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Missing Post in Double Faced Run of 8" Blockout MGS

Question
State NH
Description Text

                Good afternoon.  I hope that all is well with you and yours. 



               
Keith Cota suggested that I contact you as I have a question regarding the MGS
system with an 8” offset block.  We have a project with a gas transmission
line that we must avoid while installing median guardrail.  This is a
double faced installation.  We need to omit one post if possible.  In
the past I would have been inclined to say no problem and just do it but as I
have seen the MASH tests with existing systems I am more wary of this simple
approach.  I do not know how introducing a more flexible area in the midst
of a run that is otherwise more semi-rigid could affect the behavior of the
 impacting vehicle.  I would hate to think of it somewhat acting like
a slingshot, with the vehicle overturning.  And I am talking a MASH TL-3
scenario.  I am thinking of a 12’-6” span but could reduce that to
9’-4”?  Any thoughts that you would care to share with me?



               
Oh yes, I should state that the Department is not using high tension cable
guardrail so that is not an alternative.

Keywords
  • Guardrail
Other Keywords Median Barrier
Date February 4, 2015


Response
Response

I have thought some of your situation and further discussed with my colleagues.

 

My simple response would be to treat the situation using the same philosophy that is used for roadside obstructions. We typically recommend that when 1 to 3 posts cannot be installed due to subsurface obstructions, use the MGS Long-Span System. For the 31” tall MGS LS, three timber CRT posts are installed on both the upstream and downstream sides of the longer unsupported length, for a total of six CRT posts. We also use 12” blockouts, which were part of the original design. As such, I would expect that the MGS median system with 12” blocks could also be modified to accommodate at least 1 missing post as long as three CRT are placed on each side of longer span.

 

Further and based on significant MGS R&D as well as knowledge of other median guardrail tests on proprietary systems, we sought eligibility of a median version of the MGS, which utilized 12” blocks. I have attached the link and a pdf copy of this letter.

 

MwRSF Eligibility Request w/ 12” Blocks

http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/listing.cfm?code=long

 

When using 12” blocks for a median variation of MGS, it should be manageable to replace six W6x9 posts with six CRT posts. You will have lost 2” of internal space that needs to be addressed. One thought would be to adjust blockout depth over the 12 to 18 ft on each side of span using 10”, 11”, and 12” special blocks with 3 CRT posts.

 

Now, if you do not have the MGS median system with 12” blocks but rather the 8” blocks shown in the TTI report below, I might also suggest the use of a stepping of blockouts over the 3 CRT posts. I understand that this solution requires a few special blockout sizes and varied bolt lengths. However, it provides a reasonable solution for these special circumstances. For me personally, I am more comfortable with the use of 12” blocks in combination with CRT posts in combination with increased span systems. Unfortunately, it may be more difficult to integrate 12” blocks when one started a median system with 8” blocks, unless one starts a blockout stepping process much farther away from span with omitted post.

 

TTI Report – Median MGS w/ 8” Blocks

http://tti.tamu.edu/publications/catalog/record/?id=39225

 

Please let me know if you have any further questions or comments.

 

Date February 4, 2015
Attachment b204.pdf


Response
Response

Your response is very, very much appreciated!  Your guidance and that of your colleagues really helps.

                I certainly agree that a few custom offset blocks should not be a big deal. 

                NHDOT is trying to avoid the use of wood posts as much as is practical for the reasons that follow.  We have had posts appear sound above ground and practically had no post 6” below ground.  Some posts have been found to not be treated as advertised and have been eaten by insects in the central section of the posts.  Those conditions are not always readily discernible.   I won’t even speculate about how often they may be “field shortened”  where the soil conditions make driving difficult.  They are considered a solid waste requiring disposal at specific (expensive) sites.  Which leads to my subsequent question.

Regarding the wood CRT posts, I know that the TRP-03-288-14 report  (Universal Breakaway Steel Post for Other Applications) indicated great promise for the universal  breakaway steel post.  Has any further testing been done to “prove” the design.  And if so, has a letter gone to FHWA? 

I do not mean to take more of your and your colleagues’ time but the long span solution using CRT wood posts prompts the question.   If I wood is the answer, fine.  But if the other post could be used, that would be great as well.

Date February 5, 2015


Response
Response

You are doing very well in moving thoughts on to the Universal Breakaway Steel Post (sometimes noted as Universal Steel Breakaway Post). We already demonstrated its viability in the thrie beam bullnose system. Based on its prior success as well as the results from another MGS Long-Span study (system with CRTs showed potential promise during simulated impacts on span lengths greater than 25 ft), our Pooled Fund agreed to test and evaluate longer unsupported lengths under MASH with UBSP posts in lieu of CRTs. However, we are at the stage of programing construction/testing within our overall field project queue. This year, we will be testing UBSP posts in combination with a 31.25-ft long-span system.

 

Again and based on dynamic component testing, we believe that the UBSPs compare well to CRT posts and can likely serve as a surrogate in other designs (i.e., MGS Long Span, MGS Downstream Anchorages/Trailing Ends, etc.). However, we want to further demonstrate acceptable performance in actual crash testing, similar to what was done on the bullnose and currently planned for MGS Long-Span.

 

For your particular scenario and if all goes well, the UBSP post would not create any dimensional issues for median systems as the depth for the upper portion of the post is identical existing steel guardrail posts.

 

I have attached the FHWA Eligibility Letter for the thrie beam bullnose with UBSP posts as well as the AASHTO TF13 details for the roadside hardware guide. See below for additional information regarding other attachments.

 

Ron,

 

I went through the components list in SET03a-b (which is  the system drawing for both the wood and UBSP versions) and tried to include all the new component drawings that would not have been in the printed version of the Hardware Guide. If you need the already existing ones that were in the printed version of the Hardware Guide we will have to have a student scan them into a PDF. Let me know.

 

Karla

 

 

Date February 6, 2015
Attachment Details.zip


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