View Q&A

NJDOT Standard Construction Detail “CD-609-8A.3 Rail Height
Determination” shows rail height where GR is flush with the gutterline to be
measured from gutterline.  A guide rail offset of 4 feet or greater from
curb will be measured from top of curb.  There will be a problem
connecting to a 31” MASH terminal where guide rail is flush with curb.
  The terminal height would be measured from top of curb since we
cannot bury/lower the terminal.  We would have to do a 25’ long height
transition to connect GR to terminal. 

Based on the MASH research and Midwest questions/answers,
can 31” MGS be measured from top of curb where it is flush with 4 inch high
curb.  What about a lower curb?

I do understand that all MASH terminal manufacturers
(Softstop, SKT, Fleet, etc.) are currently mum on what to do with their
terminals with curb.  We may have to change our curb height
guidelines/details as info comes out.  But we need to address the issue
above now to the best of our knowledge.  

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.

1. The best option with our current knowledge would be to terminate the curb prior to the end terminal. This allows the terminal and MGS to be used at their as-tested heights and provides the best potential for the barriers to perform as designed. As noted in the study above, in lieu of omitting the curb, one could use a minimal or short curb in the area of the terminal to reduce the effect of the curb in that area. Sloped curbs are preferred to vertical ones. For minimal curb heights, some transitioning in height to the standard 31” MGS would be needed after the end of the terminal.
2. The next option would be to install the MGS at 31” relative to the gutterline and use a 27 ¾” terminal. This would minimize the height difference between the terminal and the MGS. The performance of the MGS would be consistent with previous testing. However, the performance of the terminal in this type of installation is unknown, and they have not been evaluated to MASH.
3. A third option would be to install a 31” terminal on top of the curb and then transition in height to 31” MGS relative to the gutter after the terminal. Here the MGS would again be installed at its nominal height for the majority of the installation. However, the performance of a 31” terminal relative to the top of curb has not been tested. The best guidance we have on that application would be the preliminary work noted above.
4. A fourth and least desirable option would be to install both the MGS and the terminal at 31” with respect to the top of the curb. For this option, the performance of both the terminal and the guardrail has not been evaluated.

Please let me know if this addresses your question or if you have further comments.

Thanks  

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.