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MGS with 1/4-Post Spacing to Shield Hazard and Placement Guidelines

Question
State
Description Text

I have another MGS question for you.  We have a situation where a sign truss foundation is located 13" from the back of guardrail posts.  MGS with standard post spacing was installed which would deflect into the concrete foundation.  The minimum deflection distance we are using for the 1/4-post spacing installation is 14" measured from the back of post to the near edge of the hazard.  The designer is proposing to add posts and to stiffen the rail by doubling up on the rail element thereby further reducing the deflection.

 

Do you have any comments/objections to this approach seeing that we only need to reduce the deflection by 1"?

 

Do you have any data on the anticipated deflection distance for this proposed installation?

 

See attached drawing of the proposed modifications.  Note that the rail is gradually stiffened by using 1/2-post spacing and then the 1/4-post spacing.  On the departure end of the 1/4-post spacing is 1/2-post spacing needed before getting back to standard spacing?

Keywords
  • Guardrail
Other Keywords none
Date December 18, 2009
Attachment SK_MAI_RAS_RFI00131_12152009.pdf


Response
Response

I have attached a pdf copy of our prior TRB journal paper for the Midwest Guardrail System (MGS). Within the paper, guardrail placement guidelines are provided for treating hazards. These guidelines pertain to the distance between the front face of the rail to the front face of the hazard. As noted, the minimum recommended distances are 1.25, 1.12, and 0.90 m (49, 44, and 35 in.) for the standard, half-, and quarter-post spacing designs, respectively. It should be noted that the width of the steel-post MGS is 0.54 m (21.25 in.). Using this information, one would need to consider using a clear distance of approximately 0.35 m (13¾  in.) between the back of the steel posts and the front face of the vertical hazard when utilizing the quarter-post spacing system.

 

Below, you noted that the available clear distance between the back of the steel posts and the front face of the rigid, vertical concrete foundation is 0.33 m (13 in.). Based on the guidelines noted above, your noted solution to use the MGS quarter-post spacing design and noted placement would result in 20 mm (¾ in.) less clear distance than that recommended in the paper (as noted above). However, I do not have significant concerns with using the basic ¼-post spacing MGS configuration to shield the noted hazard nor deem it necessary to use nested W-beam rail to cover the 20 mm (¾ in.) deficit in provided clear distance.

 

Finally, the use of the ½-post spacing in advance of the ¼-post spacing MGS seems reasonable and an appropriate transition in stiffness. In addition, I see no reason to utilize a ½-post spacing MGS on the departure end if reverse-direction impacts cannot occur.
Date January 6, 2010
Attachment 1890_MGS.pdf


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