Those look like arbitrary numbers since all are the same and there's no rotation terms.
I've seen some skid layouts where the connecting flange is at the skid boundary but is the flange, itself, is not anchored. Instead, the anchor is located back, away from the flange. This arrangement gives a good boundary and (some) flexibility at the flange.
The Codes say the system shall be analyzed "as a whole". Your approach here is common but illustrates why systems should be analyzed as a whole.
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Dave Diehl