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Thrie Beam Guardrail Transition

State KS
Description Text

I am trying to upgrade our drawings to the MGS.   I have some questions on the transition section as shown on our new drawing, RD613A (see att.).  Initial thoughts were to keep our similar thrie beam section with the 6'6" post as shown on RD613 (original drawing) for the 31'-3" total length of transition.  Note: all of our current post lengths are 6'6"; thrie or w-beam sections as shown on RD613.   For our new MGS drawing (RD613A), do we need to update the post in the thrie beam section  to 7' similar to your testing?  At the thrie to w beam section of rail, your post were shown as 6' long and we are still showing 6'6". Should this change to 6'?   I recall some conversations that states could keep their particular thrie beam transition section but wanted to verify some of these thoughts. Any help would be appreciated.

  • Approach Guardrail Transitions
  • Guardrail
Other Keywords none
Date December 1, 2010
Attachment rd613.pdf
Attachment rd613a.pdf


The original Kansas DOT thrie beam approach guardrail transition (RD613) utilized 12 ft " 6 in. of nested thrie beam and 12 ft " 6 in. of single thrie beam between the bridge end and the thrie beam side of the W-beam to thrie beam transition section. A 6 ft " 3 in. long W-beam to thrie beam transition segment was then used, followed by standard W-beam guardrail. A 1 ft " 6¾ in. post spacing was used through the nested thrie beam section, while a 3 ft " 1½ in. post spacing was used over the single thrie beam as well as the transition element. Steel and wood posts were denoted to be 6 ft " 6 in. long through and including the post at the midpoint of the transition element.


In comparison to several other NCHRP 350-approved thrie beam approach guardrail transitions, detail RD613 is a relatively long design which was intended to provide a gradual change in lateral stiffness for impacts near to and upstream from the bridge end.


In recent years, the Pooled Fund program sponsored the development and testing of a stiffness transition for the upstream end of the original thrie approach guardrail transitions. From this research, it was determined that the shorter transition designs may need to be extended in order to provide a more gradual change in lateral stiffness. However, we also observed a vehicle pocketing/rollover propensity when changes in thrie beam nesting, post spacing, and/or post type coincided.


In detail RD613A, it appears as though several posts were added beyond the W-beam to thrie beam transition section using a half-post spacing. Based on some of our prior examples for adapting the new stiffness transition to existing approach guardrail transitions, it would seem that several post could be removed beyond the W-beam to thrie beam transition segment such to have only 1 or 3 half-post spacings in this W-beam region. RD613A shows a 12 ft " 6 in. long segment of single thrie beam. Based on our adapted design variations, it would seem reasonable to utilize a 6 ft " 3 in. segment of thrie beam. Further, I may recommend that 6 steel posts be configured with a length of approximately 6 ft " 6 in. using a quarter-post spacing, similar to what is depicted in our draft transition report. It would also be worthwhile to use four shorter 6-ft long steel posts at quarter-post spacings and starting at your current post 7. All remaining steel posts would also use the 6-ft length. Section C-C depicts a 7-ft 6-in. long steel post. I believe you intended to depict it as a 6-ft 6-in. long steel post. In summary, it would seem reasonable to more closely match your detail to that proposed in Figure 96 of the draft transition report.

Date December 9, 2010

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