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Double-faced to Single Face MGS Transition

Question
State MO
Description Text Please review the attached email regarding how to transition between the median version of the MGS to two separate MGS installations and provide any guidance or suggestions. 

thanks 
Keywords
  • Approach Guardrail Transitions
  • Guardrail
Other Keywords Median barrier
Date May 5, 2016
Attachment 3132_001.pdf


Response
Response

I looked at your detail. As you noted, there is more leeway currently to omit a post in the MGS due to the test we ran last year. Thus, I follow the thinking you guys were using. However, we still have two concerns.

1.       There is some concern that the deflected guardrail can impact the back side of the offset posts. This loading of the rail across a corner or an edge of a post has shown the potential to cause rail rupture. Thus, there is some concern that the offset posts may degrade system performance.

2.       There is also concern that the posts that are offset longitudinally may interfere with each other when one side is impacted. In tested of a PCB transition system and recent research we have been doing regarding minimum light pole offsets for the MGS, we have observed that the deflected post movement can be restricted by objects just downstream of the post as it is deflected. This can potential lead to reduced deflection, pocketing, and snag.

To alleviate these concerns, we would recommend using extended blockouts on a single post as the two rails widen out at whatever flare you chose up to a maximum of 15:1.  This should be acceptable up to a blockout depth of 24”. Two separate posts can be used as soon as you reach a minimum offset from the face of the guardrail to the backside of the opposite post of 43”.

I have attached a schematic of what that looks like for a 15:1 flare starting with 8” deep blockouts.

 Let me know what you think and we can discuss things further.

Date May 5, 2016
Attachment MoDOT MGS Median Transition Schematic.jpg


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