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F-Shape in Front of Curb Adjacent to MSE Wall

Question
State KS
Description Text

I need some advice from you on a project currently being constructed.

 

This project is located in Topeka and involves a new road on new alignment that is adjacent to an active RR.  The road was designed using a 40 mph design speed with a projected traffic volume of 13,250 vpd, with 3% Trucks.

 

The roadway is elevated relative to the RR tracks.  A Mechanically Stabilized Earth Wall (MSEW) is being used.  Currently the roadway is designed with a type B curb with guard rail located about 4'-5' from the back of the curb.  The problem is that the guardrail posts get into the MSEW earth reinforcement straps and the separation fabric that is utilized to keep chloride laden runoff from coming in contact with the reinforcement straps.

 

These issues as well as future maintenance concerns with the guard fence design is pushing the need for an alternative design.  The current proposal is to install a 32" tall F-shape barrier about 3' from the face of the barrier to the back of the Type B Curb.  The barrier will be doweled into a 10" thick PCCP slab.  In most cases there will be about 3 feet behind the back of the barrier to the 3:1 slope or the MSEW.    My initial thought is that this option will be ok based on the site specific issues, such as the 40 mph design speed.  Note that we may change to a laydown curb (slope faced similar to AASHTO Type G however our curb height is only about 1 ½" flowline of curb) instead of the Type B curb.  My thought was that if it is ok for Type B curb then it should be OK for a laydown curb.  Do you agree?

Keywords
  • Temporary Barriers
Other Keywords none
Date July 21, 2008


Response
Response

You bring up a difficult problem. I am not aware of any crash test data used to determine the safety performance of vehicles launched over curbs and impacting 32-in. (813 mm) tall, safety shape barriers. Upon review of Chuck Plaxico's curb testing project, NCHRP Report No. 537, it was apparent that the bumper trajectory did not exceed 730 mm for a 2000P pickup truck vehicle impacting an AASHTO Type B curb at 70 kph and angles of 5, 15, and 25 degrees when placed within 1 m of the curb face. As such, it would seem unlikely that a 2000P (or 820C) vehicle would override a safety shape barrier with a top height of 38 in. (965 mm) above the roadway surface.

 

For us, another concern would be the effect that the forward placed curb would have on vehicle stability during vehicle redirection with a safety shape barrier. In lieu of this concern and due to the lack of sufficient test data, we cannot recommend placing a safety shape barrier behind a 6-in. tall Type B curb, even if located on a roadway with a 40 mph design speed. Instead, we would rather that you consider using a vertical, or near vertical, shape barrier for this application. If that option cannot be achieved, the next best alternative would be to utilize the single slope barrier, which offers improved safety performance over safety shape barriers.

Date July 22, 2008


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