Introduction to Civil 3D Corridors

n case you've been too glued to your monitor lately to notice the media blitz, Civil 3D is the future replacement of Autodesk's Land Desktop and Civil Design products. Civil 3D 2006, now in its second full release, is coming into its own. One of the biggest improvements is the road design system, called Corridor Design.

This system has a more general name in Civil 3D since it is equally applicable to roads, bike paths, rail corridors, levees, designed ditches and swales; just about any kind of linear design project. The system creates corridor objects -- 3D representations of your project based on a horizontal and vertical alignment and a proposed cross section (called an assembly). As with almost every aspect of Civil 3D, these corridor objects remain dynamically linked to the alignments and the assemblies they are based on. That means that if you adjust one of your alignments or assemblies, the 3D road model can update automatically, together with proposed contours and volume calculations.

Overview of Assemblies
The core elements of the corridor system are assemblies and their component subassemblies. All of you Civil Design veterans can forget Civil Design templates and subassemblies -- welcome to the world of dynamic proposed sections. The assembly object is essentially an anchor used to represent the profile grade point in the proposed cross section. This usually translates into the crown point of a two-lane roadway. Once you have located an assembly object in your drawing (Corridors / Create Assembly), you then attach subassemblies to it.

Subassemblies are segments of a proposed cross-section: a paved lane, shoulder, curb, sidewalk, etc. A number of sophisticated assembles are available out-of-the-box, and you can create your own components from polylines or links. (Creating your own custom subassemblies is a topic best left for a future article). You can browse the stock subassemblies through the Civil 3D Catalog (General / Catalog) or through several preconfigured tool palettes (figure 1).

Figure 1. Attaching a subassembly to an assembly with a tool palette.

Civil 3D subassemblies are highly customizable through their object properties. For example, the lane section subassemblies have width, depth and slope parameters (figure 2). These numeric parameters allow the same subassembly to represent dozens of different design scenarios. Plus, it means that if you want to change a lane width, for example, all you have to do is change the width parameter. Look Ma, no drafting!

Figure 2. Easily change the properties of a subassembly.

You can add subassemblies to the assembly quickly and accurately by clicking on the desired component in a tool palette and then clicking on the assembly itself (for components right at the baseline) or by clicking on the circles (markers) located along other subassemblies. You can build a completed assembly by stringing it together one subassembly at a time.

In figure 3 below, I have designed a simple subdivision roadway with two 12-foot travel lanes (called LaneOutsideSuper), curbing (UrbanCurbGutterGeneral), sidewalk sections (UrbanSidewalk --with a sidewalk and grass strip on one side and only a grass strip on the other) and a special daylighting subassembly that will allow for ditches when in a cut condition (BasicSideSlopeCutDitch).

Figure 3. The sample corridor.

Once you have a horizontal and vertical alignment and a completed assembly, your road design is ready to roll. You can create corridor objects through the Corridors / Create Corridor command (figure 4). After choosing your alignments and assembly, the Create Corridor dialog box appears. This dialog box replaces many of the section sampling and design control functions of Autodesk Civil Design Companion. You can assign each segment of the design that needs unique settings (different assemblies, different station sampling, etc.) as separate regions. Each region can have appropriate start and ending stations, different assemblies, sampling frequencies, etc., and all of this assigned data is available in a table for review and editing later on. (This beats the heck out of Civil Design Companion's Edit Design Control dialog box, where you couldn't tell what unique settings you've applied and where.) Corridors can even include multiple baselines for intersecting roads, ramps, double-barreled highways, etc.

Figure 4. The Create Corridor dialog box.

After setting the appropriate section sampling, starting and ending stations for your corridor, the real fun begins. As with many Autodesk products, the most important controls have the most subtle and mysterious names. The ellipsis button () in the Logical Name field calls up the Logical Name Mapping dialog box where you assign which surface you would like a region to daylight onto, as well as assign transition alignments and profiles (more on this later). For this first corridor we will simply assign the TargetDTMs to our existing ground surface and click OK (figure 5).

Figure 5. Configure the regions of the sample corridor.

After configuring one or more regions and setting the necessary logical names, you are ready to build your corridor -- click OK in the Create Corridor dialog box. Once Civil 3D has worked its magic, you should see a wireframe model of your 3D roadway design in the drawing, which defaults to blue linework (figure 6).

Figure 6. A wireframe model of the sample roadway design.

Again, this corridor object is dynamically linked to your alignment, vertical alignment and assembly. If you make changes to any of these design features, the corridor will update after right-clicking on the corridor icon in the Prospector and choosing Rebuild (figure 7). The Rebuild Automatic option is great if you don't make many changes to your designs or have a lot of time to kill. (If you have a number of edits to make at once, you may want to manually choose to rebuild when you're finished, rather than wait for a rebuild after each change with the automatic option.)

Figure 7. The Rebuild command.

You can view the corridor model in a shaded 3D mode by changing to a 3D viewpoint or through the AutoCAD object viewer. You can also create contours if desired for the final plan or as an analysis tool. To view the contours describing the corridor top surface, select the corridor, right-click and choose Corridor Properties. In the Surfaces tab click the Create a Corridor Surface button (far upper left) to create a surface with the name [Corridor name] Surface - (1).

The subassemblies that ship with Civil 3D already know what their top surface geometry looks like. Specify the links on the top of the corridor with the drop-downs menus, and then press the Add Surface Item button (figure 8). You could then double-click on the surface name and rename it. To view the contours describing this top surface, make sure the style shown for this surface has contours displayed (such as the example Border and Contours surface style available in the _AUTODESK CIVIL 3D IMPERIAL BY LAYER.DWT template).

Figure 8. Add surfaces to your corridor.

To complete the surface, click the Boundaries tab. Right-click on the surface you just created and automatically add a daylight boundary (figure 9), which prevents the surface from triangulating across the inside of a corner. An automatic daylight boundary is valid whenever a daylight subassembly is attached to both ends. Click the OK button to complete the surface definition. Surfaces created through corridors will also remain dynamically linked to the design alignments and assembly; rebuilding a corridor will also rebuild these surfaces.

Figure 9. Adding a daylight boundary.

Simple Transitions
Civil 3D can transition proposed cross-sections so that a single assembly can horizontally enlarge or compress to represent a changing cross-section. For example, the corridor in figure 10 is shown on the left with a constant width. The graphic on the right shows the lane section widening from one lane to two in order to make room for a right-turn lane. Civil 3D can also apply vertical transitions to allow a point in the cross-section to follow a specific profile. Applying this kind of simple transition in Civil 3D is a lot easier than it may look.

Figure 10. The corridor on the left has a constant width. The corridor on the right shows the lane section widening from one lane to two in order to make room for a right-turn lane.

Horizontal transitions are defined by additional offset alignments. In figure 10 the right edge of pavement (EOP) is defined as a new alignment. Since this alignment will not be used as a baseline, its stations and alignment direction do not need to match or even line up with the road centerline.

Our example assembly included LaneOutsideSuper subassemblies with built-in controls for width and lane outside elevation. To assign transitions select the corridor, right-click and again choose Corridor Properties. In the parameters tab press the ellipsis button ( ) in the Logical Name field for the transitioning region to again open the Logical Name dialog box. Notice the fields where you can define the width and outside elevation of each LaneOutsideSuper, as well as the width (Target_HA_Alignment) of the UrbanSidewalk subassemblies. To attach an offset (transition) alignment for the right lane, change the Object Name field from None to your transition alignment (such as R EOP, figure 11). Click OK to reprocess your corridor and voila! The corridor should transition right on cue.

Figure 11. Attaching an offset alignment.

To summarize, Civil 3D's cross-section subassemblies allow for dynamic, parameter-driven design for roadway and similar projects. Although Civil 3D could use more of them, the example subassemblies that ship with the product are fairly robust and widely applicable to many design tasks. Civil 3D creates 3D corridor objects to fully represent the proposed condition based on these assemblies, allowing for automated contour creation as well as other design tasks not covered in this article (such as quantity takeoffs). These parametric corridor objects can take into account multiple baselines for intersections as well as transitioning offset alignments and profiles for varying widths and elevations. All in all, this parametric design system is a huge and long-awaited step forward for civil designers.