Recently, I have received a lot of inquiries from friends interested in BIM quantity calculation. Based on my understanding of BIM, I would like to share my insights on how BIM is measured and what the process looks like (using Revit software as a basis).
Step 1: Build the BIM Model
Before performing any quantity calculations, it is essential to establish a BIM model. To meet the requirements for quantity takeoff, the model should be developed to a Level of Development (LOD) 300. This means including basic components such as columns, beams, slabs, walls, etc. Additionally, to facilitate the calculation of finishes and decoration quantities, room components should also be modeled. The BIM model can provide necessary information like quantities of finishing materials (e.g., wall paint), which can be derived from room data (for example, room perimeter multiplied by ceiling height). For complex elements that are difficult or impossible to model physically—such as steel reinforcement and formwork—it is recommended to include them parametrically within the BIM model.
Step 2: Add Work Item Names and Size Parameters
To accurately calculate material quantities, assign work item names to the BIM components, and supplement any missing size parameters for physical elements. For instance, room components assist with quantity calculations for interior finishes; structural elements like columns, beams, slabs, and walls support structural quantity calculations; and door and window components help with finishing quantities as well. Each relevant piece of information should be added to the corresponding component for precise quantity extraction. Since Revit has limitations defining complex units (such as kg/m), it is advisable to store such parameters as numerical values and append the units in the parameter names.
Step 3: Create Detailed Schedules and Filter Components
Use Revit’s schedule feature to create detailed lists, filtering components according to the work items to be calculated. This categorizes components containing relevant work items. Note that not all fields are available as filter criteria, and if a field cannot be filtered, it might prevent the calculation of certain items. Due to inherent limitations in Revit software, schedules for components like columns, beams, slabs, walls, and rooms must be created separately. As a result, the same material may need to be calculated in multiple schedules and manually consolidated afterward.
Step 4: Establish Quantity Calculation Formulas
Add calculation formulas within the schedules to determine the quantities of work items. Defining formulas in Revit involves setting units beforehand, and all parameters must conform to these units. However, quantity formulas can be complex, and Revit supports only a limited set of units, so some calculations may require alternative approaches or workarounds.
Step 5: Repeat Steps 3 and 4
Continue creating schedules and applying formulas for different components until quantities for all work items have been calculated.
Step 6: Consolidate All Schedules
Finally, extract quantities of the same work items from various schedules and consolidate them into a total quantity for each item.
That wraps up the overview of how BIM quantity takeoff is performed using Revit software. I hope this article provides useful guidance for everyone interested in this topic!
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