Why You Need Civil BIM, not CAD

Much of the transportation industry is still operating in a world of 2D CAD. It’s what we are familiar with and what we are comfortable with, but it’s obsolete.

CAD is a graphics display system, BIM is a feature modeling platform

CAD originally stood for Computer-Aided Drafting because its adoption basically entailed taking the traditional drafting table and Leroy machine and replacing them with MTEXT in AutoCAD. Somewhere around the mid-oughts it became fashionable to add another D and talk about Computer-Aided Drafting and Design. Whether you prefer a single D or two, CAD is primarily about geometry, dimensions and text.

The end goal is a set of construction plans, and that’s generally the end of the road. There are no attributes or metadata to provide semantic information about what is being represented. Color, lineweight and linestyle are fixed, not driven by data or attribution.

The Fallacy of Data Organization By Layer

As an aside, I’m talking about Layer in terms of an organizational construct within AutoCAD. If you come from the Bentley side of the fence, this term is interchangeable with level.

In CAD, a layer is a display property. It is all about whether or not something should show up in a particular view. Anyone with experience in a mis-configured system with poor layer display management can understand the frustration of never knowing if the sheets are going to be correct until you plot them.

Anyway, the term layer goes back to manual drafting where different types of information (topo survey, alignments, etc.) were drafted in pencil on individual mylar sheets, then stacked together in layers before final inking took place. Again, this is an example of CAD being used as a digital construct of the manual drafting era. “Here’s how we do it on the board, so let’s do the same thing on the computer”.

Over time, layers became less about managing display and more about managing information. The primary way to differentiate between different features such as survey vs. alignments was to carve out a sacred text of layer names (or numbers, pre-V8) that were to be utilized during all drafting activities. This led to more and more layers being utilized and resulted in working environments that might have upward of 2,000 standard layers.

One example I remember was a system that had separate level names for each size of each type of storm sewer pipe. This was for a DOT that had five types of pipe and maybe a dozen standard diameters. You would never have a situation where you wanted to display all of the 12” and 15” pipes but not the 18” ones on a plan sheet, so a single layer for proposed storm sewer pipe would have more than sufficed. However, this system ended up with 60 different levels to be used, requiring great amounts of effort when an iteration of storm sewer system design would change both the geometry and the pipe diameter.

As I recall, the primary driver for this arrangement was that all CAD elements were to utilize ByLevel linetypes and the system was configured to use custom linetypes for the various pipe diameters.

Civil CAD is better, but still has gaps

Legacy versions of Geopak provided rudimentary capabilities to provide flexibility beyond the “Level Name is the feature description” mantra. This was accomplished via use of adhoc attributes. I saw this used most often for quantity takeoffs and pay item information. In the example above, this would allow for the use of a single level for all proposed pipes. Integration with D & C Manager and use of plan view criteria files could theoretically drive display properties such as the custom linestyles of pipe diameters, but it was not particularly sophisticated and required expertise in text-based programming with C-based syntax. (Somewhere on a neglected server there are mountains of curly braces enjoying retirement after banging through thousands of iterations of draw, draw skip, and mark point).

Surface modeling is not BIM

Civil designers are quick to point to surface modeling and contouring from digital terrain models as 3D modeling. Since 3D modeling is an integral (but not the only!) component of BIM, we can’t really call this BIM. Two reasons:

1. A surface model is actually 2.5D, not 3D, because you can’t have a surface that backs up on itself or includes perfectly vertical edges. In other words, each X,Y point in the 2D plane can intercept the triangulation once and only once.
2. There’s no division between components. Triangles for a curb bordered by a sidewalk on one side and pavement on the other are all part of the same blob of data. These elements can’t be displayed or symbolized individually from a DTM and they can’t be utilized for detailed quantity takeoffs. For true Civil BIM, you need closed 3D meshes with metadata.

How CAD is merging with BIM

I first started developing this post a year ago, and originally had this heading as “The Future”. But truthfully, the future is now. There’s no need to wait for new tools and upcoming software releases - the current tools are more than capable of supporting Civil BIM.

This is evident in the current civil modeling platforms where model elements are specialized data types based on feature definitions. A 3D polyline isn’t just a polyline, it’s a linear feature defined as w-beam guardrail at 6’-3” post spacing. A mesh isn’t just a mesh - it’s a pavement component with material, density, and pay item definitions.

On the Bentley side, Item Types are providing the additional capability to develop specialized model elements that inherit from the basic DGN element types (line, ellipse, point) in the spirit of object-oriented programming. Symbology can be driven by item type content via display styles, regardless of what level the features reside on. Recent upgrades to the CONNECT platform have increased the capability of Item Types - particularly with the ability to limit properties to a predefined list, or domain of values. Coming back to the storm sewer example, the Pipe Type categorization would be defined as a domain-limited field with values of 1, 2, 3, 4 and 5 .

Conclusion

It’s an exciting time to be in the transportation design and construction industry. The transition from CAD to BIM is every bit as revolutionary as the previous transition from board drafting to CAD. Proper execution of a well-reasoned BIM Execution Plan not only serves the design and construction sectors, but lays the foundation for digital twins and the metaverse of the future.

(I claim bonus points for fitting in some major buzzwords there at the end.