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Flare Rates for Permanent CSB and Thrie-Beam AGT in Downstream Applications

Question
State KS
Description Text KDOT is working on evaluating different alternatives for upgrading an existing Inertial Barrier System at a mainline/ramp gore location (see attached Google Earth KMZ file for existing location). Through our evaluation we have been working with an attenuator manufacturer and have developed the draft layout shown in Attachment 1. Attachment 1 also includes details for the AGT we are planning to use as part of this installation since we will be connecting into existing 4G-1S, 28” w-beam guardrail. Basically we are proposing to construct a special concrete block similar to what is shown on page 3

As we’ve been evaluating the alternatives we need some guidance on using 8:1 flare rates (as shown in the attached PDF) for permanent CSB and Thrie-Beam AGT at this location. The 8:1 flare rate (relative to the edge of the through lane) matches approximately the existing flare rate of the w-beam at this location. The mainline and ramp posted speed is 65 mph with a traffic volume of ~25K ADT on the mainline movement. KDOT’s field staff has indicated the first couple of barrels on the existing IBS installation are hit and have to be replaced 3 to 5 times a year. Any guidance you could provide in using these types of flare rates in downstream, permanent applications would be appreciated. We were hopeful to have some guidance within the next couple of weeks, but let me know what timeframe you think is reasonable.

Call or e-mail me with any questions…
Keywords
  • End Treatments & Crash Cushions
  • Guardrail
Other Keywords Gore Area
Date April 4, 2018
Attachment Attachment 1.pdf


Response
Response I have a few thoughts/questions for you:

I’m not a huge fan of 8:1 flare rates on concrete and AGTs. So that make me a little nervous if there are ways to reduce that, I would feel better. If not, it may just be an installation site where you have go with the best you have.


How attached are you to the crash cushion you are using? I ask this because a wider crash cushion (at least one that gets wider in the back) would allow for reduced flare rates for the guardrail.


Technically, you shouldn’t need AGTs going downstream from the crash cushion. It looks like this is for one-direction traffic, so there shouldn’t be any reverse direction hits. As such, you don’t need AGTs, and you can just attach guardrail to the concrete as you would the trailing end of a one-way bridge.


Without the need for an AGT, you may want to just install MGS downstream of the concrete. The MGS has been successfully crash tested with up a 5:1 flare rate according to NCHRP TL-3. You could install MGS along any flared section of guardrail and then transition the rail down to 28” along the tangent section of the guardrail installations.


I would also encourage you to utilize the shortest concrete parapet you can, thus minimizing the exposure of flared concrete. Not sure if your concrete parapet at this location is 9 ft or 19 ft long, but it could be much shorter than that if used only to support the crash cushion. 
Date April 5, 2018


Response
Response

Scott – Thanks for the feedback… See my responses below. We’ll discuss here again at KDOT and I’ll follow up with you and Bob if we have any additional questions.

 

Tom

 

______________________________________________________________________________________________

 

I have a few thoughts/questions for you:

  • I’m not a huge fan of 8:1 flare rates on concrete and AGTs.  So that make me a little nervous  if there are ways to reduce that, I would feel better.  If not, it may just be an installation site where you have go with the best you have. We did look at an option to reconstruct all the guardrail to flatten the flare rates to 16:1 or flatter, but it pushed the attenuator out farther in the gore approximately 25’ closer to traffic. Since the first few barrels in the inertial barrel installation are already being hit fairly regularly we didn’t want to move any device closer to traffic.
  • How attached are you to the crash cushion you are using?  I ask this because a wider crash cushion (at least one that gets wider in the back) would allow for reduced flare rates for the guardrail.  We did look at the wider attenuator assemblies, but with the potential maintenance costs for impacts being higher for all the special parts we wanted to select something more typical/standard. We also ran into the same issue with the wider transition for the attenuator pushing the device out closer to traffic.
  • Technically, you shouldn’t need AGTs going downstream from the crash cushion.  It looks like this is for one-direction traffic, so there shouldn’t be any reverse direction hits.  As such, you don’t need AGTs, and you can just attach guardrail to the concrete as you would the trailing end of a one-way bridge.
  • Without the need for an AGT, you may want to just install MGS downstream of the concrete.  The MGS has been successfully crash tested with up a 5:1 flare rate according to NCHRP TL-3.  You could install MGS along any flared section of guardrail and then transition the rail down to 28” along the tangent section of the guardrail installations. I had suggested this, but didn’t get much traction. With this feedback from MwRSF I’ll bring it up again for discussion.
  • I would also encourage you to utilize the shortest concrete parapet you can, thus minimizing the exposure of flared concrete.  Not sure if your concrete parapet at this location is 9 ft or 19 ft long, but it could be much shorter than that if used only to support the crash cushion. The 8:1 flared portion of the CSB is approximately 3’ in length. Does that alleviate some of your concerns?

 

Date April 6, 2018


Response
Response

Scott – We had some additional discussions here internally at KDOT and we have a design we are moving forward with along with some documentation with everything we’ve investigated within the project/site criteria/constraints… in this situation we feel this proposal is still an improvement from the existing condition and is the best alternative of several we evaluated for this particular site. Would you mind taking one more quick look… I was wanting your thoughts on one item in particular regarding whether or not to nest the first 12’-6” of w-beam off the backside of the concrete block (see attached for details).

 

Thanks,

Date April 26, 2018
Attachment KA468501 Gore Protection Details.pdf


Response
Response

I don’t think you need to nest the w-beam coming off of the concrete barrier.  I’m not sure if it helps much in terms of system performance, but nesting the rail shouldn’t hurt in anyway.  I guess what I’m saying is that nesting isn’t necessary, but if you are concerned about the strength of the barrier in that region, you can nest the rail without negatively affecting performance.

Date April 27, 2018


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