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Bridge transitions and curb questions

State IA
Description Text

Iowa Department of Transportation bridge and structures are moving to a single slope barrier, and have a new bridge end.  On this bridge end the bolt holes for the transition are located such that the top of the thrie-beam is at 34”.  We understand that this is to accommodate a 3” overlay in the future.  I looked through TRP-03-367-19-r1 (cover attached) that discusses the testing of 34” height, and did not find any mention of the Iowa transition.  Could you provide insight on the Iowa transition as it pertains to this report and a 34” mounting height.


Another thing that came up while reviewing the new end post was how the curb lined up with both the end post and guardrail.  The end post has a taper on the bottom to prevent tire snag.  If the curb lines up with this taper (ideal for water flow) the back of the curb gets in the way of the posts.  If the curb is lined up with the thrie-beam it creates a pocket that will catch water and debris.  Special shaping so that both conditions can be met would be hard and not preferable.  From our understanding this taper, and the curb are both intended to prevent wheel snag.  This raised the question about how these two features work together or should these two be used together.  Would you please provide insight on the use of curb in association with the tapered portion of the bridge end.


Thank you,


Approach Guardrail Transitions (AGTs)


Date September 13, 2023
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Attachment Iowa_AGT.jpg

I believe the “Iowa” transition you are asking about incorporates 6.5-ft long W6x9 posts at a spacing of 18.75” (quarter post spacing).  Note, this was the post configuration used to evaluate the standardized buttress at a height of 31” – see TRP-03-369-20: . If I am incorrect, please let me know.


On page 94 of the 34” AGT report:

“Conversely, the unique shape of the standardized buttress does allow other thrie beam transitions to be installed at the increased mounting height of 34 in. (864 mm). The standardized buttress was developed to be compatible with all NCHRP Report 350 and MASH crashworthy, 31-in. (787-mm) tall, thrie beam AGTs. Thus, any other crashworthy, 31-in. (787-mm) tall AGT with a similar lateral stiffness (or stiffer) should also be considered as crashworthy when used at an increased mounting height of 34 in. (864 mm).”


Nebraska DOT’s 34” tall AGT utilized W6x15 posts at 37.5” spacing (half post spacing).  Although, some consider this AGT configuration a little stronger due to the increased post size, the difference is small and the system deflections are similar. Thus, you may consider the two AGTs to have similar strength.  To reinforce this comparison, a 34.5” tall AGT comprising of 6.5-ft long W6x9s at 18.75” was recently tested to MASH TL-3 in combination with a steel post and beam bridge rail at TTI.  Since, this system satisfied MASH TL-3, Your proposed 34” tall version of the Iowa AGT would likely be MASH TL-3 crashworthy as well.


Note, you may need to increase the length/height of your posts to account for the increase in rail height.  The NE 34” AGT simply raised the blockout and rail height relative to the post and maintained the nominal embedment depth.  However, the Iowa AGT uses a shorter post height and blockout that slopes upward from the top of the post to the top of the rail.  Thus, the contact area between the blockout and the post is already decreased.  Shifting the blockout upward would only further reduce this contact area and may have a negative effect on the safety performance of the system.



To answer your curb questions, I may need to know what kind of curb you are utilizing. But, I will try to provide some insights here.

Curbs below guardrail do generally help prevent wheel snag.  Taller and more vertical shapes tend to minimize wheel snag compared to shorter and sloped curbs.


Other DOTs have asked how to install curbs adjacent to the standardized buttress.  We have recommended that the toe of the curb be in line with the vertical face of the parapet (toe of curb flush with the back of the guardrail).  The curb would then be run directly into the lower taper of the parapet/buttress. In other words, the downstream end of the curb would have the same 4:1 taper as the parapet/buttress.  I believe other DOTS were just hand forming this short segment of curb.


If you wanted to have the curb line up with the back of the taper (4.5” farther back from the roadway) so that the curb runs directly into the upstream face of the parapet/buttress, you could increase the depth of your blockouts to 12” to avoid post-curb interference.  Some DOTs are already using 12” deep blockouts throughout their AGTs.



I hope this helps. Let me know if you have further questions.


Date September 14, 2023
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