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Asperities in Concrete Barrier

Question
State MN
Description Text

We are considering the use of asperities in a concrete bridge rail and have used NCHRP 554 as guidance. 

http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_554.pdf

The asperities on our barrier design(attached) appeared to fall within the
guidelines in the report. The only issue was the guidelines were developed for
NCHRP 350 TL-3 requirements while our barrier must meet MASH TL-2. Given the
speed of the roadway (30 mph) is much slower than NCHRP 350 TL-3 and the fact
that our barrier is vertical was enough to convince the MnDOT Bridge Design
engineer to approve it.


Any
additional thoughts on our approach?

Keywords
  • Bridge Rails
Other Keywords none
Date October 12, 2017
Attachment DOC100317-10032017110654.pdf


Response
Response

A TL-2 test condition will help some to mitigate snag risk. One thing to note is that the NCHRP Report 350 criteria used a small car impacting at a 20-degree angle, while the MASH criteria use a small car impacting at a 25-degree angle. Any snag risk with small car may be accentuated with a 5-degree angle increase.

 

At the end of the rail near posts, do you have a 2" recess with a sharp 90-degree angle coming forward? Or, do you have tapered ends on upstream and downstream ends of longitudinal recessed region?

Date October 12, 2017


Response
Response

The 2” projection on the bottom is the MnDOT standard “brush curb” that runs longitudinal with the barrier with no disruptions. The vertical posts are projected 1” at 90 degrees and what I didn’t mention in my first email is that we are changing these to include 45 degree bevels on both the upstream and downstream ends to comply with the Caltrans guidelines (included at the end of the NCHRP 554 report).  

Date October 13, 2017


Response
Response

You note that the post face to recessed rail face is only 1 in. where the interface will now include a 45-degree bevel on both side of post. For now, that may be an option until further MASH data comes forth. We may have some Hawaii DOT bridge rail testing in future that may help update on knowledge under MASH. Thanks!

Date October 14, 2017


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