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A representative from the IL Tollway called requesting guidance on the use of a trailing end terminal to protect the upstream end of the rigid concrete parapet.
|Date||November 18, 2009|
Recently, you inquired about the implementation of the MGS with trailing end terminal for use in protecting the upstream end of a rigid concrete parapet. For your special situation, the concrete parapet was positioned approximately 4 ft behind the back side of the steel line posts. It should also be noted that you were referencing the ILDOT Standard 631011-06, Traffic Barrier Terminal, Type 2.
It was stated that there are special situations where the parapet is farther away from the traveled way than desired for the guardrail offset. In addition, there may be other circumstances that do not allow for the guardrail to be flared back toward the upstream end of the parapet and anchored to it. Therefore, preliminary guidance was requested for safely positioning the downstream region of the MGS and trailing end terminal to longitudinally overlap the upstream end of the parapet so that the MGS would shield blunt end impacts on the parapet end. Based on engineering judgment and in the absence of crash test results, we believe that a reasonable positioning would be to align post no. 5 with the upstream end of the parapet. This configuration would place approximately 22 ft of guardrail past the upstream end of the parapet.
It should be noted that future research should be directed toward determining the length of need where downstream guardrail systems with trailing end terminals are effective in safely containing and redirecting high-energy impacts with pickup trucks as well as small cars under the MASH safety guidelines.
|Date||November 18, 2009|
Post #12 (the middle post in the asymmetrical transition piece) was omitted to avoid conflict with a drainage structure. See attachments. Is this allowed or what recommendation would you have for this detail?
|Date||November 25, 2009|
|Response||Over the last year or so, I have fielded a few inquiries regarding the inability to place a post in a transition region due to a pipe culvert flowing out of a curb inlet. In past inquiries and depending on the transition used, I have suggested using a simulated post at such a location. I believe you are depicting the transition that used W6x12s by 90 in. long in the noted region. If we wanted to replace the midspan capacity, it would seem reasonable to use two W6x9s by 72 in. long posts " one on each side of the lateral pipe and behind the rail. Then, a WF beam would be placed between the two posts. This beam would be used to support a deep blockout and allow the two smaller and shorter posts to serve a one post where it could not be placed. Of course, this surrogate system has not been actually designed for your case but would be a concept that likely would work in this scenario. It would give back the resistive capacity at a location in the transition that may be prone to vehicle pocketing and snag. Let me know what you think.|
|Date||December 7, 2009|
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