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Double thrie beam to two single faced thrie beams

State WI
Description Text I did some stumbling around looking for double faced thrie beam to two single faced thrie beam. The details that I did find typically had one post with extra blocks. The next post had two steel posts very close together.

I’m worried that one steel post will interfere with rotation of the steel post struck. How much room between two steel posts is adequate?

I was thinking 2 feet when I drew up my PDF.

I don’t know if there is a right answer. I know that adding a bunch of blocks has not be tested either.
  • Guardrail
Other Keywords Median barrier
Date October 28, 2015
Attachment double faced thrie beam tran drawings v1.pdf
Attachment double thrie.jpg


As a quick conservative thought, I might consider that the rail could deflect around 30 in. or more. We may not want the back of an impacted rail to deflect into the back side of an opposite-side steel post.


A single post with 12-in. blocks on each side would result in a gap of 30 in. (two 12-in. blocks plus 6 in. steel post). However, a dual-post system with 8-in. blocks would require a 44-in. gap to not allow posts to contact the backside of second rail using a 30-in. clear region.


If we extend two 8-in. blocks on each side of a single post, we end up with a 38-in. gap. This gap size still falls short of the 44-in. gap, or point where one may consider transition from 2 posts to 1 post.


I am less certain about using three 8-in. blocks on each side.


Note that these quick thoughts have not been tested/evaluated and may also be overly conservative.


As a second thought, another option would be to stager post pattern on dual-side region and use blockouts on both sides of dual posts. This options would mitigate concerns for rail rupture and may allow for narrower gap.


Date October 28, 2015

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