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Approach Guardrail Transition Attach to 8" Concrete Sidewalk

Question
State NJ
Description Text

In many of our projects, the contractors discover electric line or other utility running under the approach guardrail transition (AGT) posts.  The locations of these utility will not allow for the new posts to be installed in accordance to our design standards for the AGT.  Right now, we have a construction detail CD-609-11 for the guardrail attachment to a 8" sidewalk.  We usually use this detail when there are underground utilities that prevent the posts from meeting the required embedment length along the normal section of the MGS.  Can this construction detail be used along the AGT where there are underground utilities?  For this kind of attachment to the 8" sidewalk, the required post size is W6x20 (see attached file). Thank you for your help. 

Keywords
  • Approach Guardrail Transitions
Other Keywords none
Date September 27, 2018
Attachment 072_CD-609-11BeamGuideRailAttachments.pdf


Response
Response In theory, this type of a post connection could work within an approach guardrail transition.  However, to my knowledge, this type of system has never been evaluated and there are questions related to multiple features within this system, as detailed below.
  1. What are the effects of adding an 8" tall curb below the guardrail transition? Curbed transitions are typically tested and implemented with a 4" tall triangular shaped curb, while the MGS with curb was tested and is implemented with a 6" tall Type B curb.  Thus, the performance of guardrail (and the more sensitive transition regions) is unknown.
  2. Will a top-mounted, W6x20 post provide similar stiffness to the original transition post?  Transitions are sensitive regions that are carefully designed to gradually increase lateral stiffness and prevent snagging and pocketing.  Changes to the stiffness of the posts can negatively affect the performance of the guardrail transition. Thus, the top-mounted posts need to be designed with similar strengths to the original transition posts.
  3. How would you safely transition from these large top mounted posts to standard posts embedded in soil (with or without a curb? There will likely be a great difference in post strength at this location that must be treated properly to prevent pocketing, snag, and possible rail tearing.
  4. I also question the use of wide-flange steel sections as blockouts in transition regions.  Previous full-scale testing has demonstrated that I-shaped steel blockouts are susceptible to web buckling, which decreases stiffness, increases deflections, and increases the likelihood of snag and vehicle instabilities.

As noted above, changes to the post size, the post anchorage, blockout shapes, and adding a curb are significant changes to the guardrail transition that may have huge effects on its performance. Therefore, I would recommend that this type of a system be analyzed and evaluated prior to implementing it. 
Date October 23, 2018


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