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Comparing MGS bridge transitions

Question
State IA
Description Text

Good morning!



 



In reviewing
the Iowa and MwRSF MGS bridge transitions (attached email and pdf from May
2009), I have a couple of questions as we inch closer to MASH dates:



1.       In talking to Chris Poole about the
attached email, he suggested that I seek confirmation regarding MASH compliance
for the Iowa transition specifically. While I presume that it would be an
equivalent or even slightly stiffer design compared to the MwRSF transition,
for the sake of confirmation and supporting documents, I ask that the Iowa MGS
transition be reviewed for MASH compliance at TL-3. The as-published standard
is available at https://iowadot.gov/design/SRP/IndividualStandards/eba201.pdf.



2.       I would also like to publish the MwRSF
version for cases where we have multiple post conflicts within the thrie-beam.
The half post spacing provided would occasionally solve some of our pipe issues
that interact with multiple posts at quarter post spacing. In preparing to do
so, I’m wondering what the equivalent wood post dimension would be for the
W6x15s. We currently view 6” x 8” wood posts and W6x9 steel posts as
equivalents.



 



As always, your
time and effort are greatly appreciated.

Keywords
  • Approach Guardrail Transitions
Other Keywords none
Date May 17, 2017
Attachment Iowa Transition Design Adaptation.pdf


Response
Response

My responses are below in red

___________________________________________________________________________

Good morning!

 

In reviewing the Iowa and MwRSF MGS bridge transitions (attached email and pdf from May 2009), I have a couple of questions as we inch closer to MASH dates:

1.       In talking to Chris Poole about the attached email, he suggested that I seek confirmation regarding MASH compliance for the Iowa transition specifically. While I presume that it would be an equivalent or even slightly stiffer design compared to the MwRSF transition, for the sake of confirmation and supporting documents, I ask that the Iowa MGS transition be reviewed for MASH compliance at TL-3. The as-published standard is available at https://iowadot.gov/design/SRP/IndividualStandards/eba201.pdf.

 

The Iowa transition was previously successfully tested to MASH TL-3 as part of the NCHRP Project 22-14 (02). The test report is TRP-03-175-06 and should be available on our website. 

 

As I was looking at your standards, I noticed that you have multiple buttress options.  One of these options (Type B) appears to extend the longitudinal distance of the taper/slope on the rigid buttress and eliminates the transition post adjacent to the buttress (Post 15).  This extends the unsupported span length of the thrie beam rail by 10 inches or so, effectively reducing the rail stiffness near the buttress.  This may increase the propensity for snagging on the concrete buttress.  Transitions with longer unsupported span lengths have been successfully crash tested, but they typically involved larger posts (by cross section) spaced at 37.5” centers – similar to the transition utilized by MwRSF for the development of the W-to-thrie stiffness transition.

 

2.       I would also like to publish the MwRSF version for cases where we have multiple post conflicts within the thrie-beam. The half post spacing provided would occasionally solve some of our pipe issues that interact with multiple posts at quarter post spacing. In preparing to do so, I’m wondering what the equivalent wood post dimension would be for the W6x15s. We currently view 6” x 8” wood posts and W6x9 steel posts as equivalents.

 

There was a study conducted to develop a wood post alternative for the steel post transition.  It recommended the use of 6.5-ft long 8”x10” posts as the alternative for the 7-ft long W6x15 posts.  The report no. is TRP-03-243-11 and should be available on the website.

 

Date May 18, 2017


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