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Guardrail posts in concrete

Question
State
Description Text

Can the MGS be placed in
concrete with a leave-out behind the posts?  The contractor in Alabama
installed posts through 8” diameter holes, and our Division Office in
Montgomery wants to know 1) is  that is OK? or 2) if not, what can be done
about it?  I referenced the TTI work on GR posts in rock, and suggested
the contractor could core another 8” hole behind the post, but the MWRSF should
be the final authority on that retrofit scheme.



Keywords
  • Guardrail
Other Keywords none
Date August 16, 2013


Response
Response

In my opinion, I believe that the MGS can be placed in mow strips if an adequate leave-out is utilized to allow sufficient post rotation without hanging up along the sides or prematurely making contact at the back of the hole. I think that it would be helpful to provide between 8” to 10” of ground line clear distance behind the steel post and the back end of the concrete hole surface based on an effective post resistance occurring through a lateral distance of 16” to 20” at a mid-rail height of 25” above grade. The post rotation point was assumed to occur at 2/3 of the 40” embedment depth.

 

A second 8” diameter hole may be capable of providing the effective depth if the post sits more toward the front of the first hole. If the holes are drilled separately, then more post rotation distance can be achieved to ensure 8 to 10”. However, the wedges between two hole diameters would need to be cut/chipped off to provide smooth side walls so as to not catch the post while deflecting. We had recommended and tested something like this for the post in rock configuration that is in the same FHWA memo – Case 1. It is better to provide extra clear width on the sides of the posts as well. For this situation, I think that it can be made to work with a second hole and shaving out the material between hole. A larger diameter back hole would be even better, if that is possible. For new construction, it would be good to continue to use the TTI recommendations for leave-outs for width.

Date August 16, 2013


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