It seems there is a problem in your etabs model. In my opinion there may be column(s) and beam(s) that their nodes are not jointed ( I mean they are not working properly,load etc. is not transferred ) or at the base level your restraint definitions are not valid.But your findings quite strange because for there are many simple formulations in the litareture and codes to check first period and for such a building your results seems wrong.. For example up to 10 storey building you can use 0.1N (N=highest storey number)
for 6 floor 0.1*6=0.6 sec can be expected.
regards
pontiac
for 6 floor 0.1*6=0.6 sec can be expected.
regards
pontiac