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NJDOT Standard Construction Detail “CD-609-8A.3 Rail Height
Based on the MASH research and Midwest questions/answers,
I do understand that all MASH terminal manufacturers
|Date||August 16, 2017|
I have a few thoughts. Typically, guardrail heights adjacent to curbs with small offsets are measured from the gutterline, while larger curb offsets tend to be measured from the top of curb as noted below.
First some background information. We did a simulation project looking at curbs placed adjacent to energy-absorbing end terminals (http://mwrsf.unl.edu/researchhub/files/Report338/TRP-03-358-17.pdf ). The simulation showed some promise that a 4” tall sloped curb (1:1 slope) utilized with a 31” tall MGS measured from the top of the curb (i.e. 35” above the gutterline) would not significantly affect the performance of the end terminal when impacted end-on (MASH tests 3-30, 3-31, 3-32, 3-33). However, none of the simulation models with curbs were validated due to lack of curb traversal tests and full-scale crash tests with end terminals in these configurations. Curbs lower than 4” should further minimize the effect of the curb. For example, Wisconsin uses a 1.5” tall gently sloping curb near end terminals to minimize the effect of the curb on the end terminal performance. Additionally, this study only looked at impact on the end of the terminal. Redirective impacts were not evaluated.
One option is to use no soil backfill behind the curb, but this is usually not desired. This was also not simulated, so the curb may present a snag hazard, especially for eccentric impacts (like test 3-30) where the vehicle yaws significantly.
Although the 27 ¾” height end terminals have not been tested to MASH with this rail height, placing a 27 ¾” height end terminal (measured above the top of the curb) attached to a 31” tall MGS (measured from the gutterline of the curb) would somewhat rectify the problem you are seeing as the terminal groundline features would remain at groundline. However, the 27 ¾” tall end terminals may be a problem if you are trying to meet MASH implementation.
Another research study was done regarding the maximum mounting height for the MGS (http://mwrsf.unl.edu/researchhub/files/Report1/TRP-03-255-12%20(revised).pdf ). In that study, 1100C vehicles were safely redirected with guardrail heights of 36”. However, not 2270P vehicle testing was done in hat study and it was noted that the increased height could affect the system anchorage.
Thus, to answer your questions below, 31” MGS has only been evaluated with small cars at increased heights like mounted above a 4” curb. The performance of the increased height system was never fully evaluated under MASH. Thus, the increased heights have not been implemented at this time.
Omission of the curb or the use of lower curbs is likely a better option.
As we see it, there are several options, but some are preferred. I have listed these below in order to preference.
Please let me know if this addresses your question or if you have further comments.
|Date||August 17, 2017|
|Response||Another thought. If we had 2 inch curb at the terminals that were less than or equal to 4 feet from the curb. MGS measured from gutter line and terminal height measured from top of curb. The flared and tangent end terminals are in effect the same as MSG guide rail beginning at the point of redirection at post #3. Could we not do the vertical transition in the last 12.5 feet of the tangent or flared terminal? Instead of doing the vertical transition after the terminal. If yes, we can transition the curb height back to 4 inches within the last 12.5’ of the terminal.
This may also be another question for the terminal company.
|Date||August 18, 2017|
It would be preferred to extend a 2” tall curb the whole length of the terminal (37.5’ or 50’) and not do the rail height transition until after the whole terminal length so that the transition in curb height and rail height does not affect end-on impacts into the terminal. In some end-on crash tests, the vehicles deflect 50’ into the terminal longitudinally. Additionally, in the end-on terminal impacts where the vehicle starts yawing (5 degrees, 0 degrees with a ¼ point impact, etc.), the curb will affect the vehicle and terminal performance some (although we don’t really know how much of an effect it will have). Having a rail height transition and a curb height within the first 50’ or within the terminal length could further compromise the vehicle and terminal performance, although we don’t know how much.
|Date||August 19, 2017|
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