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RE: G-4 Guardrail: Clear Spanning of shorter distances, 18'9" & 12'

Question
State UT
Description Text

I have been asked to research
possible solutions to the the NCHRP 350 testing failure of the 18' 9"
clear span using w-beam,TTI Report/Test #405160-1-1, dated May 24, 2006. 





UDOT currently uses 18'9" & 12'6"spanned guardrail systems that
were approved using the 230 testing criteria. UDOT currently also uses the
25' span as tested and accepted under NCHRP-350, acceptance #B-58.





I am asking if the posts immediately prior to the span and after the span were
replaced using CRT post with 2 blocks would that be an acceptable alternative
to the current design of standard posts? I have modified 2 of the details
to show my proposal using crt posts and 2 blocks for your review. See
attached Span Proposal.pdf drawing.





Std. Dwg BA 4H1 has 3 details, Std. Dwg   BA 4H2 has 2 details,
if I'm remembering the discussions with Don Gripney correctly the splice
location appeared to be an issue during 230 testing.  I have also included
BA 4HI and BA 4H2 for reference.





Under the 350 testing of the 25 ft. span using CRT post and 2 blocks, the
splice joint did not appear to be an issue where it was placed in the run..



If these are not acceptable changes can you offer any suggestions that may work
in these situations?



Thanks for any assistance you can provide.

Keywords
  • Guardrail
Other Keywords none
Date January 6, 2015
Attachment BA 4H1.pdf
Attachment BA 4H2.pdf
Attachment Span Proposal.pdf


Response
Response

We have some comments regarding the use of omitted posts in G4(1S) guardrail systems.

We have addressed this topic with the states in the past and have a current project underway to investigate the omission of a single post in the MGS system. Previous research into G4(1S) long span guardrail systems with various lengths have found that the G4(1S) system require nested guardrail if posts are removed from the system, as noted in the TTI research you reference. As such, we have typically recommended that all G4(1S) systems with unsupported spans use nested rail. We have provided recommendations as to the length required nested rail. This can be found in the link below.

http://mwrsf-qa.unl.edu/view.php?id=493

With respect to the use of CRT posts in the G4(1S) system, we have typically recommended that the three CRT posts be used on each side of the unsupported span to reduce the potential for snag and pocketing when posts are omitted. It is possible that the use of CRTs could be eliminated for shorter spans or that fewer CRTs could be used. However, there is not sufficient research to fully support that at this time. Thus, we have taken a conservative approach with the recommendation.

http://mwrsf-qa.unl.edu/view.php?id=836

Thus, we would recommend that you modify your proposed installation to include the nested rail and three CRT posts adjacent to each side of the unsupported span. We do not believe that the location of a splice in the unsupported region is cause for concern.

Let me know if you have further questions.

Date January 6, 2015


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