February 1, 2012

BIM for Civil Engineers: Surfaces – Elevation Analysis Using AutoCAD Civil 3D

As an Application Specialist with Ideate, Inc., I have received many questions regarding AutoCAD Civil 3D models and how users can take advantage of the capabilities within their product. The information within a model allows designers to clearly convey design intent to their current and prospective customers. This often leads to a better understanding of the project itself, and a much smoother process leading to the end product.

The term “BIM” has been used for a few years now, however the definition of BIM has not been adapted to the recent enhancements in the design and construction industry. If you ask an Architect or Construction professional what “BIM” stands for, they may respond with “Building Information Modeling,” which is a term that focuses the intent of the model on the actual “Building” that is being designed. As engineers, when we think about “BIM” we should consider its meaning to be “Building an Information Model,” where the content is not strictly focused on the building, but on the entire model and all components making up the project.  

As a civil design engineer, AutoCAD Civil 3D is my tool of choice for creating and analyzing a Civil Engineering BIM Model. In this short article, we’ll take a look at one quick and easy way to create and analyze surface slopes using a text (.TXT) file from a Surveyor.

Step 1: Create a Civil 3D Surface using the Point File (do not import points)
  1. In your Civil 3D drawing, use the Ribbon’s Home tab → Create Ground Data panel to start the Create Surface Command
  2. Verify the Surface Type is TIN Surface, and provide a Name and select a Style for your surface. Optionally, provide a description and select a render material in this dialog box as well.
  3. Click OK to create the Surface. It is now listed in the Prospector tab of your Toolspace
  4. Expand the Surfaces collection (click on “+”) and expand the Surface object that was just created.
  5. Expand the Definition collection within the Surface object to view the different types of data that can be used to generate the model.
  6. Right-Click on Point Files and select Add
  7. Click on the “+” sign in the upper right corner to browse to your Point File (.TXT in the example).
  8. Select the Point File and click Open.
  9. Specify the Point File Format (PNEZD (comma delim) in the example) and click OK to create the surface.
The surface is automatically generated when you click OK. If you do not see the surface in your drawing, use the Zoom Extents command (double-click the mouse wheel) to zoom to the extent of your drawing.

Our Information Model now has a surface object that can be analyzed and used in our design. The power of a Civil 3D drawing does not fall in the objects you see on screen, but in the information that the drawing contains. Although we simply see contours representing our surface, we have all of the 3D information it contains available for use. In this example, we will do a quick Surface Elevation analysis to color-code the surface based on a range of elevations to find the high and low spots in our model.

The latest releases of AutoCAD Civil 3D have a much more user-friendly environment. For this example, we will take advantage of the Contextual Ribbon tab that appears when Civil 3D objects are selected. In most cases, if you wish to edit, analyze or use information from a Civil 3D object, the process begins by simply left-clicking on the object in the drawing.

Step 2: Surface Elevation Analysis
  1. Left-click on the Existing Ground Surface to activate the contextual Ribbon tab
  2. Click on the top half of the Surface Properties button in the Modify panel
  3. In the Surface Properties dialog box, switch to the Analysis tab
  4. Select Elevations from the Analysis Type drop down list
  5. Specify the number of ranges in the Ranges section (4 in the example)
  6. Click on the Run Analysis button
  7. Review the Range Details section to find out the Elevation Ranges and color scheme that was applied.
  8. Fine tune the Elevation Ranges and modify the color scheme if preferred. 
  9. Click OK to complete the Analysis.
At first glance, it may appear as though nothing happened in your drawing. What has actually happened is more information has been added to your model. In order to view the information, a modification must be made to the Surface Style. Civil 3D uses Object Styles to display the different aspects of a Surface in a drawing file.

The final step is to select or modify the Surface Style to display the new information that was just created through the Elevation Analysis.

Step 3: Displaying the Results of the Elevation Analysis
  1. Left-click on the Surface again, if the contextual tab is not already displayed.
  2. Click on the bottom half of the Surface Properties button to access the Edit Surface Style command.
  3. Switch to the Display tab of the Surface Style Dialog box and click on the light bulb symbol next to Elevations to make that feature visible.
  4. Click OK to exit and review the results of your Elevation Analysis.
It is recommended that your Civil 3D drawing template include a pre-defined style for analysis review. If it does include a pre-defined style for Elevations, you may use the Surface Properties command to assign that style to your surface instead. Editing the Surface Style using the steps above will affect all surfaces referencing that style in your drawing, and the Style must also be edited again after your review is complete to return it to the original state.
As you can see, the value in the BIM model lies in the information that it contains. Civil 3D provides users with a user-friendly way to leverage this information and convey their design intent to all parties involved in a project.
Next time you hear the term “BIM,” remember to think of it as “Building an Information Model”, rather than simply modeling a building.

Matt Miyamoto, P.E.
Infrastructure Solutions Application Specialist

Matt is a licensed Civil Engineer in the state of Hawaii. Matt obtained a BS in Mechanical Engineering from the University of Hawaii at Manoa, and has 7 years of private sector design experience which he applies in his role as an application specialist with Ideate, Inc. His project experience includes residential and commercial site development, private and public sewer, water and drainage systems, harbors improvements, and roadway improvements. While in Hawaii, Matt was involved in multidisciplinary projects for City and County agencies, State Departments, the Army COE and private developers. @MattM_PE

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