Hi ,
There are few considerations you need to follow .
(1) The Mass Modelling : The portal frame is assigned with Self-Weight X 1 , Self-Weight Y 1 and Self-Weight Z 1 .
This will instruct Staad to lump mass on all the joints from the nearby member selfweight which means it is not behaving as a SINGLE DEGREE OF FREEDOM (SDOF) system .
You could check this if you go to the same folder and open mass.txt file . This file will reports the lumped mass at each joint .
(2) Selecting right equivalent stick model : You need manually apply the total selfweight of the structure while mass modelling to a single joint where the translational displacement resistance at that joint along the first mode shape is same as the displacement resistance in case of your SDOF system . This will ensure that the concentrated mass will experience the same degree of stiffness in both the cases along the same shape of oscillation .
I have created equivalent the stick model (SDOF) by selecting a section so that its tip joint lateral displaces the same magnitude as that of the portal frame node under unit lateral load to ascertain if I have same degree of stiffness for that shape function .
Attached are the equivalent stick model and the modified Portal Frame whose fundamental NF is same as that of the Portal Frame