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MGS 8” offset block

Question
State
Description Text

Looking through MwRSF FAQ’s, the following indicates the MGS
12" block system passed



the TL-3 testing with offset  6" from the
vertical center of the curb. The FAQ also implies this



system is good for offsets varying from 0” (flush) to 6”. [ https://mwrsf-qa.unl.edu/view.php?id=479
]



 



OK here’s my question -



With successful testing of the MGS 8” offset block (TamTI), can
you tell me if this system also



applicable to same as above for 12” offset blocks?



 



Thanks.

Keywords
  • Guardrail
Other Keywords Blockout
Date June 27, 2017


Response
Response

We have not made any specific recommendations regarding the use of MGS with 8” blockouts adjacent to curb.

 

Currently the testing of 31” guardrail adjacent to curbs is limited to:

1.       NCHRP 350 TL-3 MGS with 12” deep blockouts with the face of the rail installed 6” behind the midpoint of a 6” tall Type B curb.

2.       MASH TL-3 MGS with 12” deep blockouts MGS and a top mounting height of 37 in. above the roadway, offset 8 ft behind a 6-in. high AASHTO Type B Curb – failed

3.       MASH TL-2 MGS with 12” deep blockouts MGS and a top mounting height of 37 in. above the roadway, offset 6 ft behind a 6-in. high AASHTO Type B Curb – passed

4.       MASH TL-3 T-31 with no blockout with the face of the rail installed 5” behind the midpoint of a 4” tall asphalt curb

 

We have not conducted MASH TL-3 testing with the MGS adjacent to curb. That is planned for later this summer.

 

Reduction of the blockout depth raises concerns due to increased snag on the posts and reduction in rail height during the impact that may compromise vehicle capture. We evaluated the MGS without blockout on lever terrain we noted the following with respect to curb installations.

 

The MGS was successfully crash tested and evaluated with the front face of the W-beam rail placed 6 in. (152 mm) behind the front face of a 6-in. (152-mm) tall concrete curb according to the NCHRP Report No. 350 TL-3 criteria using a 2000P pickup truck. However, vehicular impacts into guardrail placed adjacent to curbs may contact the barrier face with an increased bumper height and trajectory. As noted above, the mounting height of a blocked MGS can be critical for satisfactorily containing and redirecting a 2000P pickup truck, especially when the front bumper and impact-side wheels become airborne early in the impact event. Further, it has been noted that a non-blocked guardrail system will allow the top rail height to decrease immediately after post rotation. Therefore, a non-blocked MGS adjacent to a concrete curb is not recommended for use without further analysis and crash testing.

 

One would expect that an 8” blockout would perform similarly to the 12” blockout with respect to curbs, and that both would perform better than an unblocked system. The T-31 test above would seem to indicate that as well. However, because we have no MASH testing with the 12” blockout and the 6” offset, we cannot say for certain. We believe that blockout depth aids in capture and maintaining the rail height and reduces post snag, but we cannot say at what point reduction of the offset degrades system performance to the point that it is compromised.

 

A better option would likely be to place the face of the MGS guardrail with 8” blockouts flush with the face of the curb. Previous NCHRP 350 testing of the G4(1S) (8” blockout) found that it passed NCHRP 350 TL-3 when impacted with the face of the rail flush with a 4” wedge curb. Thus, it would seem reasonable to expect that the MGS would work as well when placed with the face of race flush with the face of curb when used with a 6” type B curb due to its increased mounting height.

 

Thanks 

Date June 28, 2017


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