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Modified Thrie Beam with Median Need to Drop One or Two Posts

State IN
Description Text

We have a project along an interstate where construction changes are causing us to place modified thrie-beam within the median.  There are a few locations where a post will need to be dropped to accommodate a crossing box structure. Can you give any guidance for possibly dropping a post within a modified thrie beam run?    Another item we would like to get your thoughts on is, where more than one post needs to be dropped, we are going to ask that they transition to an MGS long span system.  For location that have the MGS long span within the median, guardrail will be placed along both sides of the median.  Thank you


Thrie Beam Guardrails
W-beam Guardrails

Long-Span Guardrails

Date October 25, 2023
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I have several thoughts about your median modified thrie beam installation issues.

  1. We have previously researched and tested an omitted post within the MGS under MASH TL-3 impact conditions for installations like your issues with post interference. There is potential for the median modified thrie beam to perform similarly to the MGS with a single omitted post. Both systems use similar posts and the modified thrie beam uses a more robust rail section. That said, there are potential concerns with this configuration. Modified thrie beam uses steel blockouts to separate the thrie beam from the support posts. Previous testing with W-beam and thrie beam with these types of blockouts has shown that the I-section blockouts can torsional collapse and that the flange edges of the blockouts can cause stress concentrations that may lead to rail tearing. As the omitted post installation may tend to increase the deformation of the rail around the post downstream of the omitted post, there is concern that stress concentrations may arise that could compromise rail integrity. This cannot be known for certain without more analysis and testing. In order to mitigate that potential concern, one could possibly include thrie beam backup plates behind all posts in the installation (the NJ modified thrie beam was evaluated with backup plates only at non-splice post locations) and/or use shortened timber blockouts for the thrie beam that pose a reduced stress concentration risk. Both MwRSF and TTI have used shortened thrie beam blockouts successfully with thrie beam guardrail. TTI has recently developed a very similar MASH TL-3 thrie beam system using these wood blockouts that works in roadside and median configurations. It is very similar to modified thrie beam.
  2. A more straightforward option would be to use the culvert mounted w6x9 post that we developed and tested for MGS culvert installations. We believe that this post could be installed on the culvert in locations where the post install is obstructed by the box culvert as it performs similarly to a post in soil. It does require a minimum of 9” of soil fill.
  3. A long span installation of a median modified thrie beam would seem to have the potential to meet MASH TL-3, but it would likely require testing and evaluation to verify. Because the median modified thrie beam would have a stronger and taller rail section, it is there is the potential for the system to perform in a similar manner to the previous MGS long span if the system were implemented with breakaway posts adjacent to the unsupported span similar to the MGS long span. That said, median barriers have somewhat increased stiffness as compared to roadside barriers and the effect of a median barrier used as a long span has not been investigated. Further, one might want to consider the use of some kind of spacers to tie the front and back rail sections together in the unsupported span. Thus, further research is likely needed to implement this type of solution.
  4. If you have the need for multiple omitted posts, the culvert mounted strong post option noted above would be a potential solution.

Based on the points above, we would likely recommend the use of the culvert mounted posts if possible as they pose the fewest concerns/unknowns relative to the barrier performance. Of course, this has not been full-scale tested and evaluated at this time, but it would likely represent the best option without further research.

Note that the concept of median guardrail (W-beam or thrie beam) with an omitted post or as long span systems may make an interesting pooled fund program submission if you were interested in putting them forward. Other states may have similar needs or issues.


Date October 26, 2023
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Bob, Thank you for the quick response.  I did want to clarify one item.  When I mentioned using a MGS long span within the thrie beam run, I meant that we would have a section of railing that transitioned from the thrie to the w-beam and then have a MGS w-beam long span (~150 ft) then transition back to thrie beam with a section of railing that transition from w-beam to thrie beam.  Would you see a problem with that approach?  And Yes I think I should submit a problem statement, thanks for the idea. 

Date October 27, 2023
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I think you could do that while following previous guidacne for transitioning between those systems.

Date October 28, 2023
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