Practical BIM-Based Automated Cost Calculations - a scalable workflow

BIM for cost calculation is the use case; management gets glassy eyes and wants to have BIM immediately. But it's one of the most complex use cases, and most companies only achieve it when they model for themselves, if ever!

The challenge is that models need to be very consistent for automated cost calculations. The data quality varies widely! It depends on the company standard, the modeler, heck, even the mood of the modeler.

Moreover, in the early design stages, when cost calculations are impactful, architects are busy optimizing the design. Model/Data quality is not their focus.

The abstractBIM helps her, as it is automatically calculated based on minimal input from the architect. It's enough when the IFC model contains the following:

• The proper IFC structure with Project, Site, Building, and Storey, usually created by the BIM tool.
• IfcSpaces as a volume for the room and a Roomtypname in the attribute LongName (standard setting in most IFC exporters). We recommend to model the Space from top edge of the finished floor (even better top edge of raw floor, but you wont get it from the architect) to the underside of the structural ceiling.
• Optionally, the Windows and Doors as IfcWindow/IfcDoor or IfcCurtainWalls with transparent parts.

Based on this, abstractBIM's unique algorithm can create a new consistent model. The magic is due to the automated geometry generation based on the logic like an Interior wall is a wall between to IfcSpaces:

• The elements are always consistently classified.
• Every element "knows" it's direction.
• The room coverings "know" to which room they belong.
• Every wall and door "knows" between which two wall it is.
• Every window knows to which space it belongs.

So, the data quality aspect is solved. Just make sure to receive the modeled IfcSpaces early on.

After the abstraction of the architect's BIM, you get a new IFC BIM file and an Excel with all the quantities and all the sheets needed for a cost calculation.

You can use this Excel file for your calculation, adapt the Excel to your needs, or apply the logic to the tool of your choice.

Exploring the abstractBIM

We recommend checking the abstractBIM before use in the free version of BIMcollab ZOOM. Contact me for the relevant SmartViews.

In the Excel the first sheet is abstractBIM. It contains every element form the BIM model as one line, and in the columns, you can see for every Element:

• the type (Wall, Slab, Space, Covering, ...)
• quantities (Area, Volume, Length)
• position (Storey and Orientation)
• relation of every Element to the connecting Spaces. That will become important later.

The sheet "Summary" calculates some basic quantities for further use in calculations. Like total exterior wall area, Gross floor area, and so on. Some of these basic quantities are straightforward; others depend on some assumptions. E.g., To decide what is above and what is below ground, we assume a flat ground and a ground floor with 0 or a positive Z coordinate.

This summary sheet can help you to understand the project better quickly.

The calculation sheet

In the calculation sheet, all the different information is centralized. It connected the quantities from the abstractBIM, with the qualities from the Roombook, the Wallbook, and the Facadebook with your unit prices. All the formulas are ready, and you just have to enter the data into the yellowish fields.

Contact us to support you in setting up an automated connection to your cost database.

The Room-, Wall- and Facadebook

In the book Sheets you can add more information for the Spaces, Interior Walls, and Exterior Coverings. The principle is always the same. Copy the unique and insert the values (not the formulas) of the Type names into column B of the Book Sheet. Based on this information, the sheet calculates basic quantities, and you can add more information in the later columns.

All this information is already linked to the calculation sheet, so once you add the qualities, they are visible in the calculation.

The "Help Unique" Sheet supports updating the calculation

That's the odd one and needs some explanation. Doing a calculation once is easy, but it becomes interesting when you exchange the model and your calculation updates. These automations are great, but sometimes automation can lead to unexpected results. Therefore, I recommend a manual process that forces you to think.

The first manual process is to copy the new quantities after abstraction into the abstractBIM sheet, and the second one is to copy the unique type names from the Help Unique sheet into column B of your *books.

Summary

To set up a workflow for automated cost calculations from a BIM, you need to control the model quality. This we do with the automatically generated abstractBIM. All the rest is just some formulas and logic to connect the information.

As we currently update the calculation sheet frequently, you can access the latest version at Github.

So please try let us know what you think about 5D BIM in practice and if you want to participate at this open source project let us know.