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Johnson County, BRF-001-5(106)--38-52, IA 1 over Ralston Creek bridge replacement

State IA
Description Text I received a question from our bridge office about how we should handle a city request for tubular rail to match another bridge on their route. Please see the image below and the attached KMZ fle. Is this something that should even be considered? The design they have does not look too bad to me other than a couple snag points. I could try to find a detailed design of the rail if that helps. Would it be possible to design something that would look similar and be crash worthy? The speed limit is 25 MPH.  
  • Bridge Rails
Other Keywords none
Date March 16, 2015
Attachment johnson county.jpg
Attachment Tubular Bridge Rail.zip


The preferred option would be to consider a crashworthy system or at least mitigate against critical concerns, such as significant vehicle snag, launching/override, rollover, etc. In the end, it may be low enough of risk to have some leeway if posted speed or speed advisory sign denotes 25 mph or less. In the past we have been willing to give some on the urban roadside tree issue if posted speeds were 25 mph or less.


Thus, it might be feasible to post a speed advisory before bridge of 25 mph maximum. This discussion only pertains to inside separator rail.

Date March 16, 2015


I wondered if you were comfortable with also using the concrete end section shown in the photo below. It appears that the end section is only approximately 6’ long which is much shorter than our standard IDOT concrete barrier tapered end section (which is 16’ long - see attached). We are in the process of concepting a bridge replacement that has 4 adjacent entrances and using the tapered end section detail will result in having to relocate some of these entrances.  


I appreciate your help!


Date March 17, 2015


With regards to the end section, the response is largely the same. A crashworthy end section like the one shown in the attachment is preferred. However, if the speed is the area are kept below 25 mph, then concerns with vehicle vaulting or rapid deceleration upon impact with the end section shown would be reduced. That does not mean to say that the potential is eliminated, only that the use of the end section is more feasible if the speeds are limited.

Date March 18, 2015

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