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What are the guidelines/limitations for using a curb in front on the MGS transition?
Also, what is the appropriate blockout depth?
|Date||February 27, 2012|
I have some answers regarding your questions on the simplified approach transition for the MGS.
1. Using a curb in front of the MGS transition
a. Installing 6" curb in front of the MGS and upstream of the transition will cause a curb to be present throughout the length of the guardrail system. This will put us in a gray area. The MGS was tested with a curb, and the bridge transition was tested with a curb, but the upstream transition (or transition to the transition) has not yet been evaluated with a curb.
Adding a curb to the transition can lead to a number of problems for both the small car and pickup truck vehicles. The 2270P vehicle will be subjected to a vertical force component on the impacting side as the tire rolls over a curb. This vertical force combined with the changing stiffness of the transition may lead to stability issues (namely vehicle roll) as the vehicle is redirected. Theses factor could lead to vehicle rollover " similar to that seen for the recent MGS test with 6" curb placed 8 ft behind the rail. With the curb placed much closer to the rail (within a foot) the vertical force has less time to create instabilities, but this may still cause problems since the rail stiffness in the transition area is not constant (post spacing reduces and rail becomes larger/stiffer as you move downstream).
For 1100C vehicle, the small car bumper has recently shown a propensity to extend under the 31" high rail and snag on the steel posts (demonstrated by both the recent transition test, MWTSP-3, and the MGS test placed on a Gabion Wall, MGSGW-2). During the mentioned tests, the vehicle was able to bend the posts over and continue downstream without violating the ORA or OIV values. However, with inclusion of a curb and additional soil fill behind it, the post becomes stiffer and the moment arm for post bending is reduced. Thus, bending the posts over as the vehicle impacts the system will take more force and energy. Further, the as the tire rides up the curb the vehicle may become wedged between the curb and the bottom of the guardrail leading to further decelerations. The combination of these phenomena may lead to a violation of the ORA or OIV values (the transition test already saw a 14.7 longitudinal ORA and a 27.5 longitudinal OIV " recall maximum allowable values are 20.49 and 40, respectively.
Any curbs that accompany a given thrie beam transition design should remain part of the "new" system. However, the curb should be terminated ( via sloped or flared end) without extending into the w-to-thrie transition element. Again the concerns were that the addition of a curb could (1) lead to further snagging of small cars between the curb and the downward slope of the bottom of the w-to-thrie transition element, and (2) cause vehicle instabilities due compressing the suspension and creating vehicle climb. If the curb used in your existing system already terminates prior to the downstream end of the w-to-thrie-transition piece, then use it as previously designed. If not, the upstream end of the curb should be altered to meet this specification.
After identifying these potentially critical mechanisms, MwRSF is hesitant to recommend the use of the upstream transition with a curb until further evaluation is conducted (most likely full-scale crash testing). However, I can point out a few design elements that would minimize the increased risk of adding a curb.
1. Extending the 4" triangular curb throughout the upstream transition would incorporate less of a vertical force to the vehicle than would a 6" high curb. Therefore, the 4" curb should be extended upstream at least 12.5 ft (2 full post spacings) past the first 37.5" or ½ post spacing. The transition to 6" curb can then be made over the next post spacing upstream of this point.
2. To mitigate some of the increased snag potential for the small car, it may be wise not to fill in the soil behind the 4" curb in the upstream transition area (specifically from the beginning of reduced post spacings to the first 6.5 ft long post. This would eliminate the extra force and energy required to bend the posts over if the vehicle bumper gets under the rail.
2. Blockout depth in the transition
a. The blockouts used on the W6x9 posts in the transition were 12" deep. The W6x15 posts used 8" deep blockouts. 12" deep blockouts would be acceptable for use on the W6x15 posts as well if that was desired. We would not recommend the use of blockout depths shorter than those used in the tested transition.
Let me know if you have further questions.
|Date||February 28, 2012|
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