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Short Radius Guardrail

Question
State
Description Text

Last week we learned from Dean that the latest test of the short-radius guardrail failed the test with the 2000P under TL-3 conditions. Dean also mentioned that he would have no doubt that the same design would pass TL-2 conditions.

 

The State of Delaware has a situation where they need a short radius barrier. The specific question I was asked related to the NCHRP Report 230 design discussed in our Technical Advisory 5040.32: http://www.fhwa.dot.gov/legsregs/directives/techadvs/t504032.htm

 

The drawings are attached. The question was "The 8-foot design notes that the washer should be removed from the center post. Should the washers also be removed from the two posts closest to the center in the other designs?"

 

Now, my questions are:

 

1) Does your testing of the "new" short radius designs support removing those washers?

2) Should we scrap 5040.32 entirely and go with the MWRSF design even though it has not yet met Report 350?
Keywords
  • Guardrail
Other Keywords Short Radius
Date September 20, 2007


Response
Response

With regards to your first question about the posts in the 8' nose section, I have a couple of comments. First, we would not recommend any the center post in the nose of the system because it is not mounted in a foundation tube. Thus, even though it is a CRT post, it will rotate in the soil about its strong axis  and create a ramp that increases the potential for impacting vehicles to override the system. We have removed the center post in our current TL-3 design for that reason. As far as the two posts at the edge of the nose section, we have used extra strong post-rail connections at those posts in the current version of the TL-3 short-radius. We have increased the post-rail connection strength in the current design in order to keep the posts attached to the rail and prevent them from becoming debris under the wheels of the truck that can cause the vehicle to override the rail. We have observed that vehicles impacting short-radius systems display significant yaw motion as they are captured due to the geometry of the system. Breakaway posts in the system that become detached from the guardrail has been observed to get under the wheels of the vehicle as it yaws and allow the vehicle to override the rail. Thus, we would recommend that the washer be left on the post rail connection at those posts.

 

Your second question was whether or not we believe that the old design should be abandoned in favor of our current design. We believe that the old design should be abandoned for several reasons, and I will list a few of the most pressing. First, we do not believe that a W-beam system is capable of capturing both the small car and pickup truck size vehicles effectively. Second, we do not believe that the W-beam design has sufficient capacity to contain the pickup truck vehicle. Third, we do not believe that the system has sufficient anchorage to redirect vehicles along the side of the system.

 

As Dean mentioned, our current short-radius system has not met the TL-3 impact conditions at this time, however, we believe the system is much better than the older W-beam and thrie beam short-radius systems currently available. We are also confident that this design will meet TL-2. We would be willing to submit the details of the current design so that states could use it as a best available alternative as long as the following caveats are recognized.

 

1.       The MwRSF short-radius design is still under development and should not be considered the final version of the design. Further development of the system is planned and design details for the system may change in the future.

2.       Not all of the required TL-3 tests have currently been evaluated on the MwRSF short-radius design. We have been approaching the design of the system in a manner that addresses the most critical impacts first, so some tests in the required matrix remain to be resolved. Thus, the overall system behavior has not been entirely quantified at this time.

3.       The current MwRSF short-radius design places the bridge rail and approach transition on the TL-3  or primary roadway side of the system and uses an end terminal on the TL-2 or secondary roadway. This configuration was chosen based on engineering judgment as the most critical installation for testing purposes. Some installations may be different than the tested system in that they may have the bridge rail and end terminal on the opposite sides or some other configuration. These alternative configurations have not been thoroughly evaluated at this time and their behavior is not known.

4.       The performance of the MwRSF short-radius to date leads us to believe that it will certainly meet TL-2 impact criteria as designed due to the lower impact speeds and corresponding kinetic energy levels.

 

Hopefully this answers your questions as to our thoughts. Please contact me with any further comments and questions. 

Date September 20, 2007


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