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|Description Text||KDOT has a project where we are installing corral rail on a low fill culvert. We will be attaching MGS guardrail to the corral rail and have some questions concerning the 4” edge curb we would typically install with our bridges when we attach guardrail to the corral rail. Please see the attached standard drawing for the curb location. The project is scheduled to go to letting in the near future and I would like to discuss a few alternatives we are considering with you. Please give me a call when you have an opportunity.|
|Date||February 26, 2013|
Over the last few months, we have been discussing KsDOT Drawing No. RD613A which pertains to a thrie beam approach guardrail transition with the new, simplified, steel-post MGS stiffness transition. This combined system is depicted with a lower 4” wedge curb under the first 12’ 6” segment of nested thrie beam rail and adjacent to the concrete corral rail. An additional drawing, No. RD611A, provides details for the MGS.
The region adjacent to the bridge end is configured with half-post spacing of three W6x15 steel posts. Our original MWT test series utilized similar posts in this region but with the thrie beam attached to a thrie beam bridge railing that also utilized an upper channel rail. The upper channel rail was carried onto the first post or two and tapered down to shield an exposed end. When this approach guardrail transition is anchored to a concrete end, the upper channel rail would not exist, thus slightly reducing the lateral stiffness of the first W6x15 post or two.
Over the years, thrie beam AGTs with half-post spacing near the bridge end have been successfully crash tested and evaluated (or even grandfathered based on other prior testing) when attached to: (1) thrie beam bridge railings with additional upper tube or channel rails as well as W6x15 posts (Missouri and FPL systems), (2) concrete parapet with tapered end sections, a backup thrie beam, as well as large wood posts (CALTRANS system), and (3) NDOR concrete bridge end with good tapered geometry and larger steel posts at half-post spacing and one large span with simulated post via a horizontal tube and offset blockout. Per my recollection, these systems did not utilize a lower concrete curb and met NCHRP Report No. 350 impact safety standards. Although the upper channel/tube is not used with first two W6x15 steel posts, I believe that this half-post spacing system would be crashworthy with or without a lower concrete curb as long as a reasonably tapered concrete end were utilized to mitigate concerns for wheel contact and snag as well as any subsequent vehicular instabilities that lead to rollover.
In the late 90s, an Iowa thrie beam approach guardrail transition system with steel posts at quarter-post spacing near bridge end and with a lower 4” concrete curb was successfully crash tested under NCHRP Report No. 350. Later, this same thrie beam AGT with curb was successfully crash tested under MASH. Unfortunately, a similar version of this transition did not provide satisfactory safety performance when evaluated without the curb under NCHRP Report No. 350. More recently, another MASH crash test was performed on the identical system to the original Iowa transition but without the curb, and again the test results were unsatisfactory. As such, most quarter-post spacing designs adjacent to bridge end may likely require the use of a lower concrete curb. We do have a successful test on a thrie beam transition with quarter post spacing and without the curb. However, this system attached to a single-slope median end and used thrie beam on each side of the parapet.
I have also attached our comments that were previously provided to Ohio and Iowa on a similar issue. After you review the attached information, please feel free to call me to further discuss. We can set up a time for the call as well.
|Date||February 26, 2013|
|Attachment||FW MGS transition to Concrete Parapet-2 (2.01 MB).zip|
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2200 Vine Street
Lincoln, NE 68583-0853
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