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Culvert Mounted MGS Post Spacing Near End of Culvert

Question
State WI
Description Text

I have some questions about spacing of the top mounted beam guard near the end of the culvert.



 



I’m not sure how we should do the spacing when there is not enough room to use the standard spacing.



MASH
TL-3

W-beam Guardrails

Systems to Shield Culverts


Date June 30, 2022
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Attachment 2022-06-30 Top Mount.pdf
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It appears that you are looking for options when one of the ½ post spacing posts mounted to the culvert falls on or very near the edge of the culvert. I believe that we recommended that a post be no closer than 4” to the edge of a culvert.

 

We are a little reluctant to extend the post spacing in that area above 37.5” in order to be consistent with the ½ post spacing the system was evaluated with. As such, we would not recommend using the first two options you have given as the spacing gets larger.

 

The best option would likely be to shift the entire installation to prevent the post placement issue. However, there may be existing guardrail you wish to connect to without removing or the culvert length may be such that you can’t shift the system and get all off the posts to land with the right spacing on the culvert.

 

We considered a modified, offset baseplate. However, that may be difficult to design and implement without more research.

 

If you cannot shift the system, we believe the best option would be to utilize 1/4 post spacing adjacent to the obstructed post as shown below. This should eliminate the need for additional holes being added to the rail and should accommodate the obstructed post without drastic altering system performance.

 

Standard post spacing = 75”

 

½ post spacing = 37.5”

 

¼” post spacing = 18.75

 

 


Date July 1, 2022
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Attachment culvert post spacing.jpg
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Simplar question but for a skewed culvert. see attached. 


Date July 6, 2022
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Attachment 2022-07-22 Top Mount Example 02.pdf
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For the example shown, I don’t believe you would need to do anything to the spacing. See attached. I have placed an X through two of the posts on the detail that should maintain the ½ post spacing. The concern you have is with the circled post that violates our edge distance recommendations.

 

However, in the case you have shown, the internal wall below the culvert effectively thickens the edge of the culvert and provides increased concrete resistance. It would be similar to having a thick edge beam on the edge of a bridge deck. This should allow one to effectively restrain the post even with the reduced edge distance.

 

Let me know if that answers your question or if you wish to dish further.


Date July 8, 2022
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Attachment 2022-07-22 Top Mount Example 02_v2.pdf
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Following up on the previous email and our discussion.

In terms of have epoxy anchors if there are issues with washer plates or the nuts and bolts when the anchors fall near the edge of an internal wall, we encountered this during full-scale testing of the system as well. We simply used the epoxy anchor option there and saw not issues for posts in the impact region. The photos below show an example of one of those posts. No damage was noted to the culvert posttest in those locations.

In terms of a minimum offset or distance to the edge when you have the interior wall at the edge of the culvert, we believe that the culvert wall should provide increased capacity with the interior wall due to the depth of the section and the reinforcement present in that area. We believe that installation of an anchor as much as half the width of the interior wall would likely be acceptable. Recall that a minimum of 4 in. (102 mm) was recommended in the report between a free end of a culvert headwall and the center of any attachment anchor. Thus, having a shorter distance if installation in an internal wall seems resoanble.

There may be some reduction in overall anchor capacity due to the proximity to a free edge, but it is believed that most of the anchor capacity could still be retained, and certainly enough to provide substantial post resistance. If one wanted to protect from culvert damage due to the loading of the anchor near the edge, one could embed the anchor deeper when installed in the interior wall to take advantage of the increased depth of the section and provide better anchor capacity. Increasing the embedment to 12” or more would help reduce the damage potential and concerns for anchor capacity. An example of this can be seem of epoxt anchor testing we conducted for the BR27C bridge rail for Iowa. These anchors were able to develop high loads even within a narrow parapet.  - https://mwrsf.unl.edu/researchhub/files/Report313/TRP-03-325-15.pdf

 

Thanks


Date July 11, 2022
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Attachment culvert post at interior edge wall 1.jpg Attachment culvert post at interior edge wall 2.jpg
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So this is acceptable…


Date July 12, 2022
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Attachment 2022-07-26 Top Mount Example 03.pdf
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Yes. 


Date July 14, 2022
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