**4. Challenges and Future Development**

Many vibration based SHM techniques have been proposed and developed recently, which have been reviewed in Section 2, however, their real applications in practical buildings and structures are rare. Some challenges are summarized as follows:


On the other hand, it can be concluded that although there are some ISO standards and national codes relevant to vibration based SHM system, unfortunately they cannot answer the following critical questions well:


The above issues are important and unavoidable in SHM, since a properly designed SHM system should provide early warning to potential structural collapse to ensure the safety of building and residents' lives. Therefore, it is important to fill the gap in ISO standards and national codes to clearly specify which monitoring methods should be adopted for a specific building, where to install the sensors, how to evaluate the health condition of structures based on the collected data, and how to plan maintenance strategy and predict the remaining life of the building or structure.
