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.
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.
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