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Bridge Thrie Beam transition bend

Question
State MO
Description Text
 <p>I have a technical question regarding the transition of thrie beam.  If you are able to provide any advice it would be greatly appreciated. </p><br /> <br /> <p>We have a location where a bridge with thrie beam rail would be used in place while upgrading the approach rail to MASH.  The plan is to attach to the existing thriebeam at the green highlighted joint, but the shoulder and existing rail widen out as they get further from the bridge.  </p><br /> <br /> <p>The standard shows the thrie beam transition being straight, so the question is if it’s allowed to bend out like the existing rail does? </p><br /> <br /> <p>Thanks</p>

MASH
TL-3

Approach Guardrail Transitions (AGTs)


Date July 29, 2025
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Attachment MODOT-1.jpg
Response
Response
(active)

It appears that your question is related to the allowable lateral barrier flare rates for guardrail systems. There are several items to consider.

  1. In terms of MASH compliant thrie beams, only two thrie beam guardrails have been evaluated tom MASH. The modified thrie beam system and a thrie beam guardrail system evaluated at TTI that was based on the MGS. Both of these systems are 34” tall to the top of rail. This may be taller than the thrie beam shown in the image below. As such, it may be difficult to transition between the rail types.
  2. Allowable flare rates for thrie beam have not been formally evaluated through crash testing. There are previous and ongoing studies of the MGS when flared. These studies would provide the best guidance for thrie beam and could be used to estimate allowable flare rates for thrie beam.
    1. We previously evaluated flare rates for the MGS under NCHRP Report 350 impact conditions. In that study, we were able to achieve a maximum flare rate of 5:1. Because of the increased rail section and strength of thrie beam, we would be reasonable to assume that thrie beam could achieve similar flare rates under NCHRP Report 350 systems.

                                                                              i.        https://mwrsf.unl.edu/researchhub/files/Report92/TRP-03-191-08.pdf

    1. MASH evaluation of flare rates has been underway at TTI for several years. TTI has evaluated flare rates of 11:1, 21:1, and 25:1 with the MGS. All of these tests have failed due to rail rupture or anchorage failure. As such, a MASH compliant flare rate for W-beam guardrail has yet to be established at this time. As such, it is difficult to estimate allowable MASH flare rates for thrie beam. It would seem reasonable that thrie beam should be able to better accommodate flares than W-beam to the increased rail section and strength, but we cannot define the flare rate clearly at this time.
    2. The RSI MFLEAT flared terminal has been tested along it redirective length under MASH TL-3 conditions with a flare rate of 13.3 :1. This testing was at the beginning of LON of the terminal and was relatively close to the end anchorage. Additionally, the MFLEAT uses a little over 4’ post spacing. This would suggest that flare rates are achievable, but it is hard to extend that flare rate to thrie beam without further research.

That is the best guidance we can provide at this time. In short, there is not a definitive flare rate for thrie beam.

Thanks!

 
Date August 1, 2025
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