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AGT Special Curb Design, under the Thrie-beam section

Question
State MN
Description Text

We have been looking into a new single slope barrier, AGT design
option, with a back taper at the bottom to reduce snagging. 
However, this option is not preferred because
of constructability and water drainage (curb flow line consistency) concerns.



One alternative design has recently been proposed.  It uses an 8-inch high curb with a front face
at the same slope as the single slope barrier.
 
The gap between the top of the 8-inch curb and the bottom of the
thrie-beam would be 3-inches.
  The curb
would tapper to a 4-inch curb (or to zero) before the W-to-thrie transition element.



Attached is a draft drawing showing this design
approach. 
We would like to know if this
would reduce the snagging issues at the concrete end, similar to a back tapper
design.



Let me know if you need any further information.



Thank you
Keywords
  • Approach Guardrail Transitions
Other Keywords none
Date April 14, 2016
Attachment 694_AGT_type31_SingleSlope_CurbOption.pdf


Response
Response
Previous full-scale testing has shown that the addition of a curb has generally reduced the amount of snag on the rigid buttress.  That being said, the face of the buttress has typically been tapered back in conjunction with the curb.  I don't believe any AGT has been successfully tested without a taper of the rigid buttress (of the toe or the entire front face). Thus, snagging may still be an issue. The smallest taper that i could find that was successfully tested on a SS buttress was 4" laterally, by 22" longitudinally - reference TRP-03-47-95.  That AGT configuration did not utilize a curb, so there is a chance that adding a curb could help reduce snag, but it has not been tested/evaluated.

My biggest concern with the proposed idea is a potential for vehicle instability with an 8" tall curb.  All of the previously crash tested (350 and MASH) AGT designs utilized 4 inch tall curbs. Testing of W-beam guardrail with a curb has used curb heights up to 6". There is no data available on vehicle interactions with an 8" tall curb.  As such, my concerns are that excessive climbing and rolling may occur during vehicle impacts.  Again, there is no data proving that this is a problem, but there is also no data demonstrating the crashworthiness of an 8" curb. However, I would feel better about leaving the curb height at 4 inches throughout its length - the curb height typical of AGT's that has been shown to be crashworthy.

If you do decide to utilize a taller curb, you are correct in transitioning it down to a 4" tall curb prior to the curb extending below the W-to-thrie beam transition element.  Previous testing has demonstrated the sensitivity of this region of the AGT, and we would not want to induce further vertical climb and increased vertical rail loads.
Date April 25, 2016


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