WFL Parapet Wall Design (with .x30 Criteria)

Federal Lands Highway has a variety of retaining walls available within the FLH criteria library. The basic design and layout of these walls are all very similar. The criteria is intelligent enough that as the wall stem increases in height, the footing thickness and width increases dynamically using standard structural formulas that can be modified as necessary. I will attempt to illustrate a step by step procedure for designing what we refer to as a Concrete Parapet Wall, a wall commonly used here in Western Federal Lands Highway.

An example of a typical WFL Concrete Parapet Wall, (including a description of the wall adhoc attributes) is shown below:

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Adhoc Name: Description:
Parapet Thickness Parapet wall thickness at top of wall, expressed in master units.
Parapet Height Parapet wall height from top of pavement to top of wall, expressed in master units.
Stem Thickness Parapet wall stem thickness, expressed in master units.
Wall Width Factor Value used for mathematically determining the minimum width of the wall based on the wall height.
Footing Thickness Thickness of parapet wall footing, expressed in master units. This is considered a minimum and will be compared against the height of the wall using the formula (wall height / 10). The greater of the two will be used.
Embedment Depth Minimum depth below existing ground measured to the bottom of the footing. Expressed in master units.
Surcharge Height Parapet wall surcharge height, expressed in master units. This simply gives our Structures engineers the option to design an additional load on the wall structure. Obviously, this is a value that needs to be assigned by the Structures engineer.
Structure Exc Width Excavation width beyond the parapet wall footing edges, expressed in master units.
Wall Excavation Slope Slope of the wall excavation on the back side of the wall, expressed in rise:run.
BottomFootingProfName Name of the designed proposed bottom of footing profile. This is used in the second phase of the wall design.
TopFootingProfName Name of the designed proposed top of footing profile. This is used in the second phase of the wall design.
WallChainName Name of the chain used for the face of wall stationing, (independent stationing from centerline).

FLH has two sets of criteria now available for use:

  • .X21 criteria (uses redefinable variables for everything, including plan view elements). This version of the criteria does not include the ability to design the walls using chains and profiles along the wall, where the .x30 criteria does.
  • .X30 criteria (replaces redefinable variables for plan view elements with adhoc attributes).

The .X30 criteria is the most recent set of scripts, so these instructions will be using the .X30s.

Retaining walls available with the .x30 criteria are shown below:

This design concept consists of a multi-step process that involves not only the designer, but also the Structural and/or Geotechnical Engineer, with the final result being a set of proposed cross sections that reflects a constructible wall with stepped footing elevations, stepped footing widths and accurate earthwork volume calculations.

The "Steps" in Brief

  1. Create the initial proposed cross sections that reflect a retaining wall resulting from default values such as:
    • Embedment Depth
    • Wall Excavation Slope
    • Footing Thickness
  2. From these proposed cross sections, use GEOPAK's Profile Grade Report and store several chains and profiles from "search text" (automatically drawn on the cross sections by the criteria) reflecting:
    • Gutter line
    • Top of Wall Footing
    • Bottom of Wall Footing "Front Face"
    • Bottom of Wall Footing "Back Face" (chain only, profiles actually are not required for both edges of the bottom of the footings)
  3. Create an elevation view layout that reflects the 3 profiles. Provide this to the Structural and/or Geotechnical Engineer.
  4. Easily determine minimum footing width requirements. Provide this to the Structural and/or Geotechnical Engineer.
  5. From the elevation view and plan view layouts, the Structural and/or Geotechnical Engineer will perform the actual FINAL wall layout, reflecting stepped footing elevations, stepped footing widths and optionally a parabolic top of wall profile.
  6. From the final layout, store profiles into COGO reflecting the bottom and top of the footings the top of the wall.
  7. Define the widths (stepped widths) of the footings in plan view.
  8. Delete the initial proposed cross section elements, created in Step 1.
  9. Recreate the proposed cross sections using the stored wall profiles.

Step 1 ... Create the initial Proposed Cross Sections

All of the retaining walls available to design using the criteria are driven by plan view elements. In some cases the actual horizontal offset position of this plan view element has no effect on the location of the wall in the cross sections, but simply acts as a trigger to tell the criteria to draw a wall against the edge of the pavement. The Concrete Parapet wall is one of the walls that simply uses these plan view elements as a trigger.

Note: For more information on how the other walls react to these plan view elements, see the documentation included in the "help files" available by clicking on the Description button from within the Project Manager run. A fragment of the dialog is shown below:

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In our example here the edge of the pavement is 12.0 feet from centerline. Since the actual offset of the line doesn't matter in the case of this wall, draw the "trigger" line somewhere past the edge of pavement, I suggest some round figure such as 14 feet for the offset line. This wall will be located right of centerline from 60+00 to 60+50.

Invoke the Design and Computation Manager, I'll select Prop. Parapet-1 Wall (WFL) and assure that Place Influence and Adhoc Attributes are toggled ON.

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Adjust the Adhoc Attributes "default values" as desired, by double clicking in a field under the Value column, (as shown below):

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Using Draw Transition, I'll draw a line 14 feet right from 60+00 to 62+50, as shown below.

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Below is a screen capture illustrating the Prop. Parapet Wall line, drawn 14 feet right of centerline:

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Create the proposed cross sections as usual. Below is the result at station 62+50 illustrating the Concrete Parapet wall, drawn 12 feet right of centerline, even though the plan view element was drawn at 14 feet right of centerline:

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Step 2 ... Store the Chains and Profiles

NOTE: You must have GEOPAK Road 2004 Edition version or newer installed in order for the Profile Grade Report to work properly.

As mentioned earlier, in addition to drawing all the proposed cross section elements, the criteria also places "search text" at strategic locations that can be used by other GEOPAK applications... such as Profile Grade Report. If we look closer at the corners of the footings and at the top of the wall, we will see the "search text" as illustrated below from zooming in at each corner:

Gutter Line (11)
(Could be 12 if using widening)
Top of Wall Footing
Bottom of Wall Footing Face
Bottom of Wall Footing Back
Screen shot Screen shot Screen shot Screen shot

We are going to use these pieces of search text to store Profiles and Chains into COGO using the Profile Grade Report.

The next step is to create an elevation view of the initial position of the gutter line, and top and bottom of the footing, resulting from the first cross section run.

It is important to note that at this point each cross section is independent from the other cross sections and have no relation to each other. In other words the result of the initial first run of cross sections does not produce a constructible wall design. The footing profile, footing thickness and footing width will vary depending on the height of the wall, the setback and embedment depth. In order for this wall to be constructible, the footing profile needs to be flat or stepped in flat sections and the footing thickness and width needs to be formed in stepped thicknesses and widths.

First let's store the chains and profiles that represent where the gutter line , and the top and bottom of the footings would be positioned initially using the default adhoc values.

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As you can see from above, it is possible to store all the chains and profiles at the same time. I would recommend setting the dialog to Sta. Text Alignment in order to reflect the true length of the chains/profiles.

Once the symbology for Existing Ground Line and Proposed Finish Grade are selected and Apply is pressed, the chains and profiles are stored into COGO as shown below:

NOTE: All chains and profiles will be stored with the stationing beginning at 0+000 (0+00 if your design is US Customary).

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Step 3 ... Create the elevation view layout

We can all agree that (in non-tangent wall sections) the actual length along the edges of the footings, the gutter line are all different since the offsets from centerline are different. After lengthy discussions with our Structural Engineers, we decided that for simplicity, wall face stationing should be used when designing the elevation view. Wall face stationing will provide a true length along the face of the wall.

Using the Draw Profiles tool, draw the Profiles previously stored into COGO. Create the Profile cell using the Gutter line chain. The other profiles will be "projected" to the gutter line profile cell.

The graphic below illustrates drawing the Profile cell using chain GUTTER and drawing the COGO profile GUTTER that represents the gutter line.

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The graphic below illustrates "Projecting" the other profiles to the Profile cell that was created from chain GUTTER:

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NOTE: Later, (in Phase 2) the criteria will be driven by 2 profiles:

  • Top of Proposed Footing
  • Bottom of Proposed Footing

Give this drawing (above) to the Structural/Geotechnical engineer for final elevation layout. You may want to add grid lines to make it easier for them to layout the final wall design. We'll get back to this later.

Step 4 ... Easily determine minimum footing width requirements

It is very easy to give the individual responsible for the final layout, a printout that will assist in their layout. For instance, it would be useful to know what the actual offsets are for these edges of the footings so that "ranges of stepped widths" can be determined.

To create a list of offsets from one chain to another, use COGO's Layout Offset tool as shown below:

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Adjust the Layout Offset Chain dialog to Chain (Radial Intersection) as shown below. Obviously, your chains, station ranges and increments will be different.

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COGO will display the results as shown below. What we're after here are the values in the second column. This represents the width of the footings at each cross section for the initial run.

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Create an output listing of the results using COGO as shown below:

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Create the output listing by adjusting the dialog as shown below:

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The file that is created, uses GEOPAK's file naming structure that includes the job number and operator code. For this example, the file that was just created is wdth130b.osp. This is still just a simple ASCII file and can be renamed to something else more conventional. Give this file the Structural/Geotechnical engineer for final layout for assistance in designing the "stepped" footing widths.

Step 5 ... The Final wall layout

With the information that you have now given to the Structural/Geotechnical engineer, they now know what the limits of the wall are concerning the minimum thickness that the footing needs to be, the minimum width of the footing required to support the stem height and they also have an elevation view for them to design the final layout.

Using the elevation view and the ASCII file illustrating the minimum required widths for the footing, the Structural/Geotechnical engineer would perform the following:

  • Layout the top of the wall footing.
  • Set the thickness of the wall footing (this is done by laying out the profile for the bottom of the footing).
  • Set the width(s) of the wall footing.

The Structural/Geotechnical engineer does not need to use MicroStation or GEOPAK to do this. This can be done on graph paper or whatever. You will then take these values and do the work in MicroStation/GEOPAK.

Let's imagine that after some thought the final layout has been done. You have been given the following by the Structural/Geotechnical engineer:

  • Stations and elevations for stepping the bottom of the footing.
  • Station ranges illustrating various footing thicknesses required.
  • Station ranges illustrating various footing widths required for the stem heights.

Step 6 ... Designing the final wall profiles

Once you have obtained the profile information from the Structural/Geotechnical engineer, use the Profile Generator to store the profiles into COGO. A fragment of the finished wall layout is shown below:

Screen shot

Step 7 ... Define the widths (stepped widths) of the footings

In the Plan view, you have already drawn elements (at an offset of 28 feet right) that instruct the criteria to draw a Proposed Parapet retaining wall. These initial elements (28 feet right) acted only as a "trigger" to tell the criteria to draw the wall. The widths of the footings (at that point) were based on the adhoc value (wall width factor) placed with the plan view element.

To set the actual constructible width of the footing (abrupt stepped widths) that are once again determined by the Structural/Geotechnical engineer, a second line must be drawn in plan view. Once the 2 lines are drawn in plan view the criteria looks at how far apart they are at a particular station (pattern line) and uses that for the width of the footing at that station.

Below is an example of what you might receive from the Structural/Geotechnical engineer representing stepped footing widths:

NOTE: Stations shown below are along the Gutter Line and using GUTTER stationing.

Section - A Section - B Section - C
0+00 to 0+30
Width = 9.2 ft
0+30.01 to 1+50
Width = 7.0 ft
1+50.01 to 2+50.03
Width = 7.5 ft

Open the Plan view file and using Draw Transition, and the same D&C Manager item (Prop. Parapet Wall) , draw these "second" lines (elements) at the offsets shown (above) plus the 14 feet that was drawn initially.

NOTE: The beginning and ending stations for these second lines (elements) need to be from centerline using centerline stationing.

The initial 14 foot offset line was drawn using the mainline chain HA_K5. In order to set the offset distance exactly, the second line also needs to be drawn using the mainline chain, HA_K5. In order to draw the second set of lines (elements) we need to calculate what mainline stations these gutter line stations equate to. This is easily done using COGO and the Layout Offset Chain tool, as shown below:

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Draw the second offset line (using stationing along centerline) that sets the footing widths, as shown below:

Section - A Section - B Section - C
Screen shot Screen shot Screen shot

Before drawing the second offset line (elements), adhoc attributes must be assigned via D&C Manager. These adhoc attributes will tell the criteria what wall chain and wall profiles to use for the final design that will reflect the stepped footing elevations and the stepped footing widths.

To assign these adhoc attributes, simply invoke D&C Manager as usual and enter the names of the wall chain and wall profiles that you stored in step 2, prior to drawing the elements, as shown below:

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The Plan view results are shown below, remember all we're after is to draw elements outside of the edge of pavement and the "difference" between the offsets is what sets the footing widths:

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Step 8 ... Delete the initial proposed cross section elements

Open your cross section file and delete all the proposed cross section elements (at least throughout the wall area).

For illustration purposes, I'm going to leave the proposed cross section elements from the first run, to illustrate the position of the wall from the second run. I'll change the elements from the first run to a lesser line weight and make them dashed. This illustration is shown in Step 9 below.

Step 9 ... Recreate the proposed cross sections using the stored wall profiles

Process the cross section run via Project Manager as usual. The results as shown below:

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There you have it... a retaining wall in your cross sections that reflects the exact profiles and footing widths as designed. Since the wall is in the cross sections, you can now obtain staking notes and earthwork quantities that reflect the exact design.

Below is a fragment of the earthwork log file illustrating that the volumes for Roadway Excavation, Structural Excavation and special Wall Backfill (shown as Pervious) are calculated separately from the embankment and pavement layers:

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As mentioned earlier, there are many other types of retaining walls that can be designed using our criteria in basically exactly the same manner.

Special NOTE:
Obviously, you will need many extra pattern lines in order to pick up the steps for the footings and the width breaks. Once again COGO's Layout Offset Chain tool will work perfectly for this.