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Extra Block outs

http://mwrsf-qa.unl.edu/view.php?id=205

On the link above it discusses the use of double and triple
block outs.  I read it that double block outs are OK for any number of
posts.  I thought that would be for the standard 12” block out in the W
beam, but it seems to indicate 8”.  Would the 8” block out, only apply to
the Bridge transition section?  Are there areas of bridge guardrail that
double block outs should be avoided?

Restricted Length Bridge
Guardrail

A question that arises often is one related to placing
guardrail that conflicts with a side road or entrance.  We have developed
a document for guidance, Short Radius Guardrail,
and would like your input.  We realize some of the shorter choices are
less desirable but thought they were better than not doing anything. This is a
tough subject that should probably require research and analysis, but  we
really just want to make sure we are not giving some blatantly bad direction to
designers.  Any input would be greatly appreciated.   

First, the blockouts guidance below is based on 8” blockouts. Thus, we believe that 6” blockouts are acceptable as we have used them in certain special applications without a problem. 24” blockouts are allowed in limited locations as noted in the response.

This holds true for the standard MGS system as well.

http://mwrsf-qa.unl.edu/view.php?id=267

For the short-radius document, I have a couple of comments.

1.       We have provided guidance for attachment of terminals and minimum system lengths for the approach guardrail system and terminals. I would review that relative to the guidance on the first page, as I believe that some of them may be in conflict.

“Thus, the following implementation guidelines should be followed:

1. A recommended minimum length of 12 ft – 6 in. (3.8 m) for standard MGS is to be installed between the upstream end of the asymmetrical W-beam to thrie beam transition section and the interior end of an acceptable TL-3 guardrail end terminal. This segment includes one half-post spacing for Design K and three half-post spacings for Design L.

2. A recommended minimum barrier length of 46 ft – 10½ in. (13.3 m) is to be installed beyond the upstream end of the asymmetrical W-beam to thrie beam transition section, which includes standard MGS, a crashworthy guardrail end terminal, and an acceptable anchorage system. This segment includes one halfpost spacing for Design K and three half-post spacings for Design L.

3. For flared guardrail applications, a minimum length of 25 ft (7.6 m) is recommended between the upstream end of the asymmetrical W-beam to thrie beam transition section and the start of the flared section (i.e. bend between flare and tangent sections). This segment includes one half-post spacing for Design K and three half-post spacings for Design L.”

http://mwrsf.unl.edu/researchhub/files/Report38/TRP-03-210-10.pdf

2.       When discussing the radius options for the short-radius guardrail, we would suggest a minimum radius of 8’. No radius smaller than that has ever been crash tested.

3.       It appears that you are using the Washington short-radius design. This is likely based on the FHWA memo that previously recommended that design for use until a better, crash-tested system is developed. Recently, TTI got TL-2 approval for the Yuma County short-radius design. Some states have moved to this design, and I just wanted to bring it up in case you were unaware. http://www.roadsidepooledfund.org/files/2010/11/T-Intersection-final_2010-08-17.pdf

4.       TTI has also done some recent research into a MASH TL-3 short-radius system. The system did meet the crash test criteria, but we have some concerns about impacts on the system in locations not specified in the crash test matrix. I thought you might want to look at that information as well.

http://tti.tamu.edu/documents/0-6711-1.pdf

http://tti.tamu.edu/documents/0-6711-S.pdf

5.       On page three, you discuss an area for breakaway posts for capture of errant vehicles. You have chosen an area from 5-15 degrees. We have recently been doing research with NDOR on a new safety treatment for intersecting roadways and have looked into similar issues of necessary capture area. We believe that the angle of impact in the capture area may vary from 0-25 degrees. We defined the potential impact area for errant vehicles based on a runout length calculation. NDOR liked this approach because it provided for a more justifiable definition. The report on this research should be out in a month, but I have attached a draft of the chapter describing this for your review.

Thanks