. I do not see a problem with utilizing washers underneath the culvert top slab. In fact, a similar attachment scheme was full-scale crash tested back with the original evaluation of this system – see report no. TRP-03-114-02 page 111 (available on MwRSF website/research hub). The tested configuration utilized only rectangular plate washers in the attachment of the through bolts to the underside of the culver slab. You could use this concept too, if desired. If you utilize washers of a similar size, you would not need to include the full-size washer plate in addition to the enlarged washers. The individual washers alone would suffice.
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