In previous research works, the authors have identified a key system parameter which controls the maximum rotational response under free and forced vibrations of one storey-linear elastic systems representative of asymmetric seismic isolated building structures. This parameter, called ALPHA, is defined as the mass radius of gyration of the structure multiplied by the ratio of the maximum rotational to the maximum longitudinal displacement response developed by a one-story eccentric system in free vibration. A compact exact closed-form solution for the ALPHA parameter is given for undamped systems. ALPHA parameter has also led to the identification of a simplified procedure (called ALPHA method) for the estimation of the maximum rotational response of such systems. Main goal of this paper is to verify the properties of the ALPHA parameter and the predictive capabilities of ALPHA method, when applied to one storey systems representative of generic asymmetric building structures. The verification is carried out through a comprehensive set of numerical simulations carried out with reference to a number of representative structures subjected to historically recorded earthquake ground motions, with special attention devoted to the identification of the sensitivity of the ALPHA parameter and ALPHA method upon the fundamental period of vibration of the structure (not yet considered in previous research works). The results obtained (a) confirm the effectiveness of the ALPHA parameter to capture the intrinsic propensity of an eccentric system to develop a torsional response, (b) confirm the capacity of the ALPHA method to effectively estimate the maximum rotational response of a given eccentric system under seismic excitation, and (c) indicate that the ALPHA method is only weakly sensitive upon the period of vibration of the structure, (c) indicate that the proposed ALPHA method when applied to non systems allow a conservative estimation of the maximum rotations.
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