Superelevation and cant are methods for changing the cross section of a design to keep cars and trains safely and comfortably on their paths when driving along a curve or series of curves. Superelevation tools also provide a convenient way to force the slope of a road for drainage purposes, without the need for additional assemblies.
Once you have a good grasp of alignments, assemblies, and corridors, you can add a level of sophistication to your design with the superelevation and cant tools within the Autodesk® AutoCAD® Civil 3D® software. Whether you are trying to match into an existing road's superelevation or creating new data, you will find the tools you need.
In this chapter, you will learn to
Before superelevation can be applied to the design, you will need a design criteria file appropriate for your locale, design speeds applied to an alignment, an assembly with subassemblies that recognize superelevation, and an understanding of superelevation critical stations.
Critical stations mark “milestones” in a superelevated region of an alignment. These points are located by means of calculations driven by the design criteria file.
Now that you are familiar with the terminology used to describe transition stations, you are ready to examine design criteria.
Having the correct design criteria file in place is the first step to applying superelevation to your corridor. These XML-based files contain instructions to the software on when to flag your design for geometry problems both horizontally and vertically. Design criteria files are the brains behind how your road behaves when superelevation is applied to the design.
Several design criteria files are supplied with Civil 3D upon installation. The out-of-the-box standards include AASHTO 2001, AASHTO 2004, and AASHTO 2010 for both metric and Imperial units. Several of the country kits include design criteria files for your locality if you are outside of the United States. If country or state kits do not exist for your situation, you can create your own user-defined files.
To create your own design criteria, follow these steps:
Inside the Design Criteria Editor (Figure 11.6), you will see three headings: Units, Alignments, and Profiles. The Units page tells Civil 3D what type of values it will be using in the file. The Alignments page is used for checking design, creating superelevation, and widening outside curves. The Profiles page provides tabular data for minimum K-value used to check vertical design.
Civil 3D will graphically flag alignments when the design speed specified in the alignment properties has a radius less than the value specified in the Minimum Radius Table. The Minimum Radius Tables from AASHTO use superelevation rates in the table names, but this does not lock you into that rate for applying superelevation to the corridor. In other words, just because you use a more conservative value in your radius check, that doesn't mean you can't superelevate at a steeper rate. The tables are independent of one another.
Also in the Alignments branch you will find the superelevation attainment equations. These equations determine the distance between superelevation critical stations. Familiarize yourself with the terminology and locations represented by these stations, as shown in Figure 11.7.
In the following exercise, you will modify an example design criteria file and save it:
1101_Criteria.dwg
(1101_Criteria_METRIC.dwg
) file, which you can download from this book's web page at www.sybex.com/go/masteringcivil3d2015. This exercise will also require 1101_CriteriaExample.xml
(1101_CriteriaExample_METRIC.xml
).1101_CriteriaExample.xml
(1101_CriteriaExample_METRIC.xml
) file. Click Open.There is only one superelevation table in this example for 4% maximum slope.
When you expand the SuperelevationTypeByTable branch, you will see that Civil 3D has created a new table with a default design speed of 10 regardless of the units you are working in.
Notice that you need only the numeric value; Civil 3D fills in the rest of the table name for you.
The right side of the dialog will display an empty table containing columns labeled Radius and Superelevation Rate.
Curve radius (feet) | Curve radius (meters) | Superelevation % |
300 | 90 | 6 |
1000 | 300 | 4.5 |
1500 | 450 | 3.2 |
2000 | 600 | 2.6 |
4000 | 1250 | NC |
When you have completed your data entry from the table, the US units Design Criteria Editor will resemble Figure 11.9.
Superelevation stations are connected to alignment curves (unless you create a user-defined curve in a tangent section). The design speed from the alignment properties is needed at each curve to specify which superelevation rate tables to use from the design criteria. The design speed has an effect on the distance between superelevation critical stations and the cross slope used when the road is at full-super.
It is a good idea to get your alignment geometry and design speed locations finalized before attaching superelevation. If a change is made to your alignment, the superelevation stationing will be marked as out of date.
As a general rule, if the lane subassembly has the word super somewhere in its name, it will respond to superelevation. If you want to verify that the lane you are choosing will behave the way you want it to in a superelevation situation, you can right-click it from the tool palette and access the subassembly help.
As long as you stay away from the Basic tab, all of the shoulder and curb subassemblies have parameters you can set to dictate how the assembly is to behave when an adjacent lane superelevates.
Most subassemblies that are capable of superelevating are intended for use where the pivot point for the cross section is at the center crown of the road. When the pivot point is at the center of the road, the baseline profile dictates the final elevation of the crown of the road. Figure 11.10 shows an example of a two-lane highway (top image) and a four-lane divided highway (bottom image) that are designed to be used with the superelevation tools.
The axis of rotation (AOR) subassembly can be used when the centerline of the road is not the pivot point for superelevation. The flag symbols (as shown in Figure 11.11) indicate potential pivot points on the assembly.
The flag symbols on LaneSuperelevationAOR indicate where the lane can be pinned down and used as a pivot point. When the axis of rotation is not the centerline of the road, the lane geometry is used to determine the change in elevation that will occur as a result.
When building assemblies with LaneSuperelevationAOR you may see warnings appear, as shown in Figure 11.12.
Here are some of the warnings you may encounter:
You can still add assemblies with warnings to a corridor; however, the superelevation may not behave as expected.
Civil 3D takes into account other factors such as curve station locations and assembly geometry. Superelevation information is associated with the alignment but is handled in a separate calculation area. In this section, you will put all the pieces in place that are needed for the software to dynamically apply superelevation or cant to your design.
To begin applying superelevation to the design, select your alignment:
1103_Super.dwg
(1103_Super_METRIC.dwg
) file, which you can download from this book's web page.Autodesk Civil 3D Imperial (2004) Roadway Design Standards.xml
(Autodesk Civil 3D Metric (2004) Roadway Design Standards.xml
).You should now see the Superelevation Tabular Editor appear inside Panorama with the data resulting from the wizard. Examine your alignment; you should now have labels showing the superelevation critical stations created by the wizard.
As you click in the table, you will see helpful glyphs showing you which superelevation station and corresponding curve you are editing, as shown in Figure 11.17.
Compare your work with 1103_Super_FINISHED.dwg
or 1103_Super_METRIC_FINISHED.dwg
.
It is not uncommon to have overlap warnings in your superelevation table. You should resolve the transition station overlap before you continue your design.
Overlap occurs when there is not enough room between curves to fully transition out of one curve and back into the next. Transition station overlap will always occur when a reverse curve or compound curve exists in your alignment. As you can see in Figure 11.18, Curve 1 does not complete its transition out until station 9+16.64, but according to the attainment calculations, Curve 2 will begin affecting the shoulder starting at station 5+98.21.
You have several options for fixing superelevation overlap:
To have Civil 3D clear the overlap for you, click the warning symbol that appears in the Superelevation Tabular Editor. Civil 3D resolves overlap by omitting noncritical stations and/or by compressing the transition length between certain stations. In the case of a reverse curve, Civil 3D will pivot the road from full-super to full-super, without transitioning back to normal crown. Be sure to verify that the software has made the update that meets the requirements of your locale.
In the civil engineering industry, the terms superelevation and cant are used interchangeably. Inside Civil 3D, the terms have distinct meanings. Superelevation tools in Civil 3D are used for roads; the cross-slope changes within a curve are expressed by a percentage. Unlike superelevation, cant applies to railways and is expressed as a difference in height between the outer and inner rails (Figure 11.19).
In order to work with cant, the following must be in place:
You will find the RailSingle subassembly in the Bridge And Rail tab of the tool palettes. The following exercise will walk you through creating a typical rail bed design:
1104_Rail.dwg
(1104_Rail_METRIC.dwg
), which you can download from this book's website.
This drawing contains an alignment and design profile.
Leave all other parameters at their defaults.
This will be your service road, constructed out of the same material as the subballast.
Your completed assembly will look like Figure 11.23.
Compare your work with 1104_Rail_FINISHED.dwg
or 1104_Rail_METRIC_FINISHED.dwg
to see how you fared.
Like superelevation in a roadway alignment, cant is related to a rail alignment. The following exercise will walk you through applying cant to the alignment. You should experience a distinct feeling of déjà-vu if you completed earlier exercises involving applying superelevation to the alignment.
1105_RailAlignment.dwg
(1105_RailAlignment_METRIC.dwg
), which you can download from this book's website.1105_RailAlignment_FINISHED.dwg
or 1105_RailAlignment_METRIC_FINISHED.dwg
if desired.Superelevation and cant views are a graphic representation of the roadway or rail superelevation. Grip edits to the graphical view will also edit the superelevation stations. The view itself is not intended for plotting. The superelevation view plots station against lane slope to form a graph of the left and right edges of the pavement.
In the following exercise, you will create a superelevation view:
1106_SuperView.dwg
(1106_SuperView_METRIC.dwg
), which you can download from this book's website.At first glance, the superelevation view may seem difficult to read, but with a little explanation it can shed a lot of light on what is going on with your lane and shoulder slopes. The superelevation graphic plots the station value against the percent cross-slope of each edge of pavement and edge of shoulder. The upper line shows the behavior of the right edge of the pavement, and the lower line shows the left edge of the pavement.
Where no superelevation is applied, the graph data for the lanes remains at -2% while the shoulders are shown at their default cross-slope of -5%. As the assembly twists into position during superelevation, the distances between the lines become greater as the right edge slopes up to a maximum superelevation of 4%.
There are a few more observations you can make about your superelevation view. No overlap exists between the two curves' superelevation data. You can tell this by observing the center portion of the graph; the superelevation lines go back to the default cross-slope. The very astute observer can ascertain by looking at this graph that a maximum breakover slope of 8% was used on the shoulder. How can you tell? By seeing that when the lanes are in max super (i.e., the lane is at +4% slope), the shoulder slope jumps up to -4%.
Next, you will use the superelevation view to edit superelevation data. Editing superelevation data by the superelevation view is an alternative to editing the data in tabular form, as you learned about earlier.
The diamond-shaped grips can be slid in one axis to modify stationing (the horizontally oriented grips) or slope (the vertically oriented grips). The rectangular grip can be moved to reduce the maximum lane slope when it is in a full-super state, as shown in Table 11.1.
Table 11.1 Superelevation view grips
Superelevation View Symbol | Meaning |
Grip (blue) is at a superelevation critical station and a grade break occurs at that station. Vertically oriented grips can be moved up or down to change the slopes associated with them. | |
Grip is at a superelevation station. Horizontally oriented grips change the value of superelevation stations. | |
Grip will appear at locations of constant slope. These can be moved up or down to change the superelevation cross-slope. | |
Grip (gray) is at a superelevation critical station, but no grade breaks occur at the location. Vertically oriented grips can be moved up or down to change the slopes associated with them. | |
The plus sign next to any grip indicates that more than one item is the same slope at that station. |
In the following exercise, you will use the superelevation view to remove the normal crown area in the middle of the alignment. In other words, you will force the curves to transition directly from one to the other. You will also adjust the rate of maximum superelevation.
1106_SuperView.dwg
(1106_SuperView_METRIC.dwg
). You need to have completed the previous exercise before continuing.1106_SuperView_FINISHED.dwg
or 1106_SuperView_METRIC_FINISHED.dwg
if desired.MasterIt1101.dwg
(MasterIt1101_METRIC.dwg
) file, which you can download from www.sybex.com/go/masteringcivil3d2015. Verify that the design speed of the road is 20 miles per hour (35 km per hour) and apply superelevation to the entire length of the alignment. Use AASHTO 2004 design criteria with an eMax of 6% 2-Lane. Use the option to automatically resolve overlap. For the remainder of the options, use the default settings unless otherwise directed.MasterIt1101.dwg
(MasterIt1101_METRIC.dwg
). You must have completed the previous exercise before starting this one. Create an assembly similar to the one in the top image shown earlier in the chapter in Figure 11.10. Set each lane to be 14′ (4.5 m) wide and each shoulder to be 6′ (2 m) wide. Leave all other options at their defaults. If time permits, build a corridor based on the alignment and assembly.MasterIt1102.dwg
(MasterIt1102_METRIC.dwg
), create a Railway assembly with the RailSingle subassembly using the default parameters for width and depth. Add a LinkSlopeToSurface generic link with 50 percent slope to each side. Add cant to the alignment in the drawing using the default settings for attainment. Create a corridor from these pieces.MasterIt1103.dwg
(MasterIt1103_METRIC.dwg
). Create a superelevation view for the alignment. Show only the left and right outside lanes as blue and red, respectively.3.137.223.10