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4" Edge Curb (MGS Guardrail Approach Transition)

Question
State KS
Description Text After our meeting on Wednesday I checked the Nebraska and CALTRANS transition design and found the following information (See attachments for more detail). See page 3 in the Caltrans attachment and pages 3 and 4 in the Nebraska attachment. 3’-1.5” - KDOT distance from beginning of bevel to center of 1st post ? - Nebraska – Unclear from details, but appears to be similar to Caltrans (i.e. less than 3’-1.5”) 33” (953 mm) – Caltrans distance from beginning of bevel to center of 1st post 3” KDOT – distance from end of bevel to back of rail 14” (350 mm) Nebraska – distance from end of bevel to back of rail 5” (125 mm) Caltrans – distance from end of bevel to back of rail From this information it appears KDOT’s 1st post is offset a greater distance from the bevel than the other two designs and the end of the bevel is located closer to the back of the thrie-beam than the other two designs. According to our discussion on Wednesday it’s my understanding KDOT should not remove the curb in place with the transition design we currently use. Is this correct?
Keywords
  • Approach Guardrail Transitions
Other Keywords Curb
Date March 1, 2013


Response
Response

The Nebraska AGT detail uses a distance of approximately 37.5” based on center of splice to first simulated post via tube rail and blockout. The slope and lateral offset of tapered concrete end has worked well. An earlier version with less slope and offset was tried but resulted in excessive wheel snag and floorboard deformation.

 

Thus, it would be good for KDOT longitudinal distance to be similar or less than others.

 

It would be beneficial to have longer flatter flare with more offset that short steep flare with minimal offset. A larger lateral offset reduces tendency for wheel to impact on end of concrete. A flatter flare reduces scrubbing forces along the tapered section as well as helps to prevent a rapid wheel redirection which can contribute to vehicle roll during redirection. In other words, a wheel snag on the front face of a short taper may behave like a wheel contacting the upstream end.

 

For half-post spacing design, I believe that it could be installed with or without curb if end section contact is reduced, taper design is conservative, and distance to first post within range of those noted. Knowledge of crash tests of several systems gets us to this general opinion.

 

We also noted that standardization of concrete ends with half- and quarter-post spacing designs and w/ and w/o curbs would be desirable in future. Some MASH testing with 1100C and 2270P vehicles may possibly be necessary.

 

Date March 1, 2013


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