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MGS Long Span installed on 2:1 Slopes

Question
State WI
Description Text We are currently looking at options for installation of the MGS long span at the slop-break-point of a 2:1 slope.

What are your recommendations regarding this type of installation?
Keywords
  • Guardrail
Other Keywords Long Span
Date January 26, 2016


Response
Response

I looked through past guidance we have given regarding the MGS long span and its use adjacent to slopes.

 

In the past, we have fielded inquiries regarding the use of the long span at the slope break point. In those discussions, we have noted that the previous testing of the MGS with steel and wood posts suggests that there is potential to use the long span installed at the slope break point if 8 or 9 ft W6x8.5 steel posts are used or 7.5 ft SYP posts are used. Both of these options previously were evaluated to MASH and believed to be acceptable for use on 2:1 slopes. Similarly, we have recommended that the CRT posts used in the long span be extended to 7.5 ft long posts to ensure their proper breakaway function similar to the previously tested systems. All of this guidance assumed standard post spacing and assumed that the lateral offset between the back of the post and the culvert head wall  and the height of the headwall relative to the unsupported span section was maintained. It should be noted that this guidance was provided using our best engineering judgment in the absence of full-scale crash testing, computer simulation, dynamic component testing, or combination thereof. If new information becomes available, MwRSF may deem it necessary to revise this guidance.

 

You had inquired about the potential to a half post spacing MGS long span system in a similar manner. Previous research at MwRSF has suggested that 7’ long posts at half post spacing adjacent to a 2:1 slope should perform in a similar manner to the standard posts and spacing on level terrain. Thus, from a stiffness standpoint, the use of half post spacing seems reasonable. The issue with half post spacing comes from the CRT posts. In order to maintain similar stiffness and deflection of the system, we would likely need to extend the half post spacing to the CRT’s. However, the use of energy dissipating CRT posts at half post spacing may affect the function of the long span. The extra CRT’s may provide similar deflection, but they may also change the behavior of the CRT’s in terms of preventing pocketing and how the CRT’s affect the entry and exit of the vehicle with respect to the unsupported span. Without further study, it is difficult to assess how much of an affect the half post spacing of the CRT’s may have. Thus, while there is potential for a half post spacing pf the MGS long span to work it would likely require further study.

 

Thanks 

Date January 26, 2016


Response
Response

I was working with you about a long-span issue  the region put together some drawings (see attached).  I was wondering about the holes for the CRT posts.  Are they located correctly?  I’ll probably need to call to explain.

 

Date March 29, 2016
Attachment 021001_CD_Rocky Run Creek Culvert.pdf


Response
Response

We don’t have a definitive answer for you question regarding the holes in the CRT post. It is true that the lower hole in the CRT post is designed to fracture to post below grade for weaker soils. However, our previous discussion recommended increased post embedment to account for the reduced soil forces due to the slope. Thus, it is currently our best guidance to leave the holes in their standard location.

 

While it may be possible to optimize that hole location when used adjacent to slopes, determining that hole location would require further research.

 

Date March 31, 2016


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