08-22-2009, 12:26 AM
(08-01-2009, 07:59 AM)fashion Wrote: Which is the fundamental period of the structure in Etabs out of the, say 100, that it produces?
Fundamental period can be misleading if you don't understand what dof response you required. Many textbook said that fundamental period is the period of the first mode. Well it is correct if the structure is 2D and the response you want is in the horizontal direction which is mostly example in the books for rigid beam plane frame under lateral earthquake.
In 3D structure with rigid diaphragm with 3 floor dof then there will be 3 fundamental period and of course you have to see the mode shape in which dof it moves then the period of the mode is the fundamental period in that dof direction.
Another interesting problem is as follow. If you want to design a transfer beam or cantilever beam under vertical earthquake excitation and this beam is part of a whole large structure what is the fundamental period for vertical response spectrum? Our professors had no idea of what is the fundamental period because of the associated fundamental period only to 1st mode of all those mode periods of the structures.
I would suggest you to ask people who main works is in vibration they are the one who invent this modes and fundamental period things :D