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Bridge Approach Section

State NE
Description Text I have questions about the use of 29” bridge rail & 28“Bridge Approach Section (BAS) taper to 31” over 50’

Has any thrie-beam BAS been tested at 28”?
Should it work properly?
  • Approach Guardrail Transitions
Other Keywords none
Date September 17, 2015


We have been asked previously about reduced height transitions. We would expect that a slightly lower transition height would allow for redirection of the vehicle. However, all thrie beam transition testing that I have seen was conducted at 31" or at the metric height of 31 5/8". In addition, lower transition heights typically would expose more of the concrete barrier at the downstream end of the transition and create a potential snag hazard. This would not be an issue for you as you are using lower bridge heights as well.


With respect to transitions, the issue has not been adequately addressed with respect to the current MASH vehicles and impact conditions.


We cannot say with certainty that reducing the rail height of the MGS transition will not affect its performance. Reducing the height of the transition 2"-3" will increase the impact load height on the post and the relation of the vehicle front to the barrier. This in turn could increase the moment on the posts and affect the lateral deflection and stiffness of the system. In addition, there would be concerns for vehicle stability as well as an increased potential for wood posts in the transition to fracture. Thus, we would generally recommend a height of 31" for the MGS transition as well.


We have noted that for TL-2 applications, there is increased potential that a reduction in the rail height for the transition of two inches would likely be acceptable.


Let me know if you need anything else.

Date September 18, 2015

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