The CHECK-RAS program determines the abutment stations of a bridge at the locations where the low chord line from the Deck/Roadway data crosses the ground line of the internal bridge section. The abutment stations are then used to compute the left and right ineffective flow stations. CHECK-RAS then compares the abutment stations, ineffective flow stations, and encroachment stations to make sure that the bridge opening area is not blocked.

CHECK-RAS determines the abutment stations for both upstream and downstream bridge internal sections. If the low chord line from the Deck/Roadway data crosses the ground line of the internal bridge section at more than two locations in the model, the CHECK-RAS program will not be able to find the abutment stations. CHECK-RAS will print the message "ST GD 04" and terminate the checking process for that structure. The user should not ignore this message because there can be errors for that structure.

At this time we have identified two situations where the low chord line may cross the ground line at more than two locations:

  1. Piers are formed by using the low chord and ground elevations, rather than by using the pier option.
  2. The areas outside the bridge opening are not properly blocked.

An example of the latter case is provided in this bulletin. Please reply to this mailing list if you have questions or feedback concerning other situations that are not documented here.

Please download the project file, plan files, geometric files, and the flow file, by clicking here (File STGD04.zip in WinZip format). Unzip the files to one directory on your local computer.

Problem

Open the project file (STGD04.prj) with the HEC-RAS program and review plan 01 from the "Steady Flow Analysis" window. Next, open the "Geometric Data" window. The resulting schematic diagram will show a stream with four cross sections.

Click the "Brdg/Culv" on the left-hand side of the "Geometric Data" window, then click the "Deck/Roadway" button in the "Bridge Culvert Data" window. The "Deck/Roadway Data Editor" window will be displayed. Notice that the elevation of the low chord is specified for all of the road stations. The low chord elevation changes for the upstream bridge section, from 228.82 at station 4969.2 to 235.94 at station 4969.3, and the low chord elevation changes back from 234.66 at station 5025.2 to 226.84 at station 5025.3

Click "OK" to close the "Deck/Roadway Data Editor" window and select "Internal Bridge Cross Sections..." from the "Options" menu of the "Bridge Culvert Data" window. This will display the "Bridge Cross Sections" window. The ground elevation at the Upstream Inside Bridge Section, at station 4969.2, is 228.82. The ground elevation at station 4969.3 is at 228.79; the ground elevation at station 5025.2 is at 226.83; and the ground elevation at station 5025.3 is at 226.84.

From the upstream Deck/Roadway data and the upstream Internal Bridge Section data we can say that the low chord line should cross the ground line between stations 4969.2 and 4969.3 on the left side and between stations 5025.2 and 5025.3 on the right side. The same observation can be made from the downstream Deck/Roadway data and the downstream Internal Bridge Section.

The above observation is only at the bridge opening area. Checking all of the other road and ground stations to determine whether the low chord line crosses the ground line would take a considerable amount of time. Because the CHECK-RAS program checks the crossing of the low chord line and the ground line, we should use CHECK-RAS to determine whether there are other locations where the low chord line and the ground line cross.

Please close all open windows in HEC-RAS, with exception of the main interface, and run plan 01 to generate the report file for use with the CHECK-RAS program. After closing the "Report Generator" window, please save the project.

Open the saved project (STGD04.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway and select the Structure check button on the program menu bar. When prompted with the message: "You need to specify allowable surcharge..." click the "Yes" button.

Review the resulting report and you will see the message STGD04 at RS 3.25. Highlight the message number STGD04 and CHECK-RAS Help will explain the problem and the solution. Please note that there are no other messages displayed. This does not mean that the bridge has no errors. It only indicates that CHECK-RAS cannot check the messages because of insufficient data.

Solution

The solution to this situation is provided in plan 02. Please select plan 02 from the "Steady Flow Analysis" window. Open the "Geometric Data" window and click the "Brdg/Culv" button. The "Bridge Culvert Data" window will open. As discussed in the "Problem" section of this message, the low chord line will cross the ground line between stations 4969.2 and 4969.3 on the left side and between stations 5025.2 and 5025.3 on the right side. Verify these locations at the Upstream and Downstream bridge cross sections displayed in the Bridge Culvert Data window. The low chord line is shown in green and the internal section ground line is shown in blue. In order to have only two intersection points, the low chord elevations before road station 4969.3 and the low chord elevations after road station 5025.2 should either be made blank or specify an elevation below the minimum elevation of the cross section. HEC-RAS will consider blank as zero elevation. If the minimum elevation of the cross section is below zero, then the minimum elevation of the cross section should be specified as the low chord elevation for these stations.

Click the "Deck/Roadway" button to display the "Deck/Roadway Data Editor" window. You will notice that the low chord elevation is deleted from starting station 3944.4 to station 4969.2 and from station 5025.3 to ending station 5541.2 for the Upstream Deck/Roadway data, and from starting station 4044.1 to station 4969.2 and from station 5025.3 to ending station 5151.6 for the Downstream Deck/Roadway data.

Close the "Deck/Roadway Data Editor" window and run plan 02 in HEC-RAS to generate the report file for use with the CHECK-RAS program. After closing the "Report Generator" window, please save the project. Open the saved project (STGD04.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway and select the Structure check button on the program menu bar. When prompted with the message: "You need to specify allowable surcharge..." click the "Yes" button.

Review the resulting report and you will see that the message STGD04 is no longer there. Messages are now displayed for the ineffective flow check and the floodway check. We will resolve these messages in the next exercise.

View the archive page for all Flood Hazard Mapping listservs.

Last Modified: Friday, 15-Jun-2007 11:57:20 EDT