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NU Rail

Question
State NE
Description Text

NDOR has had a few impacts to our new (NU) rail that have resulted in the bottom of the slab falling off under the posts.  We are currently considering the following changes to the post and slab reinforcing:

 

Move the horizontal leg of the back post bars down from just beneath the top layer of longitudinal slab steel to just above the bottom layer of longitudinal slab steel.

 

Move the horizontal leg of the front post bars up from just beneath the bottom layer of longitudinal slab steel to just above the bottom layer of longitudinal slab steel.

 

Flare the horizontal legs of both the front and back post bars from perpendicular to the post for the middle bars to 45° for the outside bars.

 

Extend the horizontal legs 1' for both the front and back post bars from 3' " 2" to 4' " 2".

Add additional longitudinal # 5 bars in the bottom of the overhang (from 12" spacing to 6").

 

Could you please review the attachment showing our existing design and these proposed changes?  We would like your opinion on whether or not these changes would require new crash tests for acceptance and any other comments you would wish to share.

Keywords
  • Bridge Rails
Other Keywords none
Date July 1, 2010


Response
Response

We have reviewed your proposed changes to the deck reinforcement for the TL-4 version of the NDOR open concrete bridge rail.  We have also reviewed photos from 3 different crash sites that were sent to MwRSF over the last 9 months or so. However, most of these photos show only the damage to the rail itself and do not illustrate the deck cracking that you are attempting to mitigate.  Therefore, the following comments are based on general structural design and the understanding that cracking is occurring in the deck.  They are not specific to damage at an individual crash site.

 

I am unclear as to the benefits of fanning the transverse steel.  It may cover a greater area, but the lateral stress that each bar can take is reduced.  Further, it may be a pain to layout/tie the steel in a consistent manner when all of the angles are changing.  If you are wanting the transverse steel to cover a greater area and protect the deck at the post edge locations, I would recommend that you instead place an extra transverse (lateral) steel bar in the deck on both side of the post.  These bars would be parallel to current legs and could extend 4'-2" into the deck to match the new proposed length of the legs.

 

The additional longitudinal steel (along the length of the bridge) near the bottom surface of the deck should provide additional resistance to bending and punching shear.  These bars in combination with the additional transverse bars described above should help mitigate cracking.

 

Lowering the bag leg of the post reinforcement to bottom level of the deck may provide some additional steel near that surface to resist shear cracking, but it may also cause punching shear problems.  Those back legs are carrying a compression load during impacts, and moving the bend to the bottom of the deck leaves the bottom of the deck susceptible to compression force punching through  that surface " creating cracks and possible concrete spalling.  Therefore, I would recommend leave these back legs in their current position.

Date September 1, 2010


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