# BIM-Based Management System for Off-Site Construction Projects
## 4 System Implementation
### System Functions and Implementation Detail
#### 4.3.6 Progress Payment Management  
The proposed system has a progress payment management function that can rapidly and accurately settle costs according to the project progress. The progress payment management consists of three subfunctions: data management for billing, the automatic creation of installed quantities using BIM-based information, and the status of past invoices. The data required for billing consist of the quantity and unit price. The default value of the component quantity is derived from the number of individuals with the same attribute by the CDM. Moreover, the system can handle any quantity calculation formulas or premium conditions included in the project contract. Instead of directly inputting a list of components, quantity, and unit price required for an invoice, users can easily retrieve it from the system through the object-based attribute information. A subfunction is introduced to derive the project progress and performance indicators from past payment details. This function enables the querying of past payment history and can derive performance indicators such as the cumulative amount and quantity of a component or budget execution rate of a project.  
The system linked related data through a BIM-based parametric modeling function to manage progress payment rapidly without errors. The contract amount, component model, and past payment details are linked in the DB, and the resulting values are calculated using this information. Figure 7 shows a sequence diagram of the user interaction with the main detailed functions in the progress payment management module.  
The diagram illustrates the progress payment management in three main parts: contract amount inquiry, new invoice creation, and past payment history management. The contract amount is used to automatically calculate the unit price information required for billing (Figure 7A). Users can check the unit price in the BIM attribute information of the management system, the raw data of which are the contract amounts. Because these contract amounts often vary during the project, the system can save the change records and retrieve the most recent value when creating an invoice. Moreover, the system links the contract amount and the BIM component model to automate the cost calculation (Figure 7B). Although this system can automatically calculate the billing quantity by the CDM stored as a sub-property of the components, it uses the contract bill-of-materials value instead because the exact quantity and the cost of the materials required for one  
Figure 7: Sequence diagram for progress payment management.  
complete component depend on the contract's terms and formulas. For example, in certain project contracts, the number of sills for columns or beams is extremely small, and they are not counted independently. In addition, a premium may be applied to the actual quantity in the case of parts that are difficult to manufacture or which require a large amount of materials. Therefore, the proposed system calculates the value closest to the actual cost by integrating the component ID, quantity, and contract through the CDM. Finally, the system can manage indicators such as the completed quantity and cost per entire project or each component (Figure 7C). An inquiry regarding past payment details is also possible, and the detailed information is displayed by clicking on the desired details in the list.