We are considering increasing the height of our standard F-shape 32” concrete bridge rail.
Because of the TTI crash test (Test Report 9-1002-5), we have decided that 36” would be the minimum height. The F-shape however has the instability issue with the bottom “heal” section, so we are also considering a single slope solution similar to Wisconsin’s design.
However, for both cases there are some concerns about the Head Ejection Criteria, based on the MwRSF report TRP-03-194-07.
Based on your knowledge and experience on this issue, does the Head Ejection concern apply to 36” tall barriers (f-shape or single slope)?
This is a hot and urgent issue, so a quick response is appreciated, if you can.
Thanks
The following is a summary of our discussion we had over the phone today, April 10th.
You had questions about how to apply the Head Ejection Envelope (as detailed in report no. TRP-03-194-07) to a 36” tall single slope barrier. We discussed 2 possible solutions to this issue. The first, and more safety conservative, approach would be to apply the Head Ejection Envelope directly to the single slope barrier even though it was initially created for vertical faced barriers. This process includes cutting out a triangular wedge from the top front corner of the barrier to keep the 36” tall barrier from violating the Head Ejection Envelope. The cut out wedge would measure 1.5” vertically (as the envelope begins at 34.5”) and would extend 4” laterally into the top of the barrier. This cut out should not alter the performance of the barrier for TL-4 impacts as the 36” height will still be there to apply a vertical force the SUT vehicle’s box and prevent the vehicle from rolling over the barrier.
The second option would be to leave the 36” single slope barrier as is – no cut out at the top. This could be justified by the fact that the barrier is single sloped, not vertical faced. The sloped face of the barrier would induce a small amount of vehicle climb for the impacting side front tire. This little bit of climb coupled with slight vehicle roll away from the barrier that would go with it, may be enough to keep an occupant’s head from impacting the top of the barrier. However, I cannot say this with any form of certainty. Further, the vehicle may not roll or climb much at all during non-tracking impacts (vehicle is yawing just prior to impacts). As such, this approach is not as safety oriented as the first option.
We also briefly discussed the head ejection effects on 36” tall F-shape barriers. Due to the bottom toe the barrier, vehicles impacting F-shapes tend to climb up the face of the barrier and roll away from the barrier. As such, head ejection is not a problem for safety shaped barriers (NJ and F-shape). Side note: The vehicle roll and climb that eliminates concerns for head slap is the same instability issues that have led to vehicle rollovers and the main reason that we now recommend single slope and vertical faced barriers over safety shaped barriers.
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