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Traffic Barrier Terminal (AGT) MASH Guidance

State IL
Description Text IDOT is currently reviewing our Traffic Barrier Terminal (TBT/AGT) standards to make sure they meet MASH requirements. We had a couple questions we wanted to run by you guys as guidance on a few items which will be described below.

I’ve attached the following documents for your review:
• Our current TBT Type 6 (NCHRP 350)
• Drafted changes to our TBT Type 6
• Permanent Concrete Barrier Design that TBT Type 6 will be attached

Questions we have:
1. Is the curb required or can it be an option?
2. Is the wedge plate required? If so, is our design okay to use?
3. Is the vertical face taper under the connection of the AGT required when there is NOT curb present? (See page 2)
4. Is the vertical face taper under the connection of the AGT required when there is a curb present? (See page 2)
5. Can we use 3’- 1 ½” spacing for the first 10 post instead of the 1’-6 ¾” spacing?
6. Block out widths for post 1 – 11 can be either 12”, 8” or either?
7. Block out material can be wood or composite for entire run?
8. Would the height have to be at 34” tall to match the proposed AGT design for the steel railing you guys are developing for us if we want to be consistent?

Please let me know if you have any questions or need any additional information from us. We look forward to discussion these changes and any recommendations you guys may have for us. Thank you! 
  • Approach Guardrail Transitions
Other Keywords none
Date August 31, 2018
Attachment Detail_conc barrier dblface 44in (replaces Std 637006).pdf
Attachment Traffic Barrier Terminal Type 6 Current Standard_NCHRP 350.pdf
Attachment Traffic Barrier Terminal Type 6_MASH_updates_8_31_2018.pdf


I have responses to your questions below in in the same order as numbered in your email.  However, before we get too far into the AGT design, I want to remind you of the standardized transition buttress that was recently designed here at MwRSF. This buttress has been evaluated to MASH and is compatible with a variety of transitions, curbs, and adjacent concrete barrier configurations.  We highly recommend utilizing the standardized buttress in your approach transitions in the future.  I have attached a TRR Journal paper to this email that explains the design and some implementation issues.


Questions (and answers):

  1. The standardized buttress was tested without a curb (critical snag configuration), so it can be utilized within both curbed or non-curbed installations.  This is one of the biggest benefits to the new buttress design.  If you wish to stay with older transition parapet designs that were tested with a curb, you should keep the curb within the design to prevent snag.
  2. Wedge plates are required for all transitions that incorporate sloped-faced buttresses.  Testing of thrie beam transitions attached to safety shape and single slope barriers without a wedge plate has resulted in failures.  As such, the thrie beam needs to remain vertical throughout the transition.  Note, a wedge plate is not required for the standardized buttress since it utilizes a vertical face (another benefit of the new buttress)
  3. Removing a taper, flare, or chamfer from the upstream end of a concrete buttress can easily result in increased snagging that can lead to vehicle instabilities, excessive crush, and/or excessive vehicle decelerations. As such, you should stick to the buttress as it was initially designed and crash tested.  Speaking specifically about the new standardized buttress – both the upper and lower chamfers/tapers should be utilized regardless of the presence of a curb.  A curb by itself is likely not enough to prevent snag on the buttress.  Curbs may be placed directly adjacent to (or in contact with) the sloped face of the lower chamfer/taper, so they don’t have to be offset back from the roadway.
  4. See #3 above.
  5. The thrie-beam transition you are showing incorporates W6x9 posts at 18.75” spacing.  There are other transition designs that incorporate larger posts at 37.5” spacing.  The new standardized buttress is compatible with either of these types of transition systems.  However, the upstream stiffness transition from W-beam MGS to stiffened thrie beam was only designed to use standard W6x9/W6x8.5 guardrail posts at very specific spacings.  As such, only the posts 1-6 would be able to go to a larger post spacing.  The proper way to attach the upstream stiffness transition to various thrie beam transitions is described in Chapter 14 of the research report – linked below:


  1. Blockouts on posts 1-6 may be either 12” or 8”.  Blockouts within the upstream MGS stiffness transition (posts 7-14 on your drawing) should be 12” deep to prevent vehicle snag.
  2. Blockouts can be wood or composite (granted that the composite blockouts have been previously shown to be crashworthy).  We have also designed tubular steel blockouts for use within transitions – if you are interested in steel blocks, I can get you the details.
  3. The transition design to the TL-4 steel tube bridge rail we are currently developing has not been designed yet.  One of the design goals will be to allow the thrie beam rail to be mounted at 34” so that the system can remain in place after 3” roadway overlays.  However, until we are further along in the design process, it is difficult to know what this transition will look like and you will likely need a separate standard for this transition.  If you are asking if all of your thrie-beam transitions need to be mounted at 34”, the answer is no.  You could have both 31”  and 34” AGTs within your standards – Nebraska has done this.  Alternatively, you could raise the heights of all your transitions to 34” in anticipation of overlays if so desired.


Please review these comments, attached paper, and the MGS stiffness transition report and let me know if you have further questions.


Date August 31, 2018
Attachment TRR_2018_Standardized_Buttress_AGT.pdf

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