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MGS Transition Maximum Height

State IN
Description Text INDOT has a bridge mounted thrie rail that has a top rail height of 2'-10" (34").  INDOT would like to use the MASH tested MGS stiffness transition but we were not sure if the top rail height of the transition could be raised to 34"?  We see that the MGS guardrail passed the MASH testing for the car at 36" and the truck at 34" but was sure if the transition could be raised to 34".  I thought I saw something in the FAQ about raising the transition for overlays but could not find it.  Attached is a PDF of INDOT's current transition between the bridge thrie rail and strong post w-beam.
  • Approach Guardrail Transitions
Other Keywords MGS Transition
Date June 27, 2017
Attachment Thrie Beam Bridge Rail E706-BRTR.pdf


Recently, MwRSF successfully tested a 34" tall thrie beam transition to a concrete parapet to MASH TL-3 (both small car and pickup truck tests were conducted). The tested design utilized a symmetric W-to-thrie transition segment to transition from the 34" tall thrie beam down to 31" tall MGS, similar to what you are showing here. The standardized transition buttress had a tapered front edge below the rail with a lateral offset of 4.5".  From your drawing, it appears that your bridge rail utilizes 8" blockouts on the bridge rail posts. Since 8>4.5,  this should mitigate vehicle/tire snag at the transition to bridge rail location. Thus, I think this transition design could be safely implemented with a few important criteria listed below.


First, I do not know much about this bridge rail you are using and don't recall ever seeing crash tests on a 34" thrie beam bridge rail.  So, my first comment is that the bridge rail should be a crashworthy system. Second, the spacing between the outside bridge post and the first transition post is very important when analyzing snag potential.  The 34" system had an unsupported span length of 29.5" between the transition post and the concrete buttress.  Thus, you will want to utilize a similar or reduced distance for the length between the bridge post and the 1st transition post.

Date June 30, 2017

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