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MGS transition on 2:1

State OH
Description Text ODOT is upgrading existing guardrail to MGS on an Interstate route.  For the attached location, the District would like to use long posts (7.5' -8') for posts 7-13 of the attached drawing and also for the transition at the trailing end of the structure.  Previous research for Wisconsin addressed a retrofit option by placing additional posts behind the existing posts.  I didn't see any guidance for new installations.  Can an MGS transition be installed on a 2:1 with long posts?  Are longer posts an option for posts 1-6 of the bridge transition?
  • Approach Guardrail Transitions
Other Keywords none
Date December 21, 2015
Attachment BTA on 2-1.JPG
Attachment MGS-3-1.JPG


I need to start this response by clarifying that evaluation of this transition placed on slopes has never been performed (testing or numerical analysis). The report you referred to on retrofitting transitions (TRP-03-266-12) was focused on two different transitions utilized by the the state of Wisconsin. Additionally, that project only focused on the downstream end of the transition. Evaluation of the w-to-thrie beam stiffness transition has never been performed on sloped terrain. Thus, a crashworthy transition system has never been developed for use on or adjacent to fill slopes. In order to ensure system crashworthiness, you would need to provide 2-ft of level grading behind the transition posts. Of course, it would be costly and labor intensive to add the required soil to the roadside. If you chose to install a transition without the required grading, the recommendations below are our best guess at what it would take to make the transition perform as intended. Please do not take these recommendations as a standard for new construction.  Rather, these should only be utilized to improve existing systems without the possibility of significant grading work to the site.

 Report TRP-03-266-12 provided guidance pertaining to additional posts to be driven behind existing 7-ft transition posts. These replacement posts were conservatively designed assuming the original posts would provide no additional resistance compared to the new post. Thus, the same "additional" posts would be recommended as replacement posts. As described in the report, the size and embedment depth of the replacement posts depends on the slope of the roadside. Steeper slopes will require larger posts. So, an 8.5-ft long W6x12 post was recommended for 3:1 slopes, while a 12-ft long W6x16 was recommended for 2:1 slopes. Please use this report to guide your selection of post for use as Post nos. 1-6 depending on the terrain slope on site.

The MGS has been successfully crash tested on 2:1 slopes with 6-ft, 8-ft, and 9-ft posts. Due to the lack of grading behind the posts, working widths and system deflection were increased due to the slope. However, increasing post lengths and embedment depths helped reduce the working width of the system back near values obtained for MGS on level terrain testing. Because transitions are sensitive to changes in lateral stiffness and can result in vehicle snag and pocketing when not designed properly, you want to install a post that best replicated the stiffness of a 6-ft W6x9 on level terrain.  Therefore, we would recommend using the longer 9-ft posts to replace the standard 6-ft W6x9 posts in the transition region.  This increased post length should be carried all the way through the transition region (post nos. 7-13 on your drawing set) and for the next 25 ft of MGS upstream of the w-to-thrie transition element. 
Date December 21, 2015

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