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CRT posts behind a curb

Question
State IA
Description Text

We have an MGS post conflict at an existing curb intake.  As you have recommended, we will be installing the MGS long-span system to skip one post at the intake, placing three CRT posts upstream and downstream of the unsupported section. 

Since the CRT posts will be installed behind a curb, must we adjust the location of the weakening holes so that the center of the top hole is flush with the ground line?  Or can we still use the CRT posts that were utilized in the long-span crash tests (top hole centered 32" from top of post)?

Keywords
  • Guardrail
Other Keywords MGS, long-span
Date November 20, 2012


Response
Response In this situation, we would recommend that both the CRT holes be adjusted upward to account for the additional soil behind the curb. Thus, if a 6" curb was used in the installation, we would recommend that both the upper and lower holes in the CRT post be shifted 6" up on the post. Post embedment can remain the same (in this case slightly increased from a standard installation due to the curb).

The rational for shifting the holes is related to the function of the holes in the post.The holes reduce the cross-section of the post and act to weaken the post in bending and in shear. However, if the hole that is typically at groundline is buried, then the cross-section of the post is not weakened for shear loads across the base of the post at groundline, and the post may not fracture as designed. The bending section would still be reduced even with the hole being placed below groundline. Because the CRT is used with a curb in this case, the vehicle bumper may impact the post lower than a typical installation. Thus, the shear fracture of the post may become more critical. Thus, we recommend relocating the holes in the post as noted above.
Date November 21, 2012


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