What is the best practice to model curbs using feature lines

[RD-JLD-Eng]

I am knew to commercial grading and I am wondering what is the best way to model curbs using feature lines. I know that civil 3d doesn't allow to offset feature lines vertically, when you get a site plan from an architect with curbs represented in two lines, do you target the curb lines in the referenced site plan and then offset the face of curbs a small distance horizontally (say 1") and -0.5' vertically? or you draw the offset lines in the site plan and then you target everything in the referenced site plan? or there is a better way to do this? Just curious what's the best approach to this. 

Thank you and happy new year. 

[fcernst]

First you typically have to correct and tighten up the linework you get from the architect.. make sure everything is tangential, etc

Then you can use a Corridor workflow with curb subassemblies so you don’t have to worry about offsetting curb Featurelines. 

[Mohamed_Eiisa]

You can use subassemblies with curbs in it .. or you can offset feature line vertically if you give it a simple (.001m offset) horizontally first

[Domziman]

look what i have done here: For example if the kerb is 150mm wide offset the feature line 200mm higher and then 1mm to the side, the difference is so small its not easily visible

[Neilw_05]

I recommend you make that small offset a larger distance. You may find c3d has triangulation issues with very small offsets. Using 0.05' (15 mm) or more usually works well..

[Neilw_05]

You have the corridor option as mentioned. You also can use gradings which will create dynamic horizontal and vertical offset featurlines from the edge of pavement or back of curb geometry. You would use the horizontal distance with relative elevation or slope criteria to establish the gutter flowline and face of curbs. You can create criteria sets with predefined parameters to save you having to input the parameters every time you apply them. Whether you use corridors or gradings, you will have to apply a slight offset to any vertical faces as c3d cannot model perfectly vertical faces.

There are pros and cons to both approaches (corridor VS. gradings). Gradings are easier to use but are prone to corruption, but generally they are stable when modeling curbs this way in my experience.

[RD-JLD-Eng · Accepted solution]

Thank you all for your responses. This helps a lot. 

[ChrisRS]

You already got some excellent advice.

Here are my somewhat meandering thoughts.

1. Feature line based corridors can model curbs.
   This exhausts my knowledge of feature line based corridors.
2. You can use feature line grading and Stepped Offsets as suggested in posts 3 and 4.
   (Stepped or Offsetfeature) 
   The source and offset feature lines are not dynamically linked.
   1. Once added to a surface, as a breakline, or via a grading object, curve segments become tessellated.
   2. The curve radius and the mid-ordinate distance determine the tessellation segment length.
   3. As noted in post 5, C3D can have triangulation issues with very small offsets.
      Tight feature line spacing and tessellation segment length can cause feature lines to cross, and it would be bad. I am talking wonky surface bad, not total protonic reversal bad, but still bad.* 
3. As noted in post 6, The "Grade to Distance" and "Grade to Relative Elevation" grading tools can be used to create Grading Objects that model curbs.
   
   1. These grading objects are dynamically linked to the base feature line.
   2. These Grading Objects do not exhibit the triangulation issues notes in 3. c.
   3. Grading Objects are powerful. Grading Objects can be finicky, fragile, unstable, and self-corrupting.
      When using Grading Objects, you should save often. It is a good practice to save between saves. 
4.  A productivity hint: Reference Surfaces
   1. Use a reference surface ** to define the overall pavement elevations.
   2. Feature lines defining the curb flange, lip, or gutter location should be "relative" to the reference surface with Elevation (Relative) = 0.00
   3. Feature lines defining the top of curb location should be "relative" to the reference surface with Elevation (Relative) = Curb Height.
   4. These feature lines will be dynamically linked to changes made to the reference surface.
   5. ** Secondary reference surfaces can be made by pasting in the reference surface and applying an edit Raise/Lower Surface.
      i.e., Raise 0.50 feet for a 6-inch curb.
      Top of curb features would be relative to a secondary reference surface with an Elevation (Relative) = 0.00
   6. There was an example of this in a recent post:   
      Link: commercial-grading-setup-surface-vs-finish-grade-surface 
5. A Conundrum: Labeling Curbs.
   1. In my experience, curb labels show two elevations. Top of curb and "adjacent" Pavement, gutter, lip , or flange.
   2. Civil 3D surfaces never have vertical faces. A vertical curb face needs to incorporate some batter (horizontal offset).
   3. In my experience, curb labels are typically surface spot elevation labels. The surface has exactly one elevation value that is available to the label. The secondary elevation needs to come from something else.
      1.  An expression is often used to display this secondary elevation.
         T.C. = {Surface Elevation} + 0.50
         G = {Surface Elevation} - 0.50
      2. Using secondary reference surfaces, you can use a reference text component in your label definition.
         Reference Text Component:  "T.C. = (Surface Elevation)"  - target surface = "TC Ref"
         Primary Text Component:  "G. = (Surface Elevation)"  - target surface = "Pvmt Ref"
6. This discussion has been about How to model curbs.
   There is merit in considering the Who, Why, and What of modeling curbs.
   This is part of my ongoing process of reminding myself that Sometimes Good Enough  ...  Is Enough.
   
   
   
   1. Who and Why are closely liked. Possibility include:
      1. Owner, Contractor, and Surveyor
         To document my site design to an appropriate level on Production Drawings
         (Plan and profile, cross-section, etc.)
         It may help with water drop analysis.
         Object viewer might help me visualize during design, but it is a secondary concern.
      2. Other members of the design team.
         To provide an accurate, usable surface to other design team members.
      3. Specialty
         To provide a base for photorealistic renderings.
         Even here, consider that a 6" high curb is not very prominent in an elevation view of a 20-foot tall building.
   2. What do I need/want to model for curbs? Who and Why greatly influence this. 
      1. Using expressions to facilitate T.C. and P. labels for parking islands may eliminate the need to model the back of curb line. The surveyor doing the layout may want the back of curb points that can be tied to a secondary reference surface for elevation.
      2. We typically draw 3-line vertical curbs. The back of curb, used for layout and for designing attached sidewalk, may need to be modeled. Top Face of curb will need for proper display of contours crossing the gutter pan, but is that level of detail required? I do not know of any contractors that will look for it.
      3.  Does one need to model ramps fully? Probably. This is a place for extra effort.
      4. Street design typically needs a lip of pavement and the back of curb.  
   3. Tips
      1. Refrain from modeling unnecessary detail.
      2. Avoid "chasing a hundredth."
         A 0.02' simplification across a 40' wide drive aisle + parking stall is a 0.05% cross slope deviation.

Good Luck!

1.  

* Bonus:
Crossed feature line always reminds me of Ghostbusters (1984)

Egon Spengler:
There's something very important I forgot to tell you.

Peter Venkman:
What?

Spengler:
Don't cross the streams.

Venkman:
Why?

Spengler:
It would be bad.

Venkman:
I'm fuzzy on the whole good/bad thing. What do you mean, "bad"?

Spengler:
Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light.

Ray Stantz:
Total protonic reversal.

Venkman:
Right. That's bad. Okay. All right. Important safety tip. Thanks, Egon.

[Joe-Bouza]

Not to sound rude but keep in mind what we are building. If you really need to show is section then the offsets will need to comply. If all you want is to grade the back of curb offset .5.

I see some folks use three lines. I would run a curb assembly before going there

[RD-JLD-Eng]

Chris, 

Thank you for your input. These are very detailed and helpful points. I read your response multiple times, it's very informative. 

[RD-JLD-Eng · Accepted solution]

Good point Joe, thank you.