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Working width of MGS median barrier

Question
State OH
Description Text ODOT would like to install a two sided MGS barrier guardrail in a relatively narrow median.  We typically use a minimum barrier clearance to an obstacle of 5 feet, measured from the face of the barrier to the obstacle, for both barrier guardrail and normal MGS.  Would a two sided barrier type guardrail be stiffer than normal MGS?  With 12" blockouts, the width of the system would be around 3 feet.  To keep part of the barrier or a vehicle that strikes the barrier from protruding into the opposing traveled way, what would be a reasonable working width? Would there be any problems associated with using half or quarter post spacing with a barrier guardrail in the median?
Keywords
  • Guardrail
Other Keywords none
Date May 10, 2016


Response
Response A two-sided or median type MGS system would be slightly stiffer and have lower deflections than a standard MGS system due to the rail on both sides of the barrier. 

At this time, the MGS median barrier with 12" blockouts was deemed NCHRP 350 compliant with an FHWA eligibility letter based on a submission we made comparing the MGS system to other previously tested median barrier designs.  

Subsequent to that eligibility letter, TTI performed two MASH tests on a version of the 31" tall MGS median barrier with 8" deep blockouts (see attached). Testing with the 2270P found a dynamic deflection of 39" and a working width of 55". The dynamic deflection is about 11% lower than the 43.9" dynamic deflection we observed in the original MASH testing of the MGS. However, the working width is slightly higher, as the original MGS MASH test found a working width of 48.6" The difference in the working widths is likely due to the additional width of the median system as compared to the roadside version. 

Based on this, the best working width guidance we have is the 55" number from the TTI testing. We cannot recommend reducing this value using reduced post spacing without full-scale crash testing based on concerns for rail rupture observed in the 1100C test performed on the system by TTI. In the 1100C test of the 31" tall MGS median barrier with 8" deep blockouts. the vehicle was safely capture and redirected, but a tear that extended 2/3 to 3/4 through the first rail splice downstream of impact was observed. We have seen similar tearing in select MGS small car tests due to what we believe is combined loading of the splice due to the lateral rail loading and vertical loading/bending of the splice due to the small car body being wedged under the rail. For example, the first test of the upstream end of the MGS AGT stiffness transition with a 4" tall wedge curb had a similar rupture that caused the test to fail. Thus, there is concern that further stiffening of the barrier through reduced post spacing may increase the rail loads and lead to a complete rail rupture. As such, we would recommend full-scale crash testing to evaluate reduced post spacing versions of the MGS median barrier prior to implementing reduced post spacing. 

Let me know if you have further questions or comments.
Date May 10, 2016
Attachment 9-1002-12-8.pdf
Attachment B-204 - MGS Median Barrier acceptance letter.pdf


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