【正文】
fundamental periods of vibration may cause resonance effects. For this reason also their variation, as a consequence of possible building damage and/or soil non linear behavior during the motion, needs to be considered. Typically, structural and geotechnical engineers have different points of view about resonance effect and its variation during a seismic motion. Structural engineers say that whereas building and soil have initially close periods and an earthquake occurs, the building period, as a result of structural and non structural damage, is expected to increase during the motion, so that the building “hides” itself reducing the heaviest effects of resonance. Geotechnical engineers do not pletely agree with this opinion saying that also the soil period can shift towards higher values, that is in the same direction of the building one, thus the resonance condition could arise again. The question to be dealt with is: how much is the relative amount of that increase? A general answer is not possibl