parts or sub-systems: sensing sub-system, including both a fixed sensor system and a portable sensor, data acquisition and transmission sub-system composing of a data acquisition unit, data transmission network and corresponding software system, data managemen<sup>t</sup> and control sub-system containing of data managemen<sup>t</sup> software and control server, and structure performance evaluation sub-system including structural health assessment server, structural health assessment workstation, and corresponding software system.

Although SHM has become an important field in the development of civil engineering disciplines, related technical methods still need to be improved, and there is still a lack of complete technical standards and specifications for vibration-based structural health monitoring. It is difficult for engineers to design suitable SHM system for a given structure based on the existing codes because there is no explicit answer of what kind of sensors to be used, where the sensors to be installed, and how to evaluate the structure health condition by using monitored data. Therefore, this work aims to review both vibration-based SHM techniques and technical codes, and provide a certain reference for the application of technical methods and standard specifications for vibration-based structural health monitoring. In Section 2, the vibration-based SHM approaches are reviewed, and attention will be paid on the more recently developed ones due to the length limit. In addition, the advantages and drawbacks of each approach has been summarized. In Section 3, the developed technical codes including both ISO standards and national codes are reviewed, and the suitable situations that each code can be applied have been reviewed, which may help to find out most suitable SHM method for a given structure. In Section 4, the challenges and future development are discussed.
