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Guardrail Transition to MGS

Question
State IN
Description Text In Ohio, they have a standard drawing showing a
transition from existing guardrail to MGS, see attachment.  This transition adjusts for the height
difference between the two systems within 25 ft and moves the spice from the
post to the midspan.  This standard uses
a 3’-1.5” post pacing closest to the existing guardrail.  I have seen on the Q&A page that you have
addressed the transition question before (1076) and did not recommend placing a
3’-1.5” post spacing to keep the 12’-6” standard w-beam sections.  Rather you suggested that the height
transition be completed within 25 ft with splices at the posts, develop the 31
inch height for one 12’-6” w-beam section, and then drop a post to start the
midspan slices.  This would make the
transition 53’-1.5”.  Have there been
other discussions that have led to the Ohio standard being acceptable? 
Keywords
  • Guardrail
Other Keywords none
Date May 11, 2016
Attachment MGS-4.3_1-18-2013 Transition.pdf


Response
Response Hello.

The guidance in 1076 is currently our best guidance in terms of transitioning from G4(1S) systems to the MGS without using a odd length rail section to transition from splices at the posts to midspan splices. Keep in mind that these recommendation our based on our engineering judgment and past knowledge of the impact performance of W-beam guardrails systems in MASH and NCHRP 350 testing. Thus, our recommendations tend to represent our best case for performance as is is difficult to anticipate at what point we reach system limits without testing. Other design iterations may still work. So, we are not suggesting that other transitions are not feasible, but rather that we have more confidence in the recommended transition. 

That is not to say that other designs may not work. However, none of these types of transitions have ever been crash tested, so we tend to err on the side of caution when making these recommendations. The transition we suggested in question 1076 is similar to a recently tested MGS system with an omitted post that was evaluated to MASH TL-3. Thus, we have confidence that a similar scenario will work to transition between the splices. 

The transition you have shown from Ohio appears to add an additional post at reduced spacing to achieve the splice adjustment. This appears to occur in the G4(1S) section of the guardrail. This may be acceptable, but we might recommend a couple of improvements. 

1. Currently, we are not as confident in using the additional post method to transition between G4(1S) systems to the MGS in the G4(1S) section of the guardrail. Based on previous testing, the G4(1S) system is at or near its limits with respect to TL-3 MASH impacts. We recommended transitioning the splices in the MGS region as the MGS has proven more robust that the G4(1S) system in a variety of crash test scenarios, and it seems more conservative to apply any splice transition in the more robust guardrail system. 
2. The extra post shown for transitioning the splice is currently shown directly adjacent to a splice. We would adjust this slightly as the splice tends to be the weak point in a guardrail system. Thus, we would place the extra post adjacent to the midspan post and have no reduced post spacing at the splice. This should lower the loading on the critical location of the splice in the rail. - See attached.
3. As noted in 1076, we would not recommend this until you have completed the height transition and have a minimum of 12.5' of 31" rail. 

You could also do the additional post splice transition noted above in the G4(1S) section of the guardrail, but locating the transition in that region is less conservative than the approach in 1076 or the 31" guardrail post adjustment noted here. 

Please let me know if you have further questions or comments. 


Date May 19, 2016
Attachment mgs transition.jpg


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