I have tried to create an equivalent model in staad for the said problem stiffness. For this the total portal stiffness is 24EI/H3 = 488. Now i have assumed a height of 1m for column which leaves me with two variables E and I. I have further freezed the material as steel so only one variable I is left. Within I there are two variables b & h; which i have considered equal (Square section). Thus i get the value of I and hence the corresponding equivalent stiffness is generated.
This system when excited, under any excitation; the fundamental natural frequency as represented by the first mode of vibration will be similar as it is the characteristic of the structure.
Hence i should be getting the frequency equivalent to the theoritical one. But i am unable to get it.
I see that there is some fundamental lacking in understanding and hence is very interesting to study where i am falling short.