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31" Guide Rail Curb Offsets

Question
State NJ
Description Text

What criteria do you have on the 31” MGS placement behind curb? This is what I have so far:



NCHRP TL-3 used 6” Type B Curb (type of vertical curb) which passed 6” behind curb for 31” height measured from gutter line. The designer can use the lay down curb as shown in the Roadside Design Guide Figure 5-35(b) (see Section 5.6.2.1.2 2nd paragraph) in lieu of the 6” AASHTO Type B curb.



TRP-03-221-09 concluded for MASH TL-3 that an 8’ offset is not acceptable.



TRP-03-237-10 concluded that for MASH TL-2 that 4’ to 12’ max is acceptable.



Can we use the sloping curb offsets shown in the 2011 Roadside Design Guide, Section 5.6.2.1.1 which is:



· Design speed less than 45 MPH: Set flush or at least 8 feet behind the face of curb. Use 6” high or shorter sloping-faced curbs.



· Design Speed 45 to 50 MPH: Set flush or at least 13 feet behind the gutter. Use 4” high or shorter sloping curbs.



· Design Speed greater than 50 MPH: Set flush with gutter. Design speeds above 50 MPH, use 4” or shorter sloping face curb. For design speeds above 60 MPH, the sloping face should be 3:1 or flatter and no taller than 4” high.

Keywords
  • Guardrail
Other Keywords Curb
Date April 4, 2016


Response
Response


I have added some comments below regarding curbs.

 


What criteria do you have on the 31” MGS placement behind curb?  This is what I have so far:

 

NCHRP TL-3 used 6” Type B Curb (type of vertical curb) which passed 6” behind curb for 31” height measured from gutter line.  The designer can use the lay down curb as shown in the Roadside Design Guide Figure 5-35(b) (see Section 5.6.2.1.2 2nd paragraph) in lieu of the 6” AASHTO Type B curb.

This is correct, and we believe that less severe curbs such as the one sloped curb you note would be acceptable to NCHRP Report 350 as well. The MGS with curb in this configuration has not been evaluated to MASH TL-3 at this time. There is a proposal in the Year 27 Pooled Fund to evaluate this to MASH.

 

TRP-03-221-09 concluded for MASH TL-3 that an 8’ offset is not acceptable.

True. We did test this as we believed it was a critical configuration, and found that the 2270P vehicle became unstable and rolled.

 

TRP-03-237-10 concluded that for MASH TL-2 that 4’ to 12’ max is acceptable.

We did do additional analysis for larger curb offsets at TL-2 and had a single test of a MGS offset 6 ft from the curb that was successful. I should note that the 6 ft offset test used a MGS installed with a height of 37 in. from the gutter line. Based on this test and our LS-DYNA work, it was concluded that the 37 in. high MGS was acceptable for curb offsets between 4’-12’.

 

Can we use the sloping curb offsets shown in the 2011 Roadside Design Guide, Section 5.6.2.1.1 which is:

This guidance is based on slightly older research and pertains to the G4(1S) guardrail system and mounting heights. We would expect the MGS to perform as well or better than the G4(1S) systems in almost all circumstances. Thus, the RDG guidance is likely acceptable for the MGS unless we have the previous research noted above to alter it.

 

Note that none of the RDG guidance appears to consider alteration of the rail height relative to the gutter as we did in our previous studies.

 

·         Design speed less than 45 MPH: Set flush or at least 8 feet behind the face of curb.  Use 6” high or shorter sloping-faced curbs.

 

·         Design Speed 45 to 50 MPH: Set flush or at least 13 feet behind the gutter.  Use 4” high or shorter sloping curbs.

 

·         Design Speed greater than 50 MPH: Set flush with gutter.  Design speeds above 50 MPH, use 4” or shorter sloping face curb.  For design speeds above 60 MPH, the sloping face should be 3:1 or flatter and no taller than 4” high.

Date April 7, 2016


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