Sometimes supports lift-off during transportation therefore you need to add temporary supports (search for sea-fastening)
But if you have non-linear supports (+Y), the way you construct the load cases is not a valid approach. You apply a vertical acceleration over a system without weight and you will obtain non real vertical displacements where maybe there is no vertical displacement at all.
Change the loadcases definition or use linear supports. Observe the differences.
regards,
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Dan