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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

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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?

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,

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.