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I need to shorten the top strap for the MGS mounted to culvert parapet. The NDOR typical parapet is only 8” wide. What is the correct offset behind the threaded rod anchor? Should the cover over the threaded rod be 2”? or centered in the parapet?  What should the strap length be?

For the culverts in which the headwall is narrow (yours are 8”), I would not utilize the top-mounted, single-anchor design to attach the socket to the outside of the headwall.  For that design, it’s important to maintain the 7” anchor offset from the outside face to prevent concrete damage.  Unfortunately, that will not leave you enough concrete cover on the inside of the anchor, 2 inches is recommended. We never designed the top mounted attachments for offsets less than 7”.   

However, you could utilize either the wrap-around design, or the side-mounted design (through bolt).  See pages 36-41 of the report (TRP-03-277-14) for the design details of these attachment options.  The only difference you would need to make is the length of the strap or bolts to reflect the correct headwall width. 

Note, although only 2 of the 5 design concepts were included in the final drawing details, MwRSF has confidence that all five of the concepts provide adequate strength to support the system.  Thus, any of the five concepts can be utilized to satisfy the installation needs of existing culverts.