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Bullnose System to MGS

Question
State MN
Description Text

Can an asymmetrical (thrie-beam to
w-beam) section be used to connect the bullnose crash cushion, thrie-beam
section to the 31” high MGS standard? 
Our standard currently uses a symmetrical section between post 10 and
11 (to go from thrie-beam to 28” high w-beam).



Assume that no curbing will be used.  And in some cases the MGS portion will then
connect to an AGT or to another bullnose.



If this can be done, then we also
need guidance on post location from the Bullnose system to the MGS portion.



Attached are the current standards
for the MnDOT Bullnose and the two types of thrie-beam to w-beam transitions.



 



Thank you.

Keywords
  • Bullnose Median Barrier & Short Radius
  • Guardrail
Other Keywords none
Date January 31, 2017
Attachment MnDOT Std Plan 611.PDF
Attachment MnDOT Std Plan 614.PDF
Attachment MnDOT Std Plan 695.PDF


Response
Response I have some replies to your questions.

We do believe that the asymmetrical W-to-Thrie transition can be used to connect the bullnose to MGS. Our current recommendation would be that the transition section not be added until the end of rail section no. 4 or post no. 10. This is consistent with the detail that you sent. This guidance was based on previous analysis of the length of the deformed rail in the full-scale crash tests. 

If transitioning to the MGS, there will be a need to transition the splices to the mid span as well. We believe that this can be accomplished by placing the first post downstream of the asymmetrical W-to-Thrie transition piece at 1/2 spacing and then using standard spacing from that point on. This would correspond to putting in a post at 1/2 spacing after post no. 11 in your detail and then using standard post spacing afterwards.

One final comment with regards to transition from the bullnose to MGS and then to and AGT. We would recommend a minimum of 25 ft of standard MGS between the bullnose and the start of the MGS upstream stiffness transition.


Date February 7, 2017


Response
Response

Thank you for the response.   We just have one more quick question regarding post type (wood to steel) change locations.

 

We are proposing to change from a wood post (# 10) to steel post (# 11). 

Let us know if this post type change location is ok. (See Attached pdf figure).

Date February 7, 2017


Response
Response

I believe that you can start the use of CRT posts at post no. 9 if you wish. The NCHRP 350 bullnose testing with the UBSP posts used the standard steel posts starting at post no. 9.

 

Thus, I don’t see the use of steel posts at post no. 10 being an issue. We did use 78” long posts in the thrie beam sections rather than the 72” long posts used for the MGS and would recommend those posts in the thrie beam region. We would also recommend that the shorter blockouts used in the bullnose be used throughout the thrie beam region.

 

Thanks

 

Date February 13, 2017


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