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MGS Blockouts and Slopes

Question
State VA
Description Text

We are beginning our development
of the VDOT MGS standard drawings.  There is a little confusion on the
exact dimensions of the 12” blockouts.  Is there an allowable tolerance
for the depth dimension?  Some of the wood samples are routed  and
are 11” deep.  The composite sample is 11 ½” deep.



 



In addition, what is the
standard dimension from the back of the post to the hinge point?  We have
seen the research for the post being installed at the hinge point but want to
establish our standard layout for our geometrics.



 

Keywords
  • Guardrail
Other Keywords none
Date September 1, 2016


Response
Response

With respect to the MGS blockouts, we don’t typically provide tolerances for the states unless it is a safety performance issue. For the MGS, the nominal blockout depth is 12”. The blockouts on the MGS serve two purposes. First, they space the rail away from the post which reduces vehicle snag on the post. Second, the blockout helps maintain the rail height as the post rotates and promotes improved vehicle capture and stability.

 

We conducted all of our MGS with non-routed blockouts. However, we understand that several states prefer routed blocks and we do not see any issue with the slight loss of depth for the routed system.

 

Similarly, there would be no issue with using a 11 ½” deep composite blockout. The MGS has been successfully tested with 12” blocks, 8” blocks, and non-blocked configurations. Thus, small variation in the blockout depth should not be an issue.

 

I assume when you refer to the hinge point below, you mean the distance from the slope break point. That answer is somewhat dependent on the slope being shielded. We have may many recommendations regarding this issue over the years. Our standard answer has been that we believe that a 2 ft offset from the back of the post to the slope break point should provide similar performance for the MGS as when it is installed on level terrain. We have tested other versions of the MGS at the slope break point. One version had 9’ long posts at the slope break point of a 2:1 slope. TTI tested a similar system with 8’ long posts. Thus, another option has been to install the MGS with 8’ or 9’ posts at the slope break point. This provides similar levels of dynamic deflection as the level terrain system.

 

More recently, we did test the MGS at the slope break point of a 2:1 slope with standard 6’ long posts. This system met MASH but had significantly higher dynamic deflections than the standard MGS.

 

If you have specific slopes you are shielding, we may be able to provide a better answer.

 

We currently have a research project in the pooled fund to develop guidelines for the placement of the MGS adjacent to slopes. The background research is complete, but I need to finish the guidance. I have attached the proposal right up for you to review. It also contains some of our previous slope guidance to reference.

 

Let me know if you have further questions.

 

Date September 2, 2016
Attachment MGS_Slope_Guidance.pdf


Response
Response

Thanks for your assistance.  I have a few other questions..

 

Has a double sided (median) version of the MGS been developed and tested?

 

How about a weak post version of the same?

Date September 3, 2016


Response
Response

The MGS has been approved as a median barrier system. MwRSF reviewed previous testing of 31” tall W-beam median barriers and received an approval letter for the system without full-scale testing in 2010. See attached.

 

Since then, TTI tested a MGS median barrier with 8” blockouts that met MASH as well. http://tti.tamu.edu/documents/9-1002-12-8.pdf .

 

Weak post W-beam guardrail (G2) was tested to MASH under NCHRP 22-14(3) using the 2270P vehicle only. It was successful. To the best of my knowledge, a median version has not been evaluated. However, there would seem to be potential for such a system to work in terms of the capacity of the system to contain the pickup truck. One concern would be the override of the S3x5.7 posts by the 1100C vehicle. We have observed floorpan cutting in cable barrier systems when the small car vehicle overrides these posts. Similar concerns would exist for the G2 system or a median version of it. The 1100C test was not conducted during the TTI research. The G2 system was tested under NCHRP Report 350 and puncture of the gas tank of the 820C vehicle was noted. However, the test was still a pass.

 

Let me know if you need anything else. 

Date September 4, 2016
Attachment B-204 - MGS Median Barrier acceptance letter.pdf


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