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TL-4 MINNESOTA COMBINATION TRAFFIC/BICYCLE BRIDGE RAIL - TRP-03-74-98 - November 30, 1998

Question
State MN
Description Text
 With respect to referenced report, page 18 and 19 shows the cycling rail attached to the back of a Report 350 TL-4 NJ bridge rail with a top width of 230mm (9.0”), bottom width of 460mm (18.1”), and height of 810mm (31.9”). Is the 230mm offset from traffic side of top of traffic barrier to the face of the retrofit cycling rail critical for Report 350 TL-3 performance? Our older existing TL-3 NJ traffic barrier that we are proposing to add the Minnesota cycling rail to back of has a top width of 155mm (6.1”) and height of 825mm (32.5”). This would result in a reduced offset from top of traffic face of barrier of only 6” to face of cycling rail instead of the crash tested design with an offset of 9”. The TL-3 crash test does show the pickup engaging the cycling rail during the TL-3 test, which would result in more engagement during an impact with cycling railing offset only 6” vs 9” (although system brackets are designed to fail with tubes staying connected to bridge via cables).

On page 73, it was “recommended that consideration be given to modifying the design in order to reduce the potential for the vertical spindle bars from releasing from the system and decrease any hazard from flying debris. These design considerations may include the following: (1) increasing the strength or the connection between the tubular rails and the spindle bars; (2) attaching a longitudinal railing member to the
traffic-side face of the spindle bars and at the mid-height between the two rails; (3) reducing the mass of the spindle bars by using small tubes; and (4) moving the spindle bars to the back side of the tubular rails to increase the strength of tile welded connection.”

Are you aware if any of above considerations have been made implemented for this system, or whether a fifth option would be acceptable involving elimination of the spindles (assuming our code would allow a horizontal gap between the two rails of approximately 15” when placed above a 32” high NJ barrier)? It is my understanding the spindles don’t have a structural function. 


Bridge Rails
Permanent Concrete Barriers


Date February 28, 2018
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Attachment mndot_comb_rail.png
Response
Response
(active)

We do believe that the combination rail offset plays a role in the performance of the bridge rail tested in TRP-03-74-98. Reduction of that offset may adversely affect the performance of the barrier due to increased interaction with the combination rail as you noted below. While the reduced offset may potentially work, we cannot recommend it without further research and/or testing. You can see in the attached report that vehicle interaction with the combination rail can be a significant issue in these tests of a different combination rail that used a single-slope barrier and slightly less offset.

 https://mwrsf.unl.edu/researchhub/files/Report136/TRP-03-162-07.pdf

We have discussed placement of the combination rail on a 36” single slope with MnDOT. However, that modification was believed to reduce interaction with the combination railing.

 

https://mwrsf-qa.unl.edu/view.php?id=1141

 

We did note in the report that the spindle bars could be modified to reduce disengagement of the spindles. We have not seen those recommendations implemented to the best of my knowledge. MnDOT may have more thoughts on that.

 

Elimination of the spindles is a potential option. As you noted, they are not structural. They are required to meet pedestrian rail criteria for rail openings.

 

Thanks
Date March 1, 2018
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