Installing soffits

[catch22]

Im going to be installing aluminum soffits,they will be 26 inches long and ive read that it should have a center support also,my question is if you nail the soffits at the fascia board and in the center of the soffit will it buckle between the two nailing areas when it expands and contracts,because normally it can expand in the j channel when you just nail it to the fascia board only?

Thanks Mike