Misconceptions everywhere
Similar to 4D-BIM – which we covered recently – the concept of estimating in a BIM environment is also vastly misunderstood. Most of the time you have a fixed budget for your project. Rarely can you design something and only calculate in the end how much it will cost (if you happen to have such clients, lucky you!).
Cost information is something you have to plan and track, always based on the current project data available. There is special software for this, and I know companies who place dummy objects in their Revit models to track costs that are indepent of elements (e.g. designer fees, site equipment costs, reserves etc.).
But why is there such a misconception of cost planning when it comes to BIM? Did we forget how to do this simply because we gave it a fancy new name – 5D?
Fun fact – you don’t need any 3D object to make estimations in IFC. That might be hard to grasp at first, so once again: you can make your good old cost estimation and planning in IFC just as you always did it. It’s all there! Now let’s have a look how it works.
Cost items in IFC
The main part of IFC that we look at here is the IfcCostItem. It is a subclass of IfcRoot, so it has (among other attributes) a unique ID that makes it identifiable.
Any line of your spreadsheet that you used up until now could be such a cost item; totals, fees, building element costs, even operation or life cycle costs.
You can nest these cost items which finally add up to a total cost. Such a total cost item could then be linked to a cost schedule for further usage (approvals, work orders,…). All this can be done without any assignment to elements.
The main attributes of a cost item are (optional as always) cost values and quantities. Let’s say you calculate the cost for your concrete walls. We can simplify it to creating the formwork, building the reinforcement and pouring the concrete – these are your cost items.
You can estimate the cost of your concrete by multiplying the sum of your wall quantities (volume) with the sum of all cost components required to pour the concrete: labor cost, mixing the concrete on site or paying for the ready-mixed concrete etc.
That means you can go as detailed as you like: just define one value or nest them for even greater insight. The schema gives you the structure, you “just” have to apply it.
Connecting your model elements
Up until now we didn’t talk about elements per se. If you do your quantity takeoffs by hand you can document them in IFC, too, you don’t actually need to assign them to an element. Pretty old-school, but it works.
Isn’t it quite elegant that you can actually work similarly – with or without a 3d model?
If you have building elements which quantities you want to use you can assign them to your cost items and use the quantities (be aware, that doesn’t work for properties! Important to distinguish here) as your cost quantities. They must have the same type; it doesn’t make sense to add volume and area measurements, right?
Elements, processes, resources – everything’s costly
Linking elements to costs is quite useful. But you can even go a few levels deeper. Do you need to handle time-dependent costs? Link a process to your cost item. Do you want to apply costs from resource usage, like equipment or materials? You can do that. It’s a bit difficult you get your head around when reading the documentation, but the possibilities are pretty awesome.
And again (and again and again): this concept is widely ignored by software developers. But wouldn’t it be great to have a universal way to organize your cost data?
One way? Many ways!
Furthermore, there are often many different ways to set up your estimations depending on your role or the project phase. Early in the project you might do this based on room area or gross volume. Later you switch to simple element quantities like area of walls, then to detailed specifications (work breakdown). Construction companies would probably go into even greater detail. Each of these approaches could be its own cost structure in IFC.
You could – again – use classification or document references to compare different projects or estimations.
This might be a bit overwhelming for one article. I’ll dissect it in a future post. If you want to share any thoughts please feel free to do so. If you have objections or experience that estimating wouldn’t fit into the structure that IFC provides I’m even more interested to learn more about this.