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As you are aware the WisDOT is an avid user of the Culvert Mounted, Strong Post MGS System.

I was reading through the Research Report TRP-03-383-20-R1 and am hoping you can answer

a few questions I have.

In Section 8.1 “Recommendations”, on page 144 it says,

“In order to prevent interference with post rotation in soil, the first guardrail post within the

half-post spacing MGS adjacent to the culvert should have a minimum 12-in. clear distance

to any part of the culvert. The clearance should limit a rotated and displaced guardrail post

from interacting with the culvert.”

• On the two attached sketches I have shown where I interpret the 12” offset to be placed.

Is my interpretation correct?

In Section 8.1 “Recommendations”, on page 145 it says,

“The culvert mounted guardrail post should not be placed too close to the upstream or

downstream ends of a culvert. If a post and anchorage is placed near the end of a headwall,

the attachment anchors may not have enough concrete cover to develop the required shear

and/or tension loads. Thus, a minimum of 4 in. should be used between a free end of a

culvert headwall and the center of any attached anchor.” I assumed headwall meant the end

vertical wall. (If my assumption is incorrect, is there a minimum offset between end wall interior

face and the closest anchor bolt?).

• On the two attached sketches I have shown where I interpret the 4” offset to be placed.

Is my interpretation correct?

• Also when you speak of tension and shear in this paragraph, is it referring to the concrete

breakout cone for tension and the concrete breakout cone in shear which is covered in

Section 17 of ACI-318 Specifications?

My final question would be:

• When placing this system on a culver that has a skew, and trying to meet these required

offsets, is it acceptable to possibly reduce a post spacing to make everything fit?

Thanks for any information you can share.

Thanks for reaching out. Sorry for the delay in replying. I was traveling this week for an NCHRP project.    

For your first question, the sketches you have shown do interpret the guidance correctly. We generally wan to ensure that the post can rotate back freely and that there is sufficient soil behind and adjacent to the post to provide resistance. Your diagrams meet those criteria.

For the minimum distance to the headwall guidance, we likely should have been more clear with that. That guidance was intended to suggest that the anchor be a minimum of 4” from a clear edge of the concrete structure. Thus the anchor does not have to be 4” completely inside the culvert headwall, but rather from the outside edge of the concrete one is anchoring to. This is done to ensure that one can develop the anchor capacity appropriately. The 4” guidance was based primarily on the through bolted option of the culvert mounted post. As you noted below, epoxy anchored post would likely need to be based on the ACI guidance which would require 1.5 times the embedment. One could likely be more aggressive based on the fact that the culvert likely has reinforcing steel that aids in anchor development.

Your final question on skew is a good one. Erik Emerson with WisDOT is currently trying to develop a research project to analyze post spacing to deal with the skew issue using computer simulation. We cannot give a definitive answer without that research, but it likely that using a single reduced post spacing would have the potential to adjust for the skewed culvert without adversely affecting the system performance.

Thanks!