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Minimum Rail Height for MGS Long Span and MGS Transition to Rigid Barrier

State WI
Description Text

Does MwRSF has any recommendation for the minimum rail height for the MGS long span or MGS transition to rigid barrier?

On overlay projects, the can overlay the bridge or the roadway. These overlays will reduce the effective height of the thrie beam.

I believe that we will have to accept that the transitions to rigid barrier may be lower than 31".

  • Approach Guardrail Transitions
  • Guardrail
Other Keywords none
Date April 28, 2011


With regards to the MGS long span, we recommend that the system be installed at 31". The MGS long-span guardrail was tested with a top rail height 31.0 in. Previous reports on the standard MGS system have allowed installation at heights as low 27.75 in. However, the reduced post embedment of the MGS system was a major factor in the performance of the MGS long-span design. Thus, in order to retain the improved load distribution and reduced rail strains, the MGS long span should be installed with a top rail mounting height of 31.0 in. Reduction of the rail height below this value will likely reduce the performance of the barrier system.


With respect to the MGS transition, the answer is a little less clear. 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 would expose more of the concrete barrier at the downstream end of the transition and create a potential snag hazard. Thus, we would generally recommend a height of 31" for the MGS transition as well.


We understand the need for states to overlay their roadways, but it can have detrimental effects on barrier performance. 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 overlays adjacent to the MGS transition will not affect its performance. Adding a 2"-3" overlay in the transition area 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.
Date April 29, 2011

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