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What Degree of Slope of a Concrete Barrier Would Require Head Ejection Considerations

Question
State WY
Description Text

Our Bridge Program designed a 42 inch high single slope concrete barrier
to protect some bridge columns underneath the interstate.  The design speed of this roadway is only 30
mph. The slope of the barrier face is 5.8 degrees.  The barrier the pooled fund tested with head
ejection criteria is about 3 degrees.  Do
we need to consider head ejection for a barrier with a slope of 5.8 degrees
(and also considering the speeds are relatively low).  As a follow-up question, do you have an idea
of what barrier slope face would dictate head ejection criteria for a high
speed roadway?



Keywords
  • Guardrail
  • Permanent Concrete Barriers
Other Keywords head ejection, single slope
Date July 3, 2014
Attachment Head Ejection.docx


Response
Response

Although we have not studied head ejection for impact speeds below TL-3 conditions, I don't believe that an occupants head would extend very far out the side window on a 30 mph roadway.  Thus, the risk of head slap is greatly reduced, and I do not think you need to incorporate head ejection into your design for such a low speed roadway.

The head ejection envelope was developed for vertical (or near vertical) barrier geometries.  Although single slope barriers show increased vehicle stability during impacts over safety shaped barriers, existing single slope barriers (9.1 and 10.8 degrees from vertical) do cause some vehicle climb.  However, head ejection is still present with single slope barriers.  In fact, a few impacts with single slope barriers were utilized in the initial development of the ejection envelope.  We do not have a set slope angle in which the envelope should be applied as this has never been studied.  Though, the answer would probably be more of a sliding scale reduction factor that increased with an increase in slope. Unfortunately, we just don't know this answer at this time.  So, on the side of safety and being conservative, you could apply the head ejection envelope as it currently stands to any single slope barrier (MwRSF recommended method).  Or, you can choose to use engineering judgment and take a more aggressive approach.

Date July 7, 2014


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