Numerical verification of the effectiveness of the alpha method for the estimation of the maximum rotational elastic response of eccentric systems

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-story linear-elastic systems representative of asymmetric seismic base-isolated building structures. This parameter (called “ALPHA”) has also led to the identification of a simplified procedure (called “ALPHA method”) for the estimation of the maximum rotational response of such systems. The main goal of this article is to verify the properties of the ALPHA parameter and the predictive capabilities of the ALPHA method, when applied to one-story systems representative of generic asymmetric building structures. The verification is carried out through a comprehensive set of 11,600 numerical simulations developed with reference to different representative structures subjected to historically recorded ground motions, with special attention devoted to the identification of the sensitivity of the ALPHA parameter and the 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. The article also introduces a simple code-like provision for conservative estimations of the maximum rotation developed under seismic input by asymmetric structures based upon the confidence interval concepts.