Monday, January 21, 2013

What Makes a Good Estimating Model?

What makes a good estimating model?

It depends on the phase! A good estimating model in design phase or pre-construction phase?

Since BIMES is mostly involved in pre-construction phase, I would like to focus on this in this post:

1 - Model according to means and methods of construction:
The most important thing is doing the model according to the actual means and methods of the construction.   What do I mean?
Well, column height is the best example for this. You can model your columns as a single piece from bottom to top of the project. However in real life, the columns are constructed floor by floor. The right approach is modeling the columns floor by floor the way they are built. The start and end point of a column is also important. Is it from structural slab top to structural slab top or from structural slab top to structural slab bottom? You need to answer these kind of questions beofer starting the project.

2 - Model according to the output required:
You need to understand what output is required from the model before starting. Make sure that you understand it in detail. For instance, the concrete quantities! How to segregate it? Just a floor by floor element quantity might be enough! Or you might have to segregate it as per the concrete grade. Once you understand this, plan which information will be entered to which area? You can sort it by using layers or the custom IFC parameters introduced in ArchiCAD 16.
Image courtesy of adamr /

3 - What should be modeled? What should NOT be modeled? 
An important decision that depends on 2 main factors:
  • Time allocated for modeling
  • Future use of the model
For instance, you are modeling for tender which means you have very limited time! So you have to model smartly. For instance you can model the exterior walls conceptually and use the exterior surface area only and make assumptions for the insulation etc. It would work very well if you bid for the project but don't get it:)

However if you get the project, you will have to provides details of the wall skins/sub-components for accurate estimates as well as shop drawings. You might think, I will just replace the wall or increase it's detail level but most of the case, remodeling would be preferred by the modeler.

In another case, if the model will be used only for coordination and quantities then you can work with abstract elements and attach unit costs.

So we usually have a kick off meeting before starting the project to discuss these details and draw a road map. This is very important for a successful and efficient BIM Modeling process.

Diyane Koseoglu


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