## 4 BIM-Based Data Collection
### Construction of Seismic Damage Model  
The difference between a seismic building model and a general building model lies in the treatment of damaged components, such as cracks in the walls, deformations of windows and doors, and the spalling of concrete. This information plays a vital role in subsequent construction, so the treatment of these special components is crucial to the accuracy of the retrofitting model.  
The seismic damage model is constructed based on post-earthquake pictures taken at the site, post-earthquake building appraisal reports, and retrofitting design drawings. When building the model, it is necessary to use BIM software to draw the model. Revit is a mainstream BIM software used in major projects, and provides many functions and easy operation. As such, this study utilizes Revit to draw the models.  
First, a non-damage model is built in Revit by drawing the walls, columns, floors, and other components; then, the damage is represented in the non-damage model. Theoretically, all damage should be represented in the model, but the repair and retrofitting measures can vary depending on the degree of damage to the component. Therefore, the representation requirements for the model can be different. Accordingly, a multi-level expression method is used to solve this problem. The multi-level method can retain and present damage information in the context of the retrofitting requirements for different components. Regarding the relevant code [55] and appraisal results, the components are classified into three levels: repair, retrofitting, and reconstruction-retrofitting, based on the final retrofitting design. Refer to the above standards, standards of multiple levels of detail (multi-LoD) parametric model [56], and multi-LoD BIM models for Wooden architectural heritage (WAH) components [57], we propose the multi-level expression method for the damage and define three accuracy levels: Lev1, Lev2, and Lev3. The objects applicable to the model correspond to these three levels.  
Figure 2: Visualization process of post-earthquake retrofitting of building.  
Technical specification for post-earthquake urgent assessment and repair of buildings stipulates that for load-bearing walls with a width greater than 800 mm, when a crack width \(\omega\) < 5.0 mm, the crack must be repaired; when 5.0 mm \(\leq\omega\) < 10 mm, the crack should be retrofitted; when \(\omega\)\(\geq\) 10 mm, the area should be partially demolished and rebuilt before retrofitting. The following uses the load-bearing wall crack as an example to introduce the construction of the multi-level model and expression of the information in detail, as shown in Table 1.  
* Lev1 is mainly suitable for components that do not need to be repaired, or that can be simply processed. This type of crack generally does not affect the normal use of the wall, so the amount of damage can be unclear. BIM engineers only need to mark the damaged components in the model with color blocks, e.g., using green, blue, and purple to indicate repair, retrofitting, and rebuild/retrofitting.
* Lev2 is used for retrofitting needs. The length, depth, width, and development shape of the crack must be clarified for the retrofitting construction. In the model, a prism is used to reflect the length, maximum depth, and width of the crack; in addition, the outline of the crack is reflected, such as \"wide in the middle, narrow at both ends\", or \"wide at the top and narrow at the bottom\".
* Lev3 provides more detailed information for the construction of severely damaged components. This type of component must be partially demolished, rebuilt, and retrofitted. The development pattern of the crack needs to be depicted in the model. In addition to the maximum depth and width information contained in Lev2, the changes in depth and width need to be described to provide more comprehensive damage information for the retrofitting design.