Can drywall edges float on the ceiling?
In the field of an expanse of ceiling drywall nailed to the bottom cords, the drywall can rise with the chord and bend, but at 16" or less from a vertical partition one stops nailing to the bottom cord so the cord can lift without pulling the drywall with it. The drywall must be allowed to bend in this last 16" because at the intersection joint with the wall the ceiling drywall is joined to the wall drywall which cannot lift.