View Q&A

I reviewed your questions regarding the MGS median barrier and have put comments below.

1.       Can the Midwest System without block outs be used in a median application like this?  Assuming we mirror the detail and place another section of rail along the back of the post, would this negatively affect its performance?

a.       Currently, there are two approved versions of the MGS median barrier system. MwRSF sought approval of an MGS median barrier with 12” deep blockouts based on the performance of proprietary 31” tall median barrier systems that were tested under NCHRP 350. These include the NUCOR NU-GUARD 31 and the Gregory GMS 31 which have both been given FHWA approval in double-sided median configurations under the NCHRP Report No. 350 safety requirements (FHWA Approval Letters B-150B and B-162). Neither of these systems used blockouts in their median barrier configurations, and the Gregory system was tested with the guardrail splices at the posts locations. In the approval, we argued that a MGS median barrier system would be specified with splices at the midspan locations between the posts and using 12-in. deep blockouts, as used on the standard roadside MGS design. Previous testing and analysis of guardrail systems had shown that the use of blockouts and placement of the guardrail splices away from the posts tends to increase the capacity of guardrail systems and reduce the potential for vehicle snag. Thus, a MGS median barrier system would have improved safety performance as compared to the existing approved 31-in. high, median W-beam guardrail systems.

Since that time, TTI has performed research into the use of 8” deep blockouts with the MGS median barrier system. TTI successfully tested this system with both the 1100C and the 2270P vehicles under the MASH requirements, which validated our original approval. No issues were noted in those test with respect to reduced blockout depth from 12” to 8”.

No research has been done to date with respect to non-blocked MGS median barrier. However, the previous performance of the MGS median barrier with 8” blockouts and NCHRP 350 testing of non-blocked proprietary median barrier designs would suggest that the non-blocked MGS median barrier has a good likelihood of performing acceptably. The concerns with a non-blocked MGS median barrier are that the reduced offset block may increase the snag on the posts which are supported on both sides by W-beam rail. This may increase the forces developed by the snagged posts and the loads transferred to the vehicle. However, occupant ridedown accelerations (ORA) and occupant impact velocities (OIV) from previous MGS testing with and without blockouts involving wheel snag have not indicated concern and it is not believed that the additional support of the post by the backside rail would prove sufficient to increase the snag forces to unacceptable levels. Review of the 1100C testing conducted by TTI on the reduced blockout median system would seem to support this conclusion,  as occupant risk values were not increased to significantly when compared to previous testing of the roadside MGS system with 8” blockouts. However, testing of non-blocked MGS did indicate that removal of the blockouts tended to increase the ORA and OIV levels observed. While these increases were noted, they were not near critical levels. Thus, we believe that a non-blocked version of the MGS median barrier would likely perform acceptably, but there is some concern regarding the effect of vehicle snag on the posts that has not been accurately quantified. You may want to consult with FHWA and see if they would concur with this opinion.  

2.        Has the non-blocked out system been accepted by FHWA?  I found their letter for the standard 31” system but was wondering if they had reviewed it without the block outs. 

a.       The roadside version of the MGS without blockouts has been accepted. I have attached the approval. The report detailing the effort can be found at the following location.

http://mwrsf.unl.edu/researchhub/files/Report9/TRP-03-262-12.pdf

3.       Do you know of any median end treatments / terminals that could be used with the type of installation I am proposing?  All of the median end treatments that I have seen use block outs on each side of the post.  Does Midwest have a special transition detail to go from a non-blocked out section to one that uses blocks?  I’m guessing we will have to make some kind of transition like that in order to use a crashworthy end treatment.

a.       I do not know of any median terminals that do not utilize blockouts.

As far as transitioning blockout depth from existing median terminals to non-blocked guardrail, we would recommend the following approach. Begin transitioning blockout depth no sooner than 12.5’ from the end of the terminal system. At that point, you can transition the blockout depth from 8” to 0” over 25 ft of guardrail using gradually reduced blockout depth. This should only generate a 1.5 degree flare in the rail which should have minimal effect on performance.