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Strong Post W Beam Guardrail Placed Adjacent to Steep Slope

State SD
Description Text

We have a situation as shown in the pdf. Please provide guidance or a recommendation regarding the W Beam Guardrail Wood post spacing and length of posts. This situation did not fall into the categories as stated on pages 28 and 29 of  the recent Pooled Fund Quarterly Report.  

  • Guardrail
Other Keywords none
Date April 9, 2008
Attachment 9_FT_POST.pdf


I looked at your schematic of the w-beam guardrail installation on what appears to be a 3:1 slope with a 1:1 or a 2:1 slope behind it. Missouri had a similar issue in the past. We recommended to them that they use 9' posts at ½ post spacing (3'-1 ½"). I have attached the detail they made based on this recommendation.


We would recommend a similar post length and spacing for your installation.


I looked at the energy of the TL-2 impact, and I believe that you can use 7' long post at ½ post spacing in this installation for TL-2. That should help you out quite a bit.

Date April 11, 2008

Response I have a similar question as above. A local agency is adding bike lanes to a road that is posted at 40 mph (TL-2), a curvy and hilly route. Due to limited space because of steep downward side slopes, they would like to used the MGS without a blockout and place the back of post adjacent the existing 1:1 side slope. To help develop post strength would you recommend half post spacing? Or since the completion of Report TRP 03-320-16 (MGS with 6-ft posts placed adjacent to a 1:2 fill slope) and TRP 03-185-10 (MGS with 9-ft posts placed adjacent to a 1:2 fill slope) would it be reasonable to use a MGS non-blockout system, at 6'-3" spacing, with 9-ft posts and assume a working width of 6.5 ft along this TL-2 route? Thank you
Date June 14, 2018

Response To date, the MGS has only been successfully tested at slopes as steep as 2:1. This testing was all done at TL-3. 

Installation of the MGS at the slope break point of a 1:1 slope has never been investigated. Two issues arise at slopes that steep adjacent to guardrail. First, there is the structural issue of lack of foundation support for the posts which can adversely affect barrier performance. Second, the impacting vehicle would no deflect over a very steep slope which may affect vehicle capture and stability. 

Based on these concerns, it is difficult to recommend an installation for steep slopes, even when considering a TL-2 installation. I would not recommend the standard 6' post length. There is potential for the longer 9' posts and/or reduced post spacing to work, but without further research on those posts installed adjacent to that slope, we cannot recommend an installation. 

Our best current recommendation would likely be to install the non-blocked MGS with 6' long posts at standard post spacing 2' from the slope break point of the 1:1 slope. 

TTI is currently researching installation of the MGS on steep slopes at MASH TL-3. They are looking at the use of 9' posts for that installation, but I have not results or updates on it to provide. If we get updated results on this research, we may be able to provide better guidance for this type of installation in the future. 


Date June 22, 2018

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