## 3 Methodology
### Structure Deterioration Identification
#### 3.2.4 Establishment of Bridge Model Update Process  
Overview of Bridge Model Update Process  
By comparing the point cloud data of bridges at different time points, the deformation of the bridge over time can be obtained. Specifically, the deformation of the bridge over a certain period can be obtained by measuring the distances between corresponding points in the point cloud models at different times. This not only allows for the observation of changes at individual points but also provides an intuitive representation of the changes in all points within the bridge point cloud, which can be visually depicted using different colors to indicate different magnitudes of change, thus enabling the direct visualization of the deformation at various locations of the bridge. The use of three-dimensional laser scanning technology can intuitively present the deformation of various parts of the overall bridge structure, achieving deformation visualization, which aids in the rapid diagnosis of the bridge structure. This study will establish a framework for updating bridge models based on bridge point cloud data. Initially, a matrix is established that includes various static parameters of the bridge (strength indicators, deformation indicators, stress distribution, etc. ), each parameter having a range of numerical values. Using the previously obtained locations of bridge cracks and deformations, data fusion is employed to obtain an analysis of the current state of the bridge, considering \"cracks + deformation\". Further calculations determine the current stress state and damage issues of the bridge, enabling the backward completion of the updating of relevant static parameters in the finite element model of the bridge. In the framework of updating the finite element model of the bridge constructed in this article, the authors have analyzed the beam element model and solid element model based on the actual working conditions of the bridge, as well as the bridge deck and pier column slices obtained from the previous point cloud preprocessing process of the bridge. The authors focus on the responses of bridge structures under static loads when there is a linear relationship between stress and strain during small deformations, that is, the overall stress and deformation of the bridge structure. This updating process typically cannot be pinned down to an exact value, and thus, the goal is to narrow down the range of the aforementioned parameter values, completing the updating of the parameters in the bridge finite element model. Therefore, this study has established a framework for updating the parameters of the bridge finite element model, as shown in Figure 11.  
Classification of Cracks  
In bridge engineering, it is important to recognize that not all cracks compromise bridge safety. Therefore, to accurately assess bridge conditions, cracks must be categorized based on their shapes. For instance, longitudinal cracks extend along the bridge's length and are often due to expansion and contraction loads or structural issues. Similarly, transverse cracks cross the bridge's width, usually linked to traffic loads and temperature changes. Furthermore, diagonal cracks extend at an oblique angle, typically caused by structural stresses or uneven load distribution. In addition, circumferential cracks appear in a circular pattern near abutments or piers, possibly indicating foundation problems or design and construction defects. Moreover, joint cracks are found at bridge joints or expansion joints, mainly due to connecting material wear and temperature changes. However, surface cracks, often caused by environmental factors, chemical erosion, or material aging, usually do not pose a significant threat to the bridge's bearing capacity. Lastly, helical cracks, extending in a spiral pattern, are typically related to structural issues or corrosion fatigue and can affect the bridge's bearing capacity.