Improving the accuracy of BIM-based quantity takeoff for compound elements

Quantity takeoff based on Building Information Modeling (BIM) is a faster and more reliable method than the traditional 2D-based quantity takeoff approach. However, the quality of BIM models affects the accuracy of the extracted quantities. Incomplete details and inappropriate modeling methods cause deviations in extracted quantities. Compound elements, such as walls and floors, always face this issue because they contain multiple material layers, which can have different sizes. In order to obtain accurate quantity takeoffs of these layers, each compound element layer that is composed of a core layer and other layers must be created according to the actual construction. Nevertheless, this is a time-consuming and error-prone solution. Furthermore, it can be difficult to edit a model in the future if its design is not finalized. This study proposes a method to improve the accuracy of the extracted quantities of compound elements from BIM models that are incomplete or incorrect by using information from BIM-based clash detection to eliminate excess quantities and add missing quantities. The proposed method is called the BIM-based compound element quantity takeoff improvement (BCEQTI) method. The BCEQTI method is then validated through four case studies. With this method, accurate material quantities can be delivered, and the time used to edit the BIM models is saved. Furthermore, it can be applied to other building elements that have similar issues.