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Transition between G4(1S) and MGS.

Question
State MI
Description Text Per our phone conversation earlier today, I’ve attached a file containing MDOT’s proposed MGS details. If you refer to sheet 10, I’ve highlighted our proposed details for transitioning from MGS guardrail to conventional (28” tall) strong-post, w-beam guardrail and thrie-beam guardrail, respectively. We would like to introduce a single 3’-1.5” post spacing in order to transition from the splice being located between posts (MGS style) to the splice being located at post locations (conventional guardrail style). By using a single 3’-1.5” post spacing, we can use standard 13’-6.5” long beam elements throughout the entire transition. We are trying to avoid the use of w-beam elements that do not have a standard length (13’-6.5”) for ease of maintenance.



Do you foresee any issues with these details, or do you have any comments or suggestions concerning these details?
Keywords
  • Guardrail
Other Keywords none
Date March 11, 2016
Attachment R060J.pdf


Response
Response

Ron asked me to look at you details and provide some guidance regarding the transitioning from the older G4 type W-beam systems to the 31” tall MGS and dealing with the movement of the splice location. Your plans current show a height transition of the W-beam from the old G4 barrier height to the MGS height over 25’. We have previously recommended that this transition occur over a distance between 25’-50’. Thus, you plan looks acceptable there. We have more concern with the use of the additional post when relocating the splice to the midspan. Adding the post directly downstream of the splice location will tend to increase barrier stiffness and rail loads near the splice location. In previous testing, this has been shown to create a potential for rail rupture. Thus, this would not be our first choice in this area.

 

We believe that there are two options for transitioning the splice that may work better.

 

1.       As you noted, there is the option to use a non-standard rail section. This would work well, but as you noted can be undesirable in terms of inventory and maintenance.

2.       A better solution may be to proceed with the posts at the splices for one full 12.5’ rail section at 31” height. Then rather than add an extra post, we would recommend omitting the post at the next splice to make a 9.375’ long span between posts. This should place the posts in the correct spacing for the midspan splices used in the MGS. We believe this can be done because we recently conducted a successful MASH TL-3 test on the MGS with a single omitted post which created a 12.5’ long span between posts. This would suggest that using a 9.375’ long span to get the correct post alignment for the MGS should not be an issue. However, as noted previously, we would recommend not omitting the post until a full 12.5’ of 31” tall barrier is in place. See the sketch below.


Please let me know if you have any questions about this. We can provide more thoughts if needed.

 


Date March 29, 2016
Attachment G4 MGS transition.jpg


Response
Response

Thanks for looking into this and replying to my request. I agree that your suggested approach of omitting a post and creating a 9.375’ long clear span is advantageous, and MDOT will definitely look into this option.

 

Has your group prepared a report for the MASH TL-3 test where a post was omitted resulting in a 12.5’ long clear span? If so, could you share that report? Did your group use 12” or 8” offset blocks when you conducted the test? If your group used 12” offset blocks in your recent test, do you believe the same treatment could be applied to MGS with 8” offset blocks without jeopardizing the crashworthiness of the system?

 

Date March 30, 2016


Response
Response

We have not quite completed the omitted post report at this time. I can send you a link to the document when it is complete. As a side note, all of our research reports are currently archived online at the link below. You can always look there for any of our completed research.

 

http://mwrsf.unl.edu/researchhub.php

 

As for you question regarding the use of 8” blockouts, we did evaluate the system with 12” blockouts as we feel that they improve vehicle capture and stability. That said, we do not believe that the use of 8” blockouts would degrade the performance of the omitted post system sufficiently to jeopardize the crashworthiness of the system.

 

Date March 31, 2016


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