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FHWA Short Radius Beam Guard Technical Memo

Question
State WI
Description Text

I have some questions about the FHWA short radius beam guard technical memo.  From the FHWA technical memo the top of rail is 27 1/8" from the ground.  The Roadside Design Guide indicates that the top of a beam guard rail should be 27" or 28" (page 5-13 Roadside Design Guide and Figure B4.b in Appendix B) from the ground.  WisDOT standard detail drawing uses a top rail height of 27 3/4". 

 

Would it be acceptable to adjust the rail height of the FHWA short radius system to match WisDOT's use of the 27 3/4" rail height?

 

If WisDOT would switch to the MGS system, would it be acceptable to adjust the top of rail height for the FHWA technical memo to match the 31" of the MGS system?  Because the MGS system places the lap of the rail in the middle of the span between posts, would the FHWA short radius system need to be lapped similarly?

 

Are the specifications for the Wood Break Away Post on page 10 of the FHWA technical memo standard for all Wood Break Away Post (e.g. stress grade...)?

Are there standards for the CRT and regular beam guard post similar to the Wood Break Away Post (e.g. stress grade...)?

 

The FHWA details are using "A Guide to Standardized Highway Barrier Rail Hardware from 1979.  I assume that this book was the previous guide for "A Guide to Standardized Highway Barrier Hardware".  The problem I'm having is that the old nomenclature used in 1979 is not synchronized with the Online barrier hardware guide at:

http://aashtotf13.tamu.edu/Guide/nameindex2.html

 

For an example F-3 from the FHWA technical memo does not match any of the fastener hardware designations in the online manual. Is there a translation key from the 1979 manual to the online version ( i.e F-3 in the tech memo = FBB04)?

 

I was reviewing the Yuma County detail, I assume that the two CRT post that are 2-3 feet behind the railing are there to slow impact vehicle.  Would a similar set up of CRT posts help prevent the pick-up truck from sliding over the thrie beam rail in the MwRSF thrie beam system? 

 

I'm guessing that MwRSF has looked into adding multiple CRT post behind the rail like Yuma County and decided that it would not work. I would appreciate it if MwRSF could explain to me why the addition of CRT post behind the rail would not work with the MwRSF thrie beam system, or how the Yuma system uses the CRT post behind the rail during an impact.  I'm just trying to get a better handle on how these systems work.

Keywords
  • Guardrail
Other Keywords none
Date May 28, 2008


Response
Response

I have some replies to your short-radius questions.

 

First, we cannot recommend changing the height of the Yuma County short radius guardrail. We do not have any basis for evaluating the effects of the change in rail height, and our estimation of the effects of any change in rail height would be extremely limited. We know that changing the height will affect the capture of sedans that the system was designed for as well as the capture of small car and pickup truck size vehicles. Thus, without further analysis, we would recommend leaving the rail height as stated in the memo.

 

Similarly we could not recommend a change to the 31" rail height of the MGS system. One of the critical issues in the design of a short radius system is capture of the vehicles. This is why our current design uses a slotted thrie beam section as the rail element. We want to maximize the ability of the system to capture both small cars and larger cg vehicles. We have seen in our current design that we are near the limit for effectively capturing the small car even with thrie beam. Thus, we would not recommend raising he w-beam height on the FHWA recommended system.

 

The posts shown on page 10 are standard BCT end terminal posts, so you can feel free to use whatever post spec you usually use when calling out BCT posts.

 

We did look at placing a series of posts behind the system in order to further slow down the vehicle, but we did not pursue it for two main reasons.

 

  1. Testing with the small car vehicle showed that we are currently at or near the occupant risk limit for the small car. AS such, putting more posts behind the system will increase the deceleration of the small car and be detrimental.
  2. We have shown in our pickup truck testing that we have issues with debris build up causing vehicle instability. As such, we were leery of placing more posts in the system and creating more debris.

 

The Yuma county system is old and did not have to meet our more stringent test demands with two vehicles. They used the extra posts to increase vehicle deceleration, but that is not a good option for our system.

Date May 29, 2008


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