That solver option is under Criteria - General (the Ritz option is still pretty good too). The first period was right at 5 seconds and the 2nd just slightly shorter, as I recall.
There are two factors about using rigid vs semi-rigid diaphragms. The first is that the diaphragm mass is lumped to a single point when the diaphragm is rigid. The second, more important, nuance is that when the diaphragm is rigid it's out of place stiffness is neglected in the lateral load analysis. i think this also applies to the eigensolution. So I like to use semi-rigid diaphragms to all concrete two way decks. See our post on diaphragms for more: communities.bentley.com/.../ram-ss-semirigid-diaphragms
There are two factors about using rigid vs semi-rigid diaphragms. The first is that the diaphragm mass is lumped to a single point when the diaphragm is rigid. The second, more important, nuance is that when the diaphragm is rigid it's out of place stiffness is neglected in the lateral load analysis. i think this also applies to the eigensolution. So I like to use semi-rigid diaphragms to all concrete two way decks. See our post on diaphragms for more: communities.bentley.com/.../ram-ss-semirigid-diaphragms