Hi, I had been reading the book on 'Dynamics of Structures' authored by Prof.Anil K Chopra of University of Berkeley. In his book, the author has provided the natural period of vibration of some buildings like: 1) Transamerical building of San Francisco- California which is 2.90 seconds for vibration in north-south direction which was obtained by forced vibration tests My question is when under a forced vibration the building will not vibrate 'naturally'. I mean the vibration characteristics during a forced vibration are not now only dependent on the natural characteristics of the system . Right? Hence, the tiem period obtained (time required to complete one cycle of vibration) is not just the property of natural characteristics of the building but also the force applied. In such a case how natural period was obtained experimentally? Is it that they make the building go to resonance and instead measure the time period of the applied force during such an experiment? Please can anyone advise?