Bridges crossing river

Dear Robert Klemp,

For a few days we are working with RMS Program on a project for a railroad track where there are two bridges crossing river.

These bridges have multiple spans and on piers top there are very complex cap beams and, above all, these bridges are skewed relative to the direction of flow. But HEC-RAS requires that the cross- sectional properties be relative only to a plane normal to the direction of flow. HEC-RAS does not allow the user to define a cross- section skew angle, as it is written on user's manual of RMS.

My questions are:

- How could we studying the bridges structures?

- Could we cut cross-sections from 3-D topographic map? How? Normal or skewed to the direction of flow?

- At which cross-section must be the bridge structure declared and defined? Normal or skewed to the direction of flow?

- Moreover, to define mine bridges structures and its components (for example: multiple spans) that are declared using the "Opening Definitions" dialog box, which button option must be select in "Cross Section Type"? Single Opening or Multiple Opening?

Thanks, sincerely,
Mario Salab

P.S. Could you reply me to E-mail:
inass@ingegneriassociati.com
and mario.salabe@ingegneriassociati.com

(See attached file: klemp2.zip, there is 3-D drawing of the river)

Dear Mario :

Note that there is an example in the RMS User Manual that describes how to model a bridge using RMS. See Chapter 7 Advanced Modeling Lessons, Lesson 3 HEC-RAS Bridge Modeling. This example is quite easy to follow.

When you are defining a cross-section for bridge, the cross-section geometry should be perpendicular to the flow of the river. Therefore, you may have to adjust the stationing of the cross-section so that it is "unskewed". Note that you can have RMS do that by selecting Input | Ground Geometry | Adjust Geometry. You can then provide a scaling factor to adjust the left overbank, channel, and right overbank stationing to account for the cross-section skew angle. This is done using the cosine of the skew angle. Note that this adjustment simply moves the ground stationing and overbank stationing. It does not adjust any other stationing. So, you should "unskew" the cross-section before adding the bridge and roadway data.

Note that the bridge structure is always defined at the bridge downstream face cross-section. Select Input | Openings | Opening Definitions. The Opening Definitions dialog box will be displayed. Then choose "Single Opening". You can then define the roadway and low chord information by selecting "Dialog". To switch between the downstream and upstream face cross-sections, in the Edit Geometry At radio button group select "Downstream End" or "Upstream End" and RMS will then manage the defining of data at these cross-sections.

Note that you only need to select Multiple Opening if you have 2 separate bridge openings that flow passes through. From the AutoCAD drawing that you provided, it appears that the river passes through the upper bridge opening, then routes parallel to the roadway, and then crosses back through at the lower bridge opening. If that is the case, then you would have to define 2 separate bridge openings--separated by the distance that the flow parallels the roadway between the 2 bridge openings.

If you have any additional questions or need any further information, please contact me again.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com